Biochar Effects on Soil Nutrient Transformations

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                                Biochar Effects on Soil Nutrient

                        Tom H. DeLuca, M. Derek MacKenzie and Michael J. Gundale


               Nutrient transformations are influenced by a            matter of speculation. Biochar additions to
               myriad of biotic and abiotic factors.                   soil have been found to stimulate mycorrhizal
               However, to date, there have been no                    infection (Saito, 1990; Ishii and Kadoya,
               attempts to synthesize the literature regarding         1994) and influence P solubility in forest soils
               the influence of biochar on soil nutrient               (Gundale and DeLuca, 2007), which may be
               transformations. Although the major focus of            responsible for observed increases in P
               this book is to review biochar as a soil amend-         uptake. The influence of biochar on sulphur
               ment in agro-ecosystems, the majority of the            (S) transformations has received little or no
               literature that addresses the effects of biochar        attention and has not stood out as a dominant
               on nutrient transformations has originated              effect of adding biochar to natural soil envi-
               from studies in natural forest ecosystems.              ronments. However, biochar applications to
               The addition of biochar to forest soils has             mineral soils may have a noted effect on P
               been found to directly influence nitrogen (N)           and S transformations in manure-enriched
               transformations in phenol-rich acidic forest            agro-ecosystems. The mediation of nutrient
               soils of both temperate (DeLuca et al, 2006;            turnover by biochar has significant implica-
               Gundale and DeLuca, 2006; MacKenzie and                 tions for organic agricultural systems where
               DeLuca, 2006) and boreal (DeLuca et al,                 biochar may increase stabilization of organic
               2002; Berglund et al, 2004) forest ecosys-              nutrient sources (Glaser et al, 2001) and
               tems. Applying biochar to forest soils along            reduce nutrient leaching losses (Lehmann et
               with natural or synthetic fertilizers has been          al, 2003).
               found to increase the bioavailability and plant              The purpose of this chapter is to provide
               uptake of phosphorus (P), alkaline metals               a state-of-knowledge review of the influences
               and some trace metals (Glaser et al, 2002;              of biochar on N, P and S transformations in
               Lehmann et al, 2003; Steiner et al, 2007), but          soil ecosystems and to provide an overview of
               the mechanisms for these increases are still a          the known and potential mechanisms driving
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              these processes. This chapter provides a           modifies nutrient transformations; and the
              discussion of the nutrient content of biochar;     direct and indirect influences of biochar on
              the potential mechanisms by which biochar          soil nutrient transformations.

                                          Nutrient content of biochar

              Prior to considering the influence of biochar      substantial variability in the chemical compo-
              on nutrient transformations, the nutrient          sition of the resulting biochar and is
              capital associated with biochar additions          discussed in greater detail in Chapter 5.
              must be considered. In other words, does                Nitrogen is the most sensitive of all
              biochar serve as a significant source of nutri-    macronutrients to heating; thus, the N
              ents irrespective of other inputs? The             content of high-temperature biochar is
              nutrient content of biochar is discussed in        extremely low (Tyron, 1948). Pyrolysis
              depth in Chapter 5. It is important to note        conditions during the production of wheat
              that biochar is somewhat depleted in N and         straw biochar resulted in the loss of about 50
              slightly depleted in S relative to more ther-      per cent of the S at temperatures of 500°C
              mally stable nutrients. During the pyrolysis       and about 85 per cent of the S was lost in
              or oxidation process that generates biochar,       950°C pyrolysis (Knudsen et al, 2004),
              heating causes some nutrients to volatilize,       greatly reducing the S content of the resulting
              especially at the surface of the material, while   biochar. Accordingly, extractable concentra-
              other nutrients become concentrated in the         tions of NH4+ and PO4-3 generally decrease
              remaining biochar (see Chapter 5).                 with increasing pyrolysis temperature during
                   Temperature, the time a material is held      biochar generation, with a portion of NH4+
              at a given temperature and the heating rate        being oxidized to a small exchangeable NO3-
              directly influence the chemical properties of      pool at higher temperatures (Gundale and
              biochar. Individual elements are potentially       DeLuca, 2006).
              lost to the atmosphere, fixed into recalcitrant         Gundale and DeLuca (2006) evaluated
              forms or liberated as soluble oxides during        the effect of temperature on biochar forma-
              the heating process. In the case of wood-          tion from several woody substrates collected
              based biochar formed under natural                 from a Montana ponderosa pine/Douglas fir
              conditions, carbon (C) begins to volatilize        forest. High temperature (800°C) biochar
              around 100ºC, N above 200°C, S above               demonstrated higher pH, electrical conduc-
              375ºC, and potassium (K) and P between             tivity (EC) and extractable NO3- relative to
              700ºC and 800ºC. The volatilization of             low temperature (350°C) biochar (see Figure
              magnesium (Mg), calcium (Ca) and                   14.1). In contrast, density, extractable PO4-
              manganese (Mn) occurs at temperatures              3, NH4+ and soluble and total phenols were
              above 1000ºC ( Neary et al, 1999; Knoepp et        lower in high-temperature biochars relative to
              al, 2005). Biochar produced from sewage            low-temperature biochars (see Figure 14.2).
              sludge pyrolysed (heated in the absence of         These data suggest that substantial variation
              oxygen) at 450°C contains over 50 per cent         can occur in the chemical properties of
              of the original N (although not in a readily       biochar due to the temperature that the plant
              bio-available form) and all of the original P      material reaches during charring.
              (Bridle and Pritchard, 2004). The relative              Biochar additions to soil provide a
              concentration and molecular speciation of          modest contribution of nutrients depending,
              these elements during heating generates            in part, upon the nature of the feedstock
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                                                          BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS                             253

                 Figure 14.1 The pH, electrical conductivity (EC), cation exchange capacity (CEC) and density of
                     biochar produced from Douglas fir or ponderosa pine wood or bark at 350°C or 800°C
               Note: Data meeting the assumptions of normality were compared with one-way ANOVA followed by the Student-Neuman-Kuels
               post-hoc procedure where letters indicate pair-wise differences. Non-normal data were compared using the Kruskal–Wallis (K–W)
               Source: adapted from Gundale and DeLuca (2006)

                                                                                Figure 14.2 The soluble PO4-3, NH4+ and
                                                                                NO3- concentration in biochar produced from
                                                                                Douglas fir or ponderosa pine wood or bark at
                                                                                350°C or 800°C
                                                                                Note: Data meeting the assumptions of normality were
                                                                                compared with one-way ANOVA followed by the Student-
                                                                                Neuman-Kuels post-hoc procedure where letters indicate
                                                                                pair-wise differences. Non-normal data were compared using
                                                                                the Kruskal–Wallis statistic.
                                                                                Source: adapted from Gundale and DeLuca (2006)
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              (wood versus manure) and upon the temper-          as a primary source of nutrients (Glaser et al,
              ature under which the material is formed           2002; Lehmann et al, 2003). Therefore, the
              (Bridle and Pritchard, 2004; Gundale and           following discussion is focused on biochar as
              DeLuca, 2006). However, biochar is likely          a modifier of N, P and S transformations in
              more important as a soil conditioner and           mineral soils.
              driver of nutrient transformations and less so

                          Potential mechanisms for how biochar modifies
                                     nutrient transformations
              Biochar is a high surface area, highly porous,     significantly increased net nitrification
              variable charge organic material that has the      (Berglund et al, 2004). Furthermore, the
              potential to increase soil water-holding           addition of natural field-collected biochar – a
              capacity, cation exchange capacity (CEC),          soil-neutral phosphate buffer slurry – imme-
              surface sorption capacity and base saturation      diately stimulated nitrification potential with
              when added to soil (Glaser et al, 2002;            no shift in the pH of the suspension (DeLuca
              Bélanger et al, 2004; Keech et al, 2005; Liang     et al, 2006). This puts into question the
              et al, 2006).The surface area, porosity, nutri-    assumption that pH is the major driver of the
              ent content and charge density all change in       nitrification response to biochar additions to
              relation to the temperature of biochar forma-      soil. It is possible that archaeal ammonia
              tion (Gundale and DeLuca, 2006;                    oxidizers (Crenarchaeota), which have the
              Bornermann et al, 2007). Biochar additions         capacity to nitrify under low pH conditions
              to soil also have the potential to alter soil      (Leininger et al, 2006), are the primary driv-
              microbial populations and to shift functional      ers of nitrification in coniferous forest soils.
              groups (Pietikäinen et al, 2000) and have the           Bio-available C may be adsorbed to
              potential to reduce soil bulk density              biochar surfaces, thereby reducing the poten-
              (Gundale and DeLuca, 2006). The broad              tial for immobilization of nitrate formed
              array of beneficial properties associated with     under biochar stimulation of nitrification.
              biochar additions to soil may function alone       Biochar added to soil with an organic N
              or in combination in order to influence nutri-     source yielded an increase in net nitrification;
              ent transformations, described below. The          however, the addition of organic N with or
              physical characteristics of biochar are            without biochar resulted in high rates of
              discussed in Chapter 2, pH and nutrient            NH4+ production that were not immobilized
              contents in Chapter 5, and biotic influences       (DeLuca et al, 2006), reducing the likelihood
              in Chapter 6. Here, we extend this discussion      of this explanation.
              by exploring the known and potential effects            Biochar may act as a habitat or safe site
              of these bio-physico-chemical changes on           for soil microorganisms (Pietikäinen et al,
              nutrient transformations.                          2000) involved in N, P or S transformations.
                   It is well understood that autotrophic        Biochar certainly has the capacity to support
              nitrifying bacteria are favoured by less acidic    the presence of adsorbed bacteria
              soil conditions (Stevenson and Cole, 1999).        (Pietikäinen et al, 2000; Rivera-Utrilla et al,
              Thus, biochar additions to mineral soil that       2001) from which the organisms may influ-
              increase soil pH are likely to favourably influ-   ence soil processes. Both saprophytic and
              ence nitrification. However, activated biochar     mycorrhizal fungi have been observed to
              and glycine added to acid boreal forest soils      colonize soil biochar; but the significance of
              was found to have no influence on pH, but          their presence has not been clarified (Saito,
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                                                  BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS          255

               1990; Zackrisson et al, 1996). Some               DeLuca, 2006). Additional studies have
               researchers have suggested that the small         demonstrated that biochar formed during
               pore sizes of biochar might exclude grazing       wildfires or agricultural residue burning also
               protozoa and nematodes, allowing for the          functions to adsorb phenolic and various
               proliferation of fungi and bacteria (Warnock      aromatic      and      hydrophobic     organic
               et al, 2007). However, recent studies suggest     compounds (; Yaning and Sheng, 2003;
               that bacteria and fungi primarily colonize the    Brimmer, 2006; DeLuca et al, 2006; Gundale
               surface of biochar, but that limited oxygen       and DeLuca, 2006; MacKenzie and DeLuca,
               availability may limit growth inside the small    2006; Bornermann et al, 2007). Through
               internal pores (Yoshzawa et al, 2005).            these sorption reactions, biochar may reduce
                    The high surface area, porous and often      the activity of compounds that may be either
               hydrophobic nature of biochar makes it an         inhibitory to nutrient transformation special-
               ideal surface for the sorption of hydrophobic     ists, such as nitrifying bacteria (White, 1991;
               organic compounds (Cornelissen et al, 2004;       Ward et al, 1997; Paavolainen et al, 1998), or
               Bornermann et al, 2007). Numerous papers          reduce the concentration of phenolic
               have reported a reduction in soluble or free      compounds in the soil solution that would
               phenolic compounds when activated carbon          otherwise enhance the immobilization of
               is added to soils (DeLuca et al, 2002;            inorganic N, P or S (Schimel et al, 1996;
               Wallstedt et al, 2002; Berglund et al, 2004;      Stevenson and Cole, 1999).
               Gundale and DeLuca, 2006; MacKenzie and

                                Direct and indirect influences of biochar on
                                       soil nutrient transformations
               Nitrogen                                          N through the methods of ammonification
                                                                 (where NH4+ is formed) and nitrification
               Nitrogen is the single most limiting plant        (where NO3- is formed). Ammonification is a
               nutrient in most cold or temperate terrestrial    biotic process driven primarily by
               ecosystems (Vitousek and Howarth, 1991).          heterotrophic bacteria and a variety of fungi
               In soils, the majority of N exists in complex     (Stevenson and Cole, 1999). Nitrification is
               organic forms that must be ammonified to          considered to be a strictly biotic process that
               NH4+ and then nitrified to NO3- prior to          is most commonly mediated by autotrophic
               uptake by most agricultural plants                organisms, including bacteria and archaea, in
               (Stevenson and Cole, 1999). Recent studies        agricultural, grassland and forest soils
               have demonstrated that the addition of            (Stevenson and Cole, 1999; Grenon et al,
               biochar to surface mineral soils may directly     2004; Leininger et al, 2006; Islam et al,
               influence N transformations. Here we review       2007). Biochar has been found to increase
               the evidence for the direct and indirect influ-   net nitrification rates in temperate and boreal
               ences of biochar on ammonification,               forest soils that otherwise demonstrate no net
               nitrification, denitrification and N2-fixation,   nitrification (Berglund et al, 2004; DeLuca et
               and provide potential mechanisms that may         al, 2006). There is no evidence for such an
               be driving these relationships.                   effect in grassland (DeLuca et al, 2006) or
                                                                 agricultural soils (Lehmann et al, 2003;
               Ammonification and nitrification                  Rondon et al, 2007), which may already
               Nitrogen mineralization is the process            accommodate an active nitrifying commu-
               whereby organic N is converted to inorganic       nity. Results from the studies cited above are

   Table 14.1 Effect of biochar (natural biochar, lab-generated biochar or activated carbon) on nitrogen mineralization and nitrification from

                                                 studies performed in different forest ecosystems
Ecosystem            Biochar type         Nutrient source            Control                        Biochar addition                Statistical   Reference

                                          and incubation        NH4+-N            NO3--N          NH4+-N          NO3--N            difference
Ponderosa pine,      Wildfire biochar,    Glycine in          150 ± 200      200 ± 100      700 ± 400    1200 ± 500                 No NH4+       MacKenzie and

western Montana      ponderosa pine       greenhouse,       (µg N cap-1)1 (µg N cap-1) (µg N cap-1) (µg N cap-1)                    Yes No3-      DeLuca (2006)
                     wood                 resin collected,
                                          30 days
Ponderosa pine,      Lab biochar,         (NH4)SO4               NA           40 ± 5           NA           70 ± 3                  Yes No3-      DeLuca et al
western Montana      ponderosa pine       and KH2PO4       (µg N g soil-1) (µg N g soil-1)              (µg N g soil-1)                           (2006)
                                                                                                                                                                                                          Page 256

                     wood                 in lab, aerobic
                                          incubation, 15 days
Ponderosa pine,      Lab biochar,         Glycine in lab,      47 ± 4          5±1         ppw 2 20 ± 5 ppw 21 ± 4                  Yes NH4+      Gundale and
western Montana      ponderosa pine       aerobic          (µg N g soil-1) (µg N cap-1)     ppb 25 ± 6   ppb 20 ± 8                 Yes No3-      DeLuca (2006)
                     (wood and bark),     incubation,                                       dfw 32 ± 8   dfw 11 ± 8                 Yes NH4+
                     Douglas fir          14 days                                           dfb 27 ± 3    dfb 16 ± 4                Yes No3-
                     (wood and bark)                                                                                                No NH4+
                                                                                                                                    No No3-
                                                                                                                                                                   BIOCHAR FOR ENVIRONMENTAL MANAGEMENT

                                                                                                                                    No NH4+
                                                                                                                                    Yes No3-
Scots pine,          Activated carbon     Glycine in field,   20 ± 13          0.06 ± 0.02 low3 410 ± 99 low 0.12 ± 0.03            Yes NH4+      DeLuca et al
Sweden                                    resin collected, (µg N cap-1) ‡      (µg N cap-1) high 780 ± 302 high 1.89 ± 1.1           No No3       (2002)
                                          30 days                                            (µg N cap-1) (µg N cap-1)              Yes NH4+
                                                                                                                                     Yes No3
Scots pine,          Activated carbon     Glycine in lab,       46 ± 6         2.8 ± 0.4       1350 ± 50       5.5 ± 0.6            Yes NH4+      Berglund et al
Sweden                                    aerobic            (µg N g soil-1) (µg N g soil-1) (µg N g soil-1) (µg N g soil-1)        Yes No3-      (2004)
                                          60 days
Scots pine,          Activated carbon     Glycine in             20 ± 3        0.20 ± 0.20       146 ± 42          0.6 ± 0.1        Yes NH4+      Berglund et al
Sweden                                    field, resin       (µg N cap-1) ‡    (µg N cap-1)     (µg N cap-1)     (µg N cap-1)       No No3-       (2004)
                                          75 days
Note: 1 Ionic resin analysis used approximately 1g mixed bed resin in nylon mesh capsules approximately 25.4mm in diameter.
2 ppw = ponderosa pine wood; ppb = ponderosa pine bark; dfw = Douglas fir wood; dfb = Douglas fir bark biochar produced at 350ºC.
3 Low biochar application rate of 1000kg ha-1; high application rate was 10,000kg ha-1.
NA = not available.
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                                                   BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS             257

               summarized in Table 14.1, specifically focus-       tion (DeLuca et al, 2006). Nitrifier activity,
               ing on ammonification and nitrification in          as measured using an aerated slurry method
               biochar or activated carbon-amended soil            (Hart et al, 1994), was found to be extremely
               samples, field plots or mesocosms in compar-        low in soils collected from sites that had not
               ison to un-amended controls. Both activated         been exposed to fire for approximately 100
               carbon (DeLuca et al, 2002; Berglund et al,         years and relatively high in soils exposed to
               2004) and biochar collected from recently           recurrent fire (DeLuca and Sala, 2006). The
               burned forests (DeLuca et al, 2006;                 addition of field-collected biochar to soils
               MacKenzie and DeLuca, 2006) or generated            expressing no net nitrification readily stimu-
               in laboratories by heating biomass in a muffle      lated nitrifier activity in a 24-hour aerated soil
               furnace (Gundale and DeLuca, 2006) were             slurry assay (DeLuca and Sala, 2006;
               found to stimulate net nitrification in forest      DeLuca et al, 2006). The addition of biochar
               soils.                                              to grassland soils that already demonstrated
                    Nitrification was found to be below the        relatively high levels of nitrification had no
               detection limit in the acidic phenol-rich, late     measureable effect on nitrifier activity
               succession forest soils of northern Sweden.         (DeLuca et al, 2006). A small increase in
               whereas, forest sites recently exposed to fire      nitrification was observed in sterile control
               were found to have measurable levels of nitri-      samples amended with sterile biochar,
               fication (DeLuca et al, 2002). The injection        suggesting that the oxide surfaces on biochar
               of glycine (a labile organic N source) into         may stimulate some quantity of auto-oxida-
               these late succession soils readily stimulated      tion of NH4+ (DeLuca et al, 2006).Wood ash
               ammonification, but failed to stimulate any         commonly contains high concentrations of
               nitrification. The injection of activated           metal oxides, including CaO, MgO, Fe2O3,
               carbon into the humus layer induced a slight        TiO2, and CrO (Koukouzas et al, 2007).
               stimulation of nitrification (see Table 14.1),      Exposure of biochar to solubilized ash may
               but the injection of glycine with activated         result in the retention of these potentially
               carbon consistently stimulated nitrification,       catalytic oxides on active surfaces of the
               demonstrating that ammonification in these          biochar (Le Leuch and Bandosz, 2007).
               soils was substrate limited, whereas nitrifica-     These oxide surfaces may, in turn, effectively
               tion was being inhibited by a factor that could     adsorb NH4+ or NH3 and potentially catalyse
               be mitigated by adding activated carbon             the photo-oxidation of NH4+ (Lee et al,
               (DeLuca et al, 2002; Berglund et al, 2004). In      2005).
               all soils treated with activated carbon, a              The rapid response of the nitrifier
               significant reduction in soluble phenols was        community to biochar additions in soils with
               recorded (DeLuca et al, 2002; Berglund et al,       low nitrification activity and the lack of a
               2004). It is possible that the activated carbon     stimulatory effect on actively nitrifying
               adsorbed organic compounds that either              communities suggest that biochar may be
               inhibited net nitrification (White, 1991;Ward       adsorbing inhibitory compounds in the soil
               et al, 1997; Paavolainen et al, 1998) or caused     environment (Zackrisson et al, 1996), which
               immobilization of the accumulated NO3-              then allows nitrification to proceed. Fire
               (McCarty and Bremner, 1986; Schimel et al,          induces a short-term influence on N avail-
               1996).                                              ability; but biochar may act to maintain that
                    Biochar collected from forests that had        effect for years to decades after a fire (see
               been exposed to recent forest fires was found       Figure 14.3).
               to stimulate net nitrification in soils from low-       The temperature of biochar formation
               elevation ponderosa pine forests that               and the type of plant material from which the
               otherwise demonstrated little or no nitrifica-      biochar is generated also potentially influ-
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                                                                                 Figure 14.3 Hypothetical
                                                                                 change in N availability with
                                                                                 time since the last fire where
                                                                                 biochar induces a fast turnover
                                                                                 of N for years after a fire event
                                                                                 Source: adapted from MacKenzie and
                                                                                 DeLuca (2006)

              ences ammonification and nitrification            community, but not under the sedge commu-
              (Gundale and DeLuca, 2006). Gundale and           nity (see Table 14.1; MacKenzie and
              DeLuca (2006) evaluated how biochar               DeLuca, 2006). The shrub mesocosm was
              produced at two different temperatures            found to have high concentrations of free
              (350°C and 800°C) from the bark and wood          phenolic compounds whose recovery was
              of two different tree species common to west-     greatly reduced by adding biochar.The sedge
              ern North America (ponderosa pine and             mesocosm had low concentrations of free
              Douglas fir) influences N mineralization and      phenols and measurable levels of net nitrifi-
              nitrification. All biochar treatments increased   cation prior to biochar additions.
              nitrification, except for Douglas fir wood,       Nitrification in the sedge mesocosm was
              which suggests that for some species, bark        stimulated by glycine addition to soil without
              may create a more effective biochar than          biochar, suggesting that nitrification under
              wood. In these experiments, biochar addition      sedge was substrate limited (not inhibited)
              to soil also caused reduced ammonification        and thus not affected by adding biochar. In
              compared to the control (Gundale and              this study, charred material was scraped off
              DeLuca, 2006).This is possibly due to NH4+        of the outside of burned trees, making it simi-
              adsorption to biochar (Berglund et al, 2004).     lar to the charred bark material described
              Results were similar for biochar created at       above. It is possible that the wildfire biochar
              800ºC, except for ponderosa pine bark,            functioned as an inoculant, introducing nitri-
              which did not significantly increase nitrifica-   fying bacteria into the soil system; however,
              tion. It is clear that the temperature of         nitrifying bacteria are found in most forest
              formation and type of organic material pyrol-     soils, but often induce little net nitrification
              ysed are important factors to consider when       due to rapid N immobilization rates (Stark
              assessing the effects of biochar on nutrient-     and Hart, 1997) or inhibition, as described
              cycling processes in soil, and ones not easily    above.
              dealt with given the multiplicity of combina-          Biochar additions to agricultural soils of
              tions that these two factors represent.           the tropics have been reported to either
                   In a study of forest floor/mineral soil      reduce N availability (Lehmann et al, 2003)
              mesocosms collected intact from a site not        or to increase N uptake and export in crops
              exposed to fire for over 100 years in western     (Steiner et al, 2007). Reduced N availability
              Montana, biochar was found to stimulate           may be a result of the high C/N ratio of
              nitrification under the ericaceous shrub          biochar and, thus, greater potential for N
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                                                     BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS              259

               immobilization (see below) or due to biochar           Liang et al, 2006), which could induce net
               adsorption of NH4+ (described above),                  immobilization of inorganic N already pres-
               which in turn reduces the potential for N              ent in the soil solution or applied as fertilizer.
               leaching losses and sustained higher N fertil-         Low temperature biochar, in particular,
               ity over time in surface soils (Steiner et al,         would likely induce net immobilization when
               2007). It should be noted, however, that               applied to mineral soils as microbes degrade
               immobilization potential associated with               residual bio-oils (Steiner et al, 2007) or
               biochar additions to soil would be greatly             surface functional groups (Liang et al, 2006).
               limited by the recalcitrant nature of biochar          This immobilization process could create a
               (DeLuca and Aplet, 2007).                              temporary reservoir of organic N, which
                    To summarize, biochar additions to acid           would reduce the potential for leaching of
               phenol-rich soils that lack net nitrification          inorganic N in highly leached soils (Steiner et
               have the potential to stimulate nitrification.         al, 2007).
               Biochar additions to agricultural and grass-                There have been no studies that have
               land soils that already demonstrate net                directly evaluated the influence of biochar on
               nitrification will likely have no effect on nitrifi-   NH3 volatilization. Ammonia volatilization in
               cation and may express a slight decline in net         agricultural soils is favoured at alkaline pH
               ammonification due to NH4+ adsorption or               and when high concentrations of NH4+ are
               enhanced immobilization. Whether adsorp-               present (Stevenson and Cole, 1999). Biochar
               tion is sustained after biochar weathering in          and biochar mixed with ash have the poten-
               soil or decreases as shown for adsorption of           tial to raise the pH of acid soils (Glaser et al,
               polyaromatic hydrocarbons to biochar                   2002), but not to a level that would increase
               (Chapter 18) remains to be investigated.               volatilization (Stevenson and Cole, 1999).
                                                                      Biochar additions to agricultural soils, as well
               Immobilization, volatilization and                     as acid forest soils, have been found to reduce
               denitrification                                        NH4+ concentrations, which could be a result
               Little direct evidence exists to demonstrate           of volatilization; but it is more likely that
               the effect of biochar on N immobilization,             surface adsorption of NH4+ (Le Leuch and
               volatilization or denitrification. The latter is       Bandosz, 2007) reduces soil NH4+ concen-
               discussed in greater detail in Chapter 13. A           trations and reduces the potential for NH3
               few studies have suggested that biochar can            volatilization.
               adsorb both NH4+ and NH3- from the soil                     Denitrification is a biotic dissimilatory
               solution (Lehmann et al, 2006), thus reduc-            process in which NO3- is reduced to N2 (g) in
               ing solution inorganic N at least temporarily,         the absence of O2. Several intermediates
               but perhaps concentrating it for microbial             (including NO and N2O) are formed during
               use. Because biochar residing in soil becomes          this reductive process and are potentially
               occluded with organic matter (Zackrisson et            released into the soil atmosphere when condi-
               al, 1996; Wardle et al, 1998) or aggregates            tions are not favourable for complete
               both mineral and organic matter fractions              reduction of NO3- to N2. The influence of
               together into physically protected pools               biochar on denitrification is partially covered
               (Brodowski et al, 2006), the N in those                in Chapter 13, where it is demonstrated that
               organic matter pools may remain unavailable            biochar has the potential to catalyse the
               for some period of time.                               reduction of N2O to N2, potentially reducing
                    As discussed above, biochar is an N-              the emission of this important greenhouse
               depleted material having a uniquely high C/N           gas to the atmosphere.
               ratio. Some decomposition occurs when fresh                 To date, there have been few studies that
               biochar is added to soil (Schneour, 1966;              directly address the influence of biochar on
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              denitrification. Our evaluation of the fire         living N2-fixing organisms. Rondon et al
              effects literature suggests that biochar could      (2007) tested the effect of adding different
              directly or indirectly influence denitrification.   amounts of biochar to nodulating and non-
              The process of denitrification requires the         nodulating varieties of the common bean,
              presence of substrate (available C) and a           Phaseolus vulgaris, inoculated with Rhizobium
              terminal electron acceptor, such as NO3-            strains, and measured changes in N uptake
              (Stevenson and Cole, 1999). An increase in          using an isotope pool dilution technique.
              net nitrification in acid forest soils when         Biochar significantly increased N2 fixation
              biochar is added (e.g. DeLuca et al, 2006)          compared to a control; but the highest appli-
              would increase the potential for denitrifica-       cation rate, 90g biochar kg-1 soil, did not
              tion under anaerobic conditions where               produce the highest soil N concentration or
              available C is high. Adding manure with             plant biomass (Rondon et al, 2007). The
              biochar (e.g. Lehmann et al, 2003; Steiner et       study further indicates that biochar may
              al, 2007) would potentially increase bioavail-      stimulate N2 fixation as the result of
              able C in the soil solution. The combination        increased availability of trace metals such as
              of these two factors could increase denitrifi-      nickel (Ni), iron (Fe), boron (B), titanium
              cation potential in mineral soils amended           (Ti) and molybdenum (Mo). The highest
              with a mixture of biochar and manure.               rates of biochar application decreased the
                   Drawing from fire effects literature,          magnitude of the effect and, if taken to the
              Castaldi and Aragosa (2002) found that fire         extreme, might interfere with N2 fixation.
              treatments caused co-variation between              Legume nodulation might also be affected if
              moisture content, NH4+ concentrations and           added biochar interfered with signalling
              denitrification enzyme activity (DEA); but          compounds in the soil environment
              the trends were only evident during the             (Warnock et al, 2007). The formation of root
              wettest time of the year, which was                 nodules in leguminous plants is initiated by
              September to November in the studied                their release of flavonoids, which are
              Mediterranean climate. In a ‘light fire treat-      polyphenolic signalling compounds (Jain and
              ment’,      DEA       varied    with      NH4+      Nainawatee, 2002). Biochar is highly effec-
              concentration and in the ‘intense fire treat-       tive in the sorption of phenolic compounds,
              ment’, DEA varied with soil moisture content        including flavanoids (Gundale and DeLuca,
              (Castaldi and Aragosa, 2002). It is not clear       2006). Therefore, high biochar applications
              whether the high-intensity fire treatment           may interfere with signal reception and initia-
              yielded a greater amount of biochar or not, as      tion of the legume root infection process.
              higher intensity fires generally result in               Free-living N2-fixing bacteria are
              greater volatilization of C and a greater           ubiquitous in the soil environment. Agro-
              potential to deposit ash rather than biochar        ecosystems that enhance the presence of
              (Neary et al, 1999). However, the fact that         these organisms may reduce the need for
              DEA varied with moisture content and not            external inputs. Unfortunately, to date, there
              pH suggests that ash production was minimal         are no studies that directly demonstrate an
              (Castaldi and Aragosa, 2002).                       influence of biochar on free-living N2-fixing
                                                                  bacteria. In forest restoration studies involv-
              Biological nitrogen fixation                        ing prescribed fire, Burgoyne (2007) found
              Biological N2 fixation is uniquely important        no effect of fire treatments on the activity of
              in low-input agro-ecosystems where external         free-living N-fixing bacteria, although these
              N inputs are minimal. Therefore, it is impor-       same plots revealed a significant increase in
              tant to know whether biochar applications           biochar in both the forest floor and surface
              have the capacity to alter symbiotic or free-       mineral soil (DeLuca, unpublished data). It is
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                                                  BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS            261

               well understood that excess soluble N in the       •   the release of P salts from woody tissues
               soil solution reduces N2-fixation rates in free-       during charring;
               living N2-fixing bacteria (Kitoh and Shiomi,       •   biochar interference with P sorption to Al
               1991; DeLuca et al, 1996) and available soil           and Fe oxides;
               P stimulates N2 fixation (Vitousek et al,          •   biochar-induced changes in the soil ion
               2002). Therefore, it is possible that the activ-       exchange capacity; and
               ity of these N2-fixing bacteria could be           •   biochar sorption of plant and microbial
               increased in an environment where applied              chelates.
               biochar functions to increase P solubility
               (Lehmann et al, 2003; Steiner et al, 2007)         The release of P from biochar has long been
               and reduce soluble soil N concentrations           recognized (Tyron, 1948), and the mecha-
               (due to immobilization or surface adsorption       nism for direct P release from biochar is not
               of NH4+). Conversely, biochar additions to         complex. The concentration of P in plant
               forest soils that stimulate nitrification (e.g.    tissues is small relative to the large concentra-
               DeLuca et al, 2006) may ultimately down-           tion of C, and a significant portion of plant P
               regulate N2 fixation by free-living N2-fixing      is incorporated within organic molecules
               bacteria.                                          through ester or pyrophosphate bonds
                                                                  (Stevenson and Cole, 1999). This organic P
               Phosphorous                                        in dead plant tissues is not available for plant
                                                                  uptake without microbial cleavage of these
               Similar to N cycling, microbial turnover and       bonds.When plant tissue is heated, organic C
               decomposition regulate P mineralization and,       begins to volatilize at approximately 100°C,
               thus, influence how much P is available for        whereas P does not volatilize until approxi-
               plant uptake. In contrast to N cycling,            mately 700°C (Knoepp et al, 2005).
               however, P availability is also greatly affected   Combustion or charring of organic materials
               by a series of pH-dependent abiotic reactions      can greatly enhance P availability from plant
               that influence the ratio of soluble-to-insoluble   tissue by disproportionately volatilizing C
               P pools in the soil. Several studies have          and by cleaving organic P bonds, resulting in
               demonstrated enhanced P uptake in the pres-        a residue of soluble P salts associated with the
               ence of biochar; but very little work has          charred material. Gundale and DeLuca
               focused on the variety of mechanisms               (2006) demonstrated this as an increased
               through which biochar may directly or indi-        extractable PO4-3 from biochar made from
               rectly influence the biotic and abiotic            bark and bole samples of Douglas fir and
               components of the P cycle. In this section we      ponderosa pine trees from a Montana pine
               discuss a few of these mechanisms, including:      forest. Furthermore, it was found that char-
                                                                  ring at both low and high temperatures
               •   biochar as a direct source of soluble P        (350°C and 800°C) resulted in a significant
                   salts and exchangeable P;                      extractable PO4-3 pool from all substrates,
               •   biochar as a modifier of soil pH and           but that extractable P declined in biochar
                   ameliorator of P complexing metals             produced at high relative to low tempera-
                   (Al3+, Fe3+2+, Ca2+); and                      tures, where the volatilization threshold for P
               •   biochar as a promoter of microbial activ-      had been reached. Increased extractable P in
                   ity and P mineralization.                      soils amended with a variety of charred mate-
                                                                  rials has also been observed for tropical soils
               Soluble P salts and exchangeable P                 (Glaser et al, 2002; Lehmann et al, 2003).
               Altered P availability associated with biochar          In addition to directly releasing soluble P,
               is likely due, in part, to:                        biochar can have a high ion exchange capac-
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              ity (Liang et al, 2006), and may alter P avail-      these precipitation reactions occur is strongly
              ability by providing anion exchange capacity         influenced by soil pH due to the pH-depend-
              or by influencing the activity of cations that       ent activities of the ions responsible for
              interact with P. It has been demonstrated that       precipitation (Al3+, Fe2+3+ and Ca2+)
              fresh biochar has an abundance of anion              (Stevenson and Cole, 1999). In alkaline soils,
              exchange capacity in the acid pH range               P solubility is primarily regulated by its inter-
              (Cheng et al, 2008), which can initially be in       action with Ca2+, where a cascading apatite
              excess of the total cation exchange capacity         mineral pathway develops. In acid soils, P
              of the biochar. It is possible that these positive   availability is primarily regulated by its inter-
              exchange sites compete with Al and Fe oxides         action with Al3+ and Fe2+3+ ions, where
              (e.g. gibbsite and goethite) for sorption of         highly insoluble Al- and Fe-phosphates form.
              soluble P, similar to that observed for humic        Biochar may influence precipitation of P into
              and fulvic acids (Sibanda and Young, 1986;           these insoluble pools by altering the pH and,
              Hunt et al, 2007).To date, however, there is a       thus, the strength of ionic P interactions with
              noted lack of studies evaluating the effect of       Al3+, Fe2+3+ and Ca2+ (Lehmann et al, 2003;
              short-term anion exchange capacity on P              Topoliantz et al, 2005) or by sorbing organic
              cycling and availability.                            molecules that act as chelates of metal ions
                   As biochar ages, the positive exchange          that otherwise precipitate P (DeLuca,
              sites on biochar surfaces decline and negative       unpublished data; see below).
              charge sites develop (Cheng et al, 2008).The              Numerous studies have demonstrated
              biochemical basis for the high CEC is not            that biochar can modify soil pH, normally by
              fully understood, but is likely due to the pres-     increasing pH in acidic soils (Mbagwu 1989;
              ence of oxidized functional groups (such as          Matsubara et al, 2002; Lehmann et al, 2003).
              carboxyl groups), whose presence is indi-            There are few, if any, studies that have
              cated by high O/C ratios on the surface of           demonstrated a reduction in pH with biochar
              charred materials following microbial degra-         addition in alkaline soils, however, the addi-
              dation (Liang et al, 2006; Preston and               tion of acid biochar to acidic soils has been
              Schmidt 2006) and is further influenced by           observed to reduce soil pH (Cheng et al,
              the great surface area (Gundale and DeLuca,          2006). An increase in pH associated with
              2006) and high charge density of biochar             adding biochar to acid soils is due to an
              (Liang et al, 2006). Phosphorus availability         increased concentration of alkaline metal
              and recycling may be influenced by the               (Ca2+, Mg2+ and K+) oxides in the biochar
              biochar CEC over long timescales and in soils        and a reduced concentration of soluble soil
              that have inherently low exchange capacities.        Al3+ (Steiner et al, 2007). Adding these alka-
              By reducing the presence of free Al3+ and            line metals, both as soluble salts and
              Fe3+ near root surfaces, biochar may promote         associated with biochar exchange sites, is
              the formation and recycling of labile P frac-        likely the single most significant effect of
              tions. This is also an area of research that         biochar on P solubility, particularly in acidic
              deserves greater attention.                          soils where subtle changes in pH can result in
                                                                   substantially reduced P precipitation with
              Complexation                                         Al3+ and Fe3+. In contrast, adding biochar
              A significant component of the P cycle               (and associated ash residue) to neutral or
              consists of a series of precipitation reactions      alkaline soils may have a limited effect on P
              that influence the solubility of P, ultimately       availability because adding alkaline metals
              influencing the quantity of P that is available      would only exacerbate Ca-driven P limita-
              for uptake and actively recycled between             tions. In support of this, Gundale and
              plants and microbes. The degree to which             DeLuca (2007) found reduced concentra-
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                                                           BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS                               263

                    Figure 14.4 Soluble P leached from columns filled with (a) calcareous soil (pH = 8) amended
                       with catechin alone or with biochar; or (b) acid and Al-rich soil (pH = 6) amended with
                                             8-hydroxy quinoline alone or with biochar
               Note: Studies were conducted by placing 30g of soil amended with 50mg P kg-1 soil as rock phosphate into replicated 50mL leaching
               tubes (n = 3). Soils were then treated with chelate, or chelate plus biochar (1 per cent w/w) in comparison to an un-amended
               control, allowed to incubate for 16 hours moist and then leached with three successive rinsings of 0.01M CaCl2. Leachates were
               then analysed for orthophosphate on a segmented flow Auto Analyser III. Data were subject to ANOVA by using SPSS.
               Source: DeLuca, unpublished data

               tions of resin-sorbed PO4-3 in a neutral (pH                        lic acids, amino acids, and complex proteins
               6.8) forest soil when the soil was amended                          or carbohydrates (Stevenson and Cole,
               with a biochar generated by wildfire contain-                       1999).
               ing a high concentration of soluble salts                                The sorption of chelates may have a posi-
               (including Ca2+). However, these biochar                            tive or negative influence on P solubility. A
               amendments did not appear to inhibit the                            clear example of this type of interaction is
               growth of grass seedlings (Gundale and                              provided in Figure 14.4. Here, two
               DeLuca, 2007).                                                      compounds that have been reported as possi-
                   In addition to its effect on soil pH,                           ble allelopathic compounds released as root
               biochar may also influence the bioavailability                      exudates (catechin and 8-hydroxy-quinoline)
               of P through several other mechanisms asso-                         (Vivanco et al, 2004; Callaway and Vivanco,
               ciated with P precipitation, such as                                2007) have also been reported to function as
               biochar-induced surface sorption of chelating                       potent metal chelates (Stevenson and Cole,
               organic molecules. Biochar is an exception-                         1999; Shen et al, 2001) that may indirectly
               ally good surface for sorbing polar or                              increase P solubility. Catechin effectively
               non-polar organic molecules across a wide                           increased P solubility in an alkaline (pH 8.0)
               range of molecular mass (Sudhakar and                               calcareous soil and the 8-hydroxy-quinoline
               Dikshit, 1999; Schmidt and Noack, 2000;                             increased P solubility when added to an
               Preston and Schmidt, 2006; Bornermann et                            acidic (pH 5.0) and Al-rich soil (see Figure
               al, 2007). Organic molecules involved in                            14.4). The addition of biochar to these soils
               chelation of Al3+, Fe3+ and Ca2+ ions will                          eliminated the presence of soluble chelate in
               potentially be sorbed to hydrophobic or                             the soil system and, in turn, eliminated the
               charged biochar surfaces. Examples of such                          effect of the chelate on P solubility (DeLuca,
               chelates include simple organic acids, pheno-                       unpublished data). This interaction may
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              explain the observed reduction in P sorption       and pore size can occur, and this can influ-
              by ionic resins with increasing biochar appli-     ence the size of organisms able to enter
              cation rates in the presence of actively           biochar (Pietikäinen et al, 2000) and the total
              growing Koleria macrantha (Gundale and             surface area of biochar that could sorb
              DeLuca, 2007). Such indirect effects of            compounds (Keech et al, 2005). The pore
              biochar on P solubility would vary with soil       size of wood-derived biochar may range from
              type and vegetative cover and underscores          approximately 10µm2 to approximately
              the complexity of plant–soil interactions.         3000µm2, depending upon the species from
                                                                 which it is derived (Keech et al, 2005). Thus,
              Microorganisms                                     some biochars may create pore spaces for
              Biochar may have an indirect effect on P           bacteria and fungi that are safe from even the
              availability and uptake by providing a benefi-     smallest soil grazers, such as protozoa,
              cial environment for microorganisms that, in       whereas, other biochars may only restrict
              turn:                                              very large soil grazers, such as mites and
                                                                 collembola. The ability of biochar to exclude
              •     provide greater access to P from organic     soil grazers might allow soil microbes to
                    and insoluble inorganic pools;               mediate nutrient transformations more effi-
              •     produce and recycle a highly labile pool     ciently. However, it is possible that microbes
                    of organic P; and                            primarily colonize the surface of biochar and
              •     improve plants’ direct access to P           not the internal pore surface (Yoshizawa et al,
                    through improved mycorrhizal activity.       2005).
                                                                      Very little work has focused on the role of
              Several studies have demonstrated shifts in        biochar pore spaces within the context of soil
              microbial activity or community composition        food webs. Warnock et al (2007) speculated
              with biochar additions to soil (Wardle et al,      that the safe pore environment of biochars
              1998; Pietikäinen et al, 2000; DeLuca et al,       might enhance activity of mycorrhizal fungi
              2006). The mechanisms for increased micro-         or stimulate mycorrhization helper bacteria.
              bial activity remain unclear because very little   These potential mechanisms may help to
              research has focused on factors such as how        explain several studies that have demon-
              the microbial community size, community            strated higher mycorrhizal colonization in the
              structure or specific interactions within soil     presence of biochar (Saito, 1990; Ishii and
              microbial communities and soil food web            Kadoya, 1994; Ezawa et al, 2002; Matsubara
              change in the presence of biochar (see             et al, 2002;Yamato et al, 2006).
              Chapter 6).
                   Warnock et al (2007) reviewed several         Sulphur
              mechanisms through which biochar might
              affect soil microorganisms, including its          Given the similarities between the S and N
              effect on sorption of microbial signalling         cycles (Stevenson and Cole, 1999), there is a
              compounds (described above) and the physi-         significant potential for biochar to influence
              cal structure of biochar, which provides a         S mineralization and oxidation activity in the
              habitat for microbes within the porous struc-      soil. Although the majority of soil S originates
              ture of charred material. The physical             from the geologic parent material, most soil S
              structure of biochar is inherited from the         exists in an organic state and must be miner-
              plant tissue from which it is formed and,          alized prior to plant uptake (Stevenson and
              thus, can have an extremely high pore              Cole, 1999). Organic S exists as either ester
              density, such as that found in woody xylem         sulphate or as C-bonded S, the later having to
              tissue. Substantial variability in pore density    be oxidized to SO4-2 prior to plant uptake.
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                                                   BIOCHAR EFFECTS ON SOIL NUTRIENT TRANSFORMATIONS           265

               To date, no studies have directly assessed the       presence of biochar. However, these
               influence of biochar on S transformations or         autotrophic organisms have uniquely high
               S availability in agricultural or forest soils.      requirements for certain trace elements that
               However, numerous studies involving                  are in relatively high concentrations in
               biochar or biochar additions to soils have           biochar (see Chapter 5) and are increased in
               recorded changes in the soil environment that        soil when biochar is added (Rondon et al,
               suggest that biochar additions could increase        2007). Biochar additions to soil that ulti-
               soil S bio-availability. Biochar additions to        mately reduce the surface albedo of mineral
               acid agricultural soils have been observed to        soils and result in faster warming of soils in
               yield a net increase in soil pH (see Chapter         springtime may, in turn, increase S oxidation
               5), potentially as a function of the alkaline        or mineralization rates (Stevenson and Cole,
               oxides applied along with the biochar or             1999).
               potentially as a result of the influence on free          Biochar additions to mineral soils may
               Al/Ca ratios in soils amended with biochar           also directly or indirectly affect S sorption
               (Glaser et al, 2002; Topoliantz et al, 2005).        reactions and S reduction. As noted in
               Sulphur mineralization is favoured at slightly       Chapters 2 and 15, biochar improves soil
               acid to neutral pH. Sulphur mineralization           physical properties through increased
               rates have been found to increase following          specific surface area, increased water-holding
               fire in pine forest ecosystems (Binkley et al,       capacity and improved surface drainage.
               1992), much the same as that observed for N          Improved soil aeration through these
               (Smithwick et al, 2005). Separating the effect       improvements in soil physical condition
               of fire from the effect of the natural addition      would, in turn, reduce the potential for
               of biochar is difficult; but this effect is most     dissimilatory S reduction (Stevenson and
               likely due to the release of soluble S from          Cole, 1999). Sulphur is readily adsorbed to
               litter following partial combustion during fire      mineral surfaces in the soil environment and
               or heating events at temperatures in excess of       particularly to exposed Fe and Al oxides.
               200°C (Gray and Dighton, 2006).                      Organic matter additions to soil are known to
                    Sulphur oxidation is carried out by both        reduce the extent of SO4-2 sorption in acid
               autotrophic (e.g. Thiobacillus spp) and              forest soils (Johnson, 1984); therefore,
               heterotrophic organisms. Sulphur oxidation           biochar amendments may act to increase
               by acidophilic Thiobacillus spp would not be         solution concentrations of S in acid iron-rich
               favoured by pH increases induced by the              soils.

               The application of biochar to agricultural           mechanisms behind this stimulation of nitrifi-
               soils has the potential to greatly improve soil      cation remains the subject of ongoing debate;
               physical, chemical and biological conditions.        however, it is likely due to the sorption of
               In this chapter we reviewed biochar as a             compounds that otherwise lead to the inhibi-
               modifier of soil nutrient transformations and        tion of nitrification or immobilization of
               discussed the known and potential mecha-             inorganic N. In contrast, biochar has not
               nisms that drive these modifications. Biochar        been found to increase ammonification, and
               clearly has the potential to increase net nitrifi-   although biochar applications have been
               cation in acid forest soils that otherwise           found to increase plant uptake of N, there is
               demonstrate little or no nitrification. The          no evidence for an increase in N availability
ES_BEM_13-1     13/1/09     15:43    Page 266


              following harvest of the crop. This may be a            •   By what mechanisms does biochar affect
              result of the capacity of biochar to adsorb                 N mineralization and immobilization in
              NH4+ once it is formed, thereby leading to no               different ecosystems?
              measurable increases in net ammonification.             •   Does NH4+ adsorption by biochar
                   There is a distinct need for studies                   greatly reduce N availability or does it
              directed at explaining mechanisms for                       concentrate N for plant and microbial
              increased P uptake with biochar additions to                use?
              agricultural soils. Although plant P uptake             •   By what mechanism do biochar additions
              has been found to increase with increasing                  to mineral soils stimulate P availability?
              biochar added in some agro-ecosystems, this             •   How do different plant materials and
              has not been directly observed in natural                   different temperatures affect the physical
              forest soils amended with biochar. It is possi-             character and biochemical potential of
              ble that biochar additions to soils stimulate               biochar?
              mycorrhizal colonization, which may increase            •   How do biochar additions affect S avail-
              P uptake; but when applied with P-rich mate-                ability and by what mechanism(s)?
              rials, this effect may be lost.
                   The effect of biochar on soil nutrient             The answers to these questions can only be
              transformations has not been adequately                 obtained through rigorous investigation of
              studied. Some key areas that require attention          biochar as a natural component of grassland
              include:                                                and forest soils, and as a soil conditioner and
                                                                      amendment added to agricultural soils.


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