Carbon sequestration with biochar - stability and effect

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       Carbon sequestration with biochar – stability and effect on decomposition of soil organic


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       2009 IOP Conf. Ser.: Earth Environ. Sci. 6 242010


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  Climate Change: Global Risks, Challenges and Decisions                                        IOP Publishing
  IOP Conf. Series: Earth and Environmental Science 6 (2009) 242010           doi:10.1088/1755-1307/6/4/242010
Carbon sequestration with biochar - stability and effect on decomposition of soil organic matter
Sander Bruun(1), T El-Zahery(2), L Jensen(1)
(1) Plant and Soil Science Laboratory, Department of Agriculture and Ecology, Faculty of Life Sciences,
University of Copenhagen, Frederiksberg, Denmark
(2) Department of Soil Science, Faculty of Agriculture, University of Mansura, Mansura, Egypt

Thermo-chemical conversion covers several technologies which can be used to extract energy from biomass
including pyrolysis and gasification. Under high temperature and limited oxygen conditions, solid biomass
is converted into syngas, bio-oil and biochar. The syngas and bio-oil can be combusted for energy. The
biochar is also an excellent fuel, but recently the use of biochar for carbon sequestration and improvement of
soil properties has gained increasing attention. Removing additional biomass from agricultural fields for
bioenergy production may mean that the soils become depleted of nutrients and organic matter. As the
biochar contains most of the nutrients from the biomass, returning it to the land where the biomass was
produced means that the nutrients are recycled and utilised sustainably. However, biochar also has a range of
other positive effects on soil quality such as increased water holding capacity and cation exchange capacity.
In addition, biochar is highly resistant against microbial degradation, which means that it is stabilized in the
soil. Because the carbon in the biochar was recently fixed during plant growth, the process can be
considered as a means for carbon sequestration. The high stability of char in soil is incontrovertible, but
most available information comes from the natural occurrence of char in sediments and in soils from areas
with a history of vegetation burning. Laboratory incubations allow investigation of differences in stability of
chars of different origin and thermal alteration. By using isotope techniques very sensitive measurements
can be obtained. However, the high stability of char necessitates long incubations to get a better impression
of the development of the degradation rate with time. A recently published ten year litter bag experiment in
a Swedish forest (Wardle, Nielsson and Zackrisson, Science, 2008, 230, 629) indicated that the presence of
char was leading to losses of soil organic matter. This could mean that any carbon sequestered in biochar
could be offset by increased mineralization of soil organic matter. However, the study was criticized because
of several artifacts pertaining to the litter bag method (Lehman and Sohi, Science, 2008, 321, 1295). Most
importantly, the litter bags does not allow for contact with mineral soil. The purpose of the current study is
to i) compare the degradation rate of char thermally altered to different degrees and ii) test if biochar
increases the degradation of soil organic matter. Two incubation experiments were conducted; one exploring
the effect of thermal alteration on the stability of biochar and the other testing interaction between litter, char
and soil organic matter. In the first experiment, char was produced from homogeneously 14C labelled wheat
straw at 225°C, 300°C and 375°C under limited oxygen supply. The resulting char and straw were dried at
70°C and used in a two-year soil incubation where the evolved 14C was trapped in a NaOH and counted on
a scintillation counter. In the second experiment, different combinations of 14C labelled wheat straw and
char was incubated together with unlabelled straw and char. In addition, 14C soil organic matter in a soil
which had been labeled 40 years ago was incubated with unlabelled char and wheat straw. The results of the
first experiment showed that there was an initial flush of carbon evolution from all the chars included in the
study with no apparent lag phase. In contrast, there was a distinct lag phase for the straw. The small amount
of carbonates in the chars produced at 225°C and 300°C excluded carbonates as the source of the initial
flush. The initial flush is therefore more likely to be associated with oxidation of the char particle surfaces,
which has been observed in other studies. After the initial flush, the carbon evolution rate quickly decreased
to very low values for all the chars, whereas it remained high for the litter before the evolution for this
material also faded eventually. After two years of incubation, 56% of the carbon originally added as straw
had been evolved as CO2. For char produced at 225°C, 9.3% was lost while for 300°C, 3.1% had been lost.
The rate of carbon evolution after two years of incubation was 0.082 ‰ d-1 for straw, 0.060 ‰ d-1, 0.011
‰ d-1, and 0.008 ‰ d-1 for char produced at 225°C, 300°C and 375°C. Therefore it was concluded that
char is very resistant against microbial degradation and that the resistance increases with the degree of
thermal alteration, also after the initial flush of carbon evolution. This confirms that the potential for carbon
sequestration using biochar is large. However, although biochar produced at high temperatures is more
stable, it may not necessarily lead to higher carbon sequestration because the efficiency of formation is
higher at lower temperatures since less carbon is released during biochar production. Thus after two years,
44% of the carbon was sequestered in the soil when straw litter was added, whereas as much as 81% or only
20% was sequestered when char produced at 225°C or 300°C, respectively, was added to the soil. For char

  c 2009 IOP Publishing Ltd                             1
  Climate Change: Global Risks, Challenges and Decisions                                      IOP Publishing
  IOP Conf. Series: Earth and Environmental Science 6 (2009) 242010         doi:10.1088/1755-1307/6/4/242010
produced at 375°C almost all carbon was lost already during production. This means that the treatment
actually resulted in the highest carbon sequestration after two years from biochar production at 225°C.
However after longer period of time the char produced at higher temperatures is likely to become more
favorable. The results of the second experiment did not indicate any increased decomposition of either straw
or soil organic matter due to char addition. In fact, there was a small tendency for biochar to decrease
decomposition of soil organic matter. However, addition of straw resulted in higher reductions soil organic
matter decomposition rates. There are several possible explanations for the reduction in the decomposition
of soil organic matter. First of all, the added straw and char will suck up water from the soil and reduce the
water potential in the soil and thereby perhaps also the decomposition of the organic matter. A reason for the
reduction after addition of litter could be the immobilization of nutrients, which decreases the availability of
nutrients for microorganisms decomposing the soil organic matter. Whether this could also explain the
reduction of the rates of soil organic matter decomposition in the treatments where char is added is
questionable as the char has a considerably smaller potential for nutrient immobilization. Thus, based on the
high stability of char there seems to be a great potential for carbon sequestration based on biochar
production. This potential is dependant on the temperature at which the biochar is produced and upon the
time frame at which the sequestration is assessed. Also the energy recovery in the thermo-chemical
conversion will have to be considered. Finally we did not find any evidence that the biochar increases the
decomposition of soil organic matter. There is thus no indication the carbon sequestered in the biochar will
be offset by an increased release of carbon dioxide because of increased decomposition of soil organic or
recenty added plant litters. All of this support the assertion that biochar presents a potentially very effective
method for soil carbon sequestration.


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