Carbon dioxide fluxes and carbon storage in conventional and

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Carbon dioxide fluxes and carbon storage in conventional and Powered By Docstoc
					Scientific registration no : 1880
Symposium no : 26
Presentation : poster

Carbon dioxide fluxes and carbon storage in conventional
   and no-till soil in semiarid Saskatchewan, Canada
 Flux de CO2 et stokage du carbone dans des sols semi-
  arides travaillés ou non au Saskatchewan, (Canada)
CURTIN Denis (1), SELLES F(2), WANG H (2), McCONKEY B G (2), CAMPBELL
C A (2)

(1) Crop & Food Research, Private Bag 4704, Christchurch, New Zealand
(2) Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift
Current, Saskatchewan, Canada S9H 3X2


                                          Introduction
         Increasing concentrations of carbon dioxide (CO2) and other greenhouse gases in the
atmosphere are a concern because of potential climate changes that could result in an increase
in temperature and drought. Soils play an important role in regulating the atmospheric
concentrations of CO2. Reduced tillage is regarded as one of the most effective agricultural
strategies for sequestering atmospheric C (Kern and Johnson, 1993; Campbell et al., 1995,
1996). Tillage accelerates organic C oxidation to CO2 by improving soil aeration, by increasing
contact between soil and crop residues, and by exposing aggregate-protected organic matter
to microbial attack (Beare et al., 1994). The effect of reduced tillage on soil C storage can vary
widely. The effect is often greatest when no till (NT) is compared with intensive cultivation
(i.e., moldboard plowing) in humid regions (Mahboubi et al., 1993) and least in dryland
conditions where shallow cultivation is the conventional tillage method (Unger, 1991). Other
factors that can influence C storage under NT are soil texture, fertility and crop rotation (Lal
et al., 1995; Campbell et al., 1996).
         In semiarid regions of Saskatchewan, the traditional cropping system involves frequent
summer fallowing (land fallowed over the entire growing season) with several tillage operations
to control weeds during the fallow period. This system is regarded as a worst-case scenario for
soil C storage because inputs of C in crop residues are relatively low and frequent tillage
promotes organic matter decomposition. In recent years reduced tillage has gained popularity
with wheat (Triticum aestivum L.) producers in Saskatchewan (Larney et al., 1994). The
objective of this study was to quantify the effect of tillage on C fluxes in wheat-producing
systems differing in summer fallow frequency.




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                                   Materials and Methods
        Carbon fluxes were measured in a field experiment that was initiated in 1982 on a gently
sloping silt loam (Typic Haploboroll) at the Semiarid Prairie Agricultural Research Centre,
Swift Current, Saskatchewan, Canada (mean annual temperature and precipitation 3.3°C and
350 mm). In the previous 70-80 yr the land had been managed in a fallow-wheat rotation with
conventional tillage. Two spring wheat rotations were established as main treatments: fallow-
spring wheat (F-W) and continuous wheat (Cont W). Each rotation had a conventional tillage
(CT) and a no tillage (NT) sub-treatment. Details of tillage and other management practices
have been reported by McConkey at al. (1996). Herbicides were used exclusively for weed
control under NT. The CT system involved cultivation prior to seeding and, on CT fallow, one
to four tillage operations, performed during summer with a heavy-duty cultivator, were used
to control weeds. Treatments were arranged in a complete block design with four replicates.
Both phases of the F-W rotation were present each year. Each plot was 15 m wide by 76 m
long. Full-sized farm equipment was used to perform most field operations.
        In 1995 and 1996, CO2 emissions were measured using a portable infrared analyzer
(LICOR model LI-6200, Licor, Lincoln, Nebraska) to determine accumulation of CO2 in a
vented chamber that enclosed an area of 177 cm2. The chamber was attached to a collar using
quick-fit clamps (a rubber gasket was placed between the collar and the chamber to provide an
air-tight seal) and inserted 4 cm into the soil. Head space volume was 2 L. Fluxes of CO2,
expressed as µmol CO2 m-2 s-1 (1 µmol CO2 m-2 s-1 ~ 10 kg C ha-1 day-1), were estimated from
the rate of increase in CO2 in the chamber during a 60 second deployment period. Flux
measurements were made (usually once each week) from spring until freeze-up in late fall.
Generally, three measurements were made in each plot at locations 3 to 4 m apart (the distance
between sampling locations was determined from a preliminary study of spatial pattern of CO2
emissions). Designated sampling locations were marked using flags and measurements were
always made within 1 m of the flags. In-crop measurements were made by placing the chamber
between plant rows (row spacing for wheat was 17.5 cm).
        Estimates of C inputs in crop residues were obtained from above-ground biomass,
measured by taking four random samples in each plot at crop maturity. The plant material was
dried (60oC), weighed, and separated into grain and straw. Subsamples were analyzed for C
using an automated elemental analyzer (Carlo Erba, Milan, Italy). Carbon input in roots was
estimated assuming a root:straw ratio of 0.59:1 (Campbell et al., 1977).
        The amount of crop residue on the soil surface was determined by taking five 0.5 m2
samples per plot each fall. Surface residues were analyzed for C and N using an elemental
analyzer (Carlo ErbaTM, Milan, Italy).
        Weather data were recorded at a meteorological station about 0.5 to1 km from the
experimental plots.

                                   Results and Discussion
       The 1995 growing season was relatively wet with about 40% more precipitation than
normal. In 1996, precipitation was above average in April, May and June. A dry period in July
and August was followed by an especially wet September when precipitation exceeded 100 mm.



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         Fluxes of CO2 ranged from less than 0.5 µmol CO2 m-2 s-1 to > 10 µmol CO2 m-2 s-1,
though values greater than 3 µmol CO2 m-2 s-1 were uncommon (Fig. 1) and were only observed
following heavy rain events. Our values are comparable to those recorded by Fortin et al. (1996)
in a tillage experiment over two growing seasons in Ontario, Canada (range 1 to 8 µmol CO2
m-2 s-1). In general, fluxes were greater under CT than under NT (Fig.1) and in cropped versus
fallow plots. Tillage effects were most apparent following heavy rains in 1995 when
exceptionally high fluxes were measured on CT plots (Fig. 1). Within a day or two, emissions
returned to normal. These high fluxes from CT plots could not be explained by normal
respiratory output of CO2. We suspect that, under raindrop impact, the soil surface may have
sealed, temporarily trapping CO2. As the soil surface dried, trapped CO2 was released in a brief,
intense burst. Better soil structure, combined with greater surface residue cover (Fig. 2 ), may
have prevented surface sealing under NT.
         Total annual emissions of CO2-C were generally less under NT than under CT in both
1995 and 1996 (Table 1). Emissions of CO2 from the fallow phase of the F-W rotation
amounted to 58-69% of those from the cropped phase, suggesting that root respiration
accounted for about one-third of the CO2 emitted from cropped plots. Use of flux data to
estimate the soil C balance requires caution because errors in estimating the total seasonal




Fig. 1. Fluxes of CO2 at the surface of soil under continuous wheat in 1995 and 1996 as
        influenced by tillage management.


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CO2 efflux from discrete measurements could be substantial and the contribution of root
respiration to CO2 efflux is uncertain. Nevertheless, comparison of the apparent C balances of
 different tillage systems can be informative. The C balance estimates in Table 2 were obtained
assuming that one-third of the CO2 emitted from cropped plots was due to root respiration. In
1995 and 1996, as in previous years (Campbell et al., 1995), tillage had little effect on inputs
of C in crop residues (Table 2). In both years, the soil C balance for Cont W was positive (i.e.,
inputs exceeded emissions), with NT soil gaining more C than CT soil. The wheat phase of F-W
also had a positive C balance but C gained was insufficient to offset average C losses of about
2,000 kg ha-1 yr-1 from the fallow phase of the rotation. The results suggest that, in the F-W
rotation, soil C decreased in 1995-1996, especially under CT management.
        Tillage effects on the C balance, estimated from C fluxes, are generally consistent with
soil C values measured in 1994, 12 years after initiation of the experiment (Campbell et al.,
1995). In Cont W about 2.5 t ha-1 more C was stored in the soil under NT relative to CT and,
in the F-W rotation, the balance in favor of NT was about 0.9 t ha-1 (Table 3). By the time our
flux measurements were made (13-14 years after initiation of NT management) soil C levels
might be expected to have reached `steady state’ under NT (Campbell et al.,1995). However,
good growing conditions in the six years prior to our study resulted in above-average crop
residue inputs, providing conditions favorable to further C storage


Table 1.       Total emissions of CO2-C in 1995 and 1996 as influenced by tillage and crop
               rotation.

Tillage                        Rotation†                  CO2-C emitted‡

                                                       1995             1996

                                                       ---------- kg ha-1 --------
Conventional Till              Cont W                  4430a            3680a
No till                        Cont W                  2970b            2940b

Conventional Till              F-(W)                   3840c            2790b
No Till                F-(W)                   3080b           2510b

Conventional Till              (F)-W                   2610b            1610c
No Till                (F)-W                   1930d           1730c
†
 Rotation phase in parentheses is the one to which the data refer.
‡
 Values followed by the same letter within a column are not significantly different at P <
0.05.

under NT. Figure 2 shows that under NT there was an accumulation of C in surface residues
since about 1990. Relative to the traditional management system in semiarid regions of the


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Canadian prairies (F-W with conventional tillage), all systems showed increases in surface
residue C: F-W (CT) < Cont W (CT) < F-W (NT) << Cont W (NT) (Table 3). Management
effects on surface residue C were similar in magnitude to those observed for soil organic C
(Table 3).
           Lower CO2 fluxes under NT compared with CT were clearly due to slower
decomposition of crop residues placed on the surface of NT soil than when they were
incorporated by tillage. The results in Table 3 show that surface residues can be as important
as soil organic matter as a C repository under NT and should be considered when evaluating
tillage effects on C dynamics. Since surface residues are relatively easily decomposed, amounts
can fluctuate rapidly in response to environmental conditions and management practices, making
them an ephemeral C store. Even a single tillage operation might result in significant loss of this
accumulated surface residue C. A further portion of the C stored in NT soil was in light fraction
organic matter (Curtin et al., 1996), which is also labile and susceptible to decomposition in the
short term.
                                   Summary and Conclusions
            After 13-14 years of NT management, fluxes of CO2 were lower under NT than
under CT due to slower decomposition of surface versus incorporated crop residues. Based


Table 2.        Effect of tillage and rotation† on apparent soil C balance in 1995 and 1996.

Rotation                Conventional Till                                No Till

                C Input     C emitted‡     Balance         C input      C emitted‡      Balance

                ----------------------------------- kg ha-1-----------------------------------------
                1995
Cont W          3110            2970       + 140           2810             1990          + 820
F-(W)           3860            2580       +1280           3330             2060          +1270
(F)-W             0             2610       - 2610 0                1930          - 1930
                1996
Cont W          2700            2470        + 230          2850            1970           + 880
F-(W)           2780            1870        + 910          2810            1680           +1130
(F)-W             0             1610       - 1610             0            1730           - 1730
†
  Data for the F-W rotation refer to the phase in parentheses.
‡
  Emissions were adjusted for the contribution of root respiration which was assumed to
represent one-third of total CO2 emitted from wheat plots.

on our results, producers on medium textured soils in semiarid Saskatchewan who switch from
the traditional system of wheat production (F-W with conventional tillage) to continuous no
till cropping could, potentially, sequester 5-6 t C ha-1 in soil organic matter and surface residues.
 Maintaining the sequestered C may require careful management because much of it may be in


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pools that could decline rapidly if the soil is disturbed mechanically




Fig. 2. Effect of tillage on amounts of C present in residues on the surface of soil under
continuous wheat.

                                           References
Beare, M.H., Cabrera, M.L., Hendrix, P.F., and Coleman, D.C. 1994. Aggregate-protected
        and unprotected organic matter pools in conventional- and no-tillage soils. Soil Sci.
        Soc. Am. J. 58: 787-795.
Campbell, C.A., Cameron, D.R., Nicholaichuk, W., and Davidson, H.R 1977. Effects of
        fertilizer N and soil moisture on growth, N content and moisture use by spring
        wheat. Can. J. Soil Sci. 57: 289-310.
Campbell, C.A., McConkey, B.G., Zentner, R.P., Dyck, F.B., Selles, F., and Curtin, D.
        1995. Carbon sequestration in a Brown Chernozem as affected by tillage and
        rotation. Can. J. Soil Sci. 75: 449-458.
Campbell, C.A., McConkey, B.G., Zentner, R.P., Selles, F., and Curtin, D. 1996. Long-
        term effects of tillage and crop rotations on soil organic C and total N in a clay soil
        in southwestern Saskatchewan. Can. J. Soil Sci. 76: 395-401.
Curtin, D., McConkey, B.G., Campbell, C.A., Biederbeck, V.O., Schoenau, J., Lafond,
        G.P., Brandt, S., and Moulin, A.P. 1996. Response of soil carbon fractions to tillage
        on the Canadian prairies. Agronomy Abstracts, p.266.
Fortin, M.C., Rochette, P., and Pattey, E. 1996. Coil carbon dioxide fluxes from
        conventional and no-tillage small-grain cropping systems. Soil Sci. Soc. Am. J. 60:
        1541-1547.


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Kern, J.S., and Johnson, M.G. 1993. Conservation tillage impacts on national soil and
        atmospheric carbon levels. Soil Sci. Soc. Am. J. 57: 200-210.
Lal, R., Kimble, J., and Stewart, B.A. 1995. Towards soil management for mitigating the
        greenhouse effect. Pages 373-381 in R. Lal, J. Kimble, E. Levine, and B.A. Stewart
        (ed.) Soil management and the greenhouse effect. Lewis Publ., Boca Raton, FL.
Larney, F.J., Lindwall, C.W., Izauralde, R.C., and Moulin, A.P. 1994. Tillage systems for
        and water conservation on the Canadian prairies. Pages 305-328 in M.R. Carter (ed.)
        Conservation tillage in temperate agroecosystems. Lewis Publ., Boca Raton, FL.
Mahboubi, A.A., Lal, R., and Faussey, N.R. 1993. Twenty-eight years of tillage effects on
        two soils in Ohio. Soil Sci. Soc. Am. J. 57: 506-512.
McConkey, B.G., Campbell, C.A., Zentner, R.P., Dyck, F.B., and Selles, F. 1996. Long-
        term tillage effects on spring wheat production on three textures in the Brown soil
        zone. Can. J. Soil Sci. 76: 747-756.
Unger, P.W. 1991. Organic matter, nutrient, and pH distribution in no- and conventional-
        tillage semi-arid soils. Agron. J. 83: 186-189.


Table 3.       Effect of tillage and rotation on soil organic C (0-7.5 cm depth) and surface
               residue C.

Tillage        Rotation†      Soil Organic C‡                   Surface Residue C

                                       ----------------------- kg ha-1 --------------------------

Conventional Till   Cont W        14,360 (440)§                        1367 ( 686)
No Till       Cont W       16,840 (2920)                        3548 (2858)

Conventional Till      F-W            13,920 ( 0 )                      690 ( 0       )
No Till       F-W               14,780 (860)                    1667 ( 977)
†
  In the F-W rotation, values were averaged over rotation phase, which was not significant
    (P < 0.05).
‡
  Soil C data were taken from Campbell et al. (1995); residue C data were obtained from a
    sampling carried out in fall 1996.
§
  Values in parentheses represent increases in C relative to F-W/conventional tillage, the
   system with least C sequestration potential.



Keywords : carbon dioxide fluxes, carbon sequestration, reduced tillage, surface residues, soil
organic matter
Mots clés : flux de gaz carbonique, séquestration du carbone, non labour, résidus de surface,
matière organique


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