The global C cycle is constituted by a short-term biochemical cycle superimposed on a long-term geochemical cycle. Annually, anthropogenic activities distort both cycles by emitting 8.6 Pg C, which is absorbed by the atmosphere (3.3 Pg C), the oceans (2.2 Pg C) and unknown sinks (Lal, 2007). The soil C pool comprises two components: (i) the soil organic carbon (SOC) pool; and (ii) the soil inorganic carbon (SIC) pool. Agricultural activities affect mainly the SOC pool, which constitutes a potential source of GHGs with estimated current C content in the 1 m top layer two times larger than the atmospheric pool (Lal, 2007). The global C and N cycles are connected.
Farming alters the C cycle and management of cropping systems will determine the amount of CO2 emissions in the atmosphere as well as the potential for C sequestration in the soil. Marland et al. (2003) distinguished four sources of CO2 emissions in agricultural systems: (i) plant respiration; (ii) the oxidation of organic C in soils and crop residues; (iii) the use of fossil fuels in agricultural machinery such as tractors, harvesters and irrigation equipment; and (iv) the use of fossil fuels in the production of agricultural inputs such as fertilizers and pesticides. Therefore, C sequestration in soil, C storage in crop residues and CO2 emissions from farming activities should be considered, as well as the hidden CO2 costs of energy use and C emissions for primary fuels, electricity, fertilizers, lime, pesticides, irrigation, seed production and farm machinery (Wang and Dalal, 2006), to evaluate the atmospheric CO2 mitigation capacity of different farming practices.
C levels in soil are determined by the balance of inputs, as crop residues and organic amendments, and C losses through organic matter decomposition. Management to build up SOC requires increasing the C input, decreasing decomposition, or both (Paustian et al., 1997). The C input may be increased by intensifying crop rotations, including perennial forages and reducing bare fallow, by retaining crop residues, and by optimizing agronomic inputs such as fertilizer, irrigation, pesticides and liming. Decomposition may be slowed by altering tillage practices or including crops with slowly decomposing residue in the rotation.
Tillage can influence bulk density of the topsoil. Ellert and Bettany (1995) therefore suggested basing calculations of SOC stocks on an equivalent soil mass rather than on genetic horizons or fixed sampling depths in order to account for differences in bulk density.
To better understand the influence of different management practices (with special emphasis on tillage, crop rotation and residue management) on C sequestration, Govaerts et al. (2009) did an extensive literature review. Some of the already existing reviews on the influence of agriculture and management on C sequestration were used as a basis and the review was completed through a further literature search.
Crop residues are precursors of the SOC pool. The decomposition of plant material to simple C compounds and assimilation and repeating cycling of C through the microbial biomass with the formation of new cells are the primary stages in the process of humus formation (Collins et al., 1997). Returning more crop residues can be associated with an increase in SOC concentration (Govaerts et al., 2009).
Furthermore, the decomposition rate of organic material is controlled by the quality of the substrate that is available for soil microorganisms (Mosier et al., 2006). The C:N ratio is one of the most often used criteria for residue quality (Vanlauwe et al., 1994), together with initial residue N, lignin, polyphenols and soluble C concentrations (Trinsoutrot et al., 2000; Moretto et al., 2001). As decomposition proceeds, the recalcitrant components will accumulate in the material. Due to this change in organic-matter quality, the decomposition rate of fresh plant litter may decrease and it can thus be expected that when residues are retained the decomposition rate and CO2 flux will decrease over time. The quality of the substrate is, besides the recalcitrant components, also determined by the nutrient composition of the organic material (Lavelle et al., 1993).
It has often been reported that during the decomposition of organic matter, especially when organic material with a large C:N ratio is added to soil, decomposition may limit microbial activity and thereby decrease the CO2 flux (Lavelle et al., 1993).
The largest contribution to reducing the CO2 emissions associated with farming activities is made by the reduction of tillage operations. Reduced tillage practices influence greatly the use of fossil fuels by agricultural machinery as well as the electricity consumed in the production, the transportation and the reparation of the machines. In a wheat-fallow system in semi-arid subtropical Queensland, Australia, practising zero tillage reduced fossil fuel emissions from machinery operation by 2.2 million g CO2/ha over 33 years or 67 kg CO2/ha/year (four to five tillage operations with a chisel plough to 10 cm during fallow each year were replaced by one herbicide spray) (Wang and Dalal, 2006). West and Marland (2002) reported estimates for C emissions from agricultural machinery averaged over maize, soybean and wheat crops in the USA of 69.0, 42.2 and 23.3 kg C/ha/year for conventional tillage, reduced tillage and zero tillage, respectively. Robertson et al. (2000) studied fields under maize-wheat-soybean rotations in the Midwest, USA and calculated slightly lower fuel costs for zero tillage systems than for conventional tillage. While enhanced C sequestration will continue for a finite time, the reduction in net CO2 flux to the atmosphere, caused by the reduced fossil-fuel use, can continue indefinitely, as long as the alternative practice is continued and could more than offset the amount of C sequestered in the soil in the long term (West and Marland, 2002).
Govaerts et al. (2009) evaluated most of the available case studies on C sequestration. Based on the review of research constraints for C sequestration the authors decided to include only those results that came from measurements done to at least 30 cm deep after at least 5 years of continuous practice. In seven of the 78 cases retained, the soil C stock was lower in zero compared to conventional tillage, in 40 cases it was higher and in 31 of the cases there was no significant difference. Results do not always point in the same direction. Doran et al. (1998) report a positive effect of zero tillage on SOC stocks, whereas Halvorson et al. (2002) and Thomas et al. (2007) did not find a significant difference between zero and conventional tillage, and Black and Tanaka (1997) even reported a negative effect from a conversion to zero tillage. There is no consensus between the studies in wheat-fallow systems reported about the effect of a conversion to zero tillage on SOC stocks. West and Post (2002) for example found that moving to zero tillage in wheat-fallow rotations showed no significant increase in SOC and, therefore, may not be a recommended practice for sequestering SOC. Conversely, Alvarez (2005) reported in his compilation study that soils from wheat-fallow (n = 13) under reduced and zero tillage had a mean SOC content that was 2.6 t C/ha higher than under conventional tillage, an increase similar to that for the other rotations. The mechanisms that govern the balance between increased or no sequestration after conversion to zero tillage are not clear, although some factors that play a role can be distinguished, for example soil physical properties, such as soil aggregation, bulk density and porosity, root development and rhizodeposits, baseline soil C content, climate, landscape position and erosion/ deposition history.
It is known that aggregation physically protects soil organic matter that would otherwise decompose rapidly (Beare et al., 1994; Six et al., 2002). Due to more stable macro-aggregates in zero tillage compared to conventional tillage (Six et al., 2000), the soil organic matter is more trapped inside the soil aggregates and therefore not accessible to microbial action (Beare et al., 1994; Six et al., 2000). Tillage brings microorganisms in direct contact with crop residue, thereby increasing decomposition. Additionally, tillage disrupts aggregates liberating physically stabilized organic material (Buchanan and King, 1992; Six et al., 2002). Each tillage operation will thus induce a flush in C mineralization and subsequent C loss. Lal (2004) reported that conventional tillage increased the emission of CO2 by 30-35 kg C/ha compared to zero tillage.
Crop root-derived C may be very important for C storage in soil (Holanda et al., 1998; Flessa et al., 2000; Gregorich et al., 2001; Tresder et al., 2005; Baker et al., 2007 all as cited in Govaerts et al., 2009). Zero tillage practices can produce greater horizontal distribution of roots and greater root density near the surface (Ballcoelho et al., 1998; Qin et al., 2006).
VandenBygaart et al. (2002) concluded that soil erosion and redistribution over a prolonged period also affects SOC storage under zero tillage. Soils that had lost SOC through soil erosion had a high potential to gain SOC when converted from conventional tillage to zero tillage, whereas in depressed landscape positions (with high SOC from a history of soil deposition) the potential to gain SOC was lower when converted to zero tillage, with some soils even losing SOC.
Altering crop rotation can influence soil C stocks by changing the quantity and quality of organic matter input. Increasing rotation complexity and crop intensity is expected to increase the SOC stocks. In the literature review reported by Govaerts et al. (2009) however, the soil C stock increased in 28 of the 55 retained cases, showed no significant difference in five cases and decreased in 22 cases. West and Post (2002) calculated from a global database of 67 long-term experiments that enhancing rotation complexity (i.e. changing from monoculture to continuous rotation cropping, changing crop-fallow to continuous monoculture or rotation cropping, or increasing the number of crops in a rotation system), did not result in sequestering as much SOC (15 ± 11 g C/m2/year) on average as did a change to zero tillage, but crop rotation is still more effective in retaining C and N in soil than monoculture (Yang and Kay, 2001). The increased input of C as a result of the increased productivity due to crop intensification will result in increased
C sequestration. VandenBygaart et al. (2003) reported in their review of Canadian studies that, regardless of tillage treatment, more frequent fallowing resulted in a lower potential to gain SOC in Canada. Also eliminating fallows by including cover crops promotes SOC sequestration by increasing the input of plant residues and providing a vegetation cover during critical periods (Franzluebbers et al., 1994; Bowman et al., 1999), but the increase in SOC concentration can be negated when the cover crop is incorporated into the soil (Bayer et al., 2000). Crop residue mass may not be the only factor in SOC retention by agricultural soil. The mechanism of capturing C in stable and long-term forms might also be different for different crop species (Gál et al., 2007).
Conservation agriculture: the combined effect of minimum tillage, residue retention and crop rotation on SOC stocks
Conservation agriculture is defined as a cropping system that combines the following principles: (i) reduction in tillage; (ii) retention of adequate levels of crop residues on the soil surface; and (iii) use of crop rotations. These conservation agriculture principles seem to be applicable to a wide range of crop production systems under low-yielding, dry rainfed and high-yielding irrigated conditions. Obviously, specific and compatible management components (weed control tactics, nutrient management strategies and appropriately scaled implements) will need to be identified through adaptive research with active farmer involvement to facilitate farmer adoption of appropriate conservation agriculture-based technologies for contrasting agroclimatic/production systems. Therefore, by applying the three components conservation agriculture has the potential to increase C stock through the increased input from crop residue retention, increased crop intensifications and crop rotation and the reduced C decomposition through reduced tillage.
The soil C case study results reported in Govaerts et al. (2009) are not conclusive. More research is needed, especially in the tropical areas where good quantitative infor mation is lacking. The mechanisms that govern the balance between increased or no sequestration after conversion to zero tillage are not clear, although some factors that play a role can be distinguished (e.g. root development and rhizodeposits, baseline soil C content, bulk density and porosity, climate, landscape position and erosion/ deposition history). However, even if C sequestration is questionable in some areas and cropping systems, conservation agriculture remains an important technology that improves soil quality, controls erosion and reduces tillage-related production costs.
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