Article

Conservation Agriculture and Soil Carbon Sequestration: Between Myth and Farmer Reality

Critical Reviews in Plant Sciences (Impact Factor: 5.44). 05/2009; 28(3):97-122. DOI: 10.1080/07352680902776358

ABSTRACT

Improving food security, environmental preservation and enhancing livelihood should be the main targets of the innovators of today's farming systems. Conservation agriculture (CA), based on minimum tillage, crop residue retention, and crop rotations, has been proposed as an alternative system combining benefits for the farmer with advantages for the society. This paper reviews the potential impact of CA on C sequestration by synthesizing the knowledge of carbon and nitrogen cycling in agriculture; summarizing the influence of tillage, residue management, and crop rotation on soil organic carbon stocks; and compiling the existing case study information. To evaluate the C sequestration capacity of farming practices, their influence on emissions from farming activities should be considered together with their influence on soil C stocks. The largest contribution of CA to reducing emissions from farming activities is made by the reduction of tillage operations. The soil C case study results are not conclusive. In 7 of the 78 cases withheld, 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. 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. Altering crop rotation can influence soil C stocks by changing quantity and quality of organic matter input. More research is needed, especially in the tropical areas where good quantitative information is lacking. However, even if C sequestration is questionable in some areas and cropping systems, CA remains an important technology that improves soil processes, controls soil erosion and reduces production cost.

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Available from: John Dixon, Aug 25, 2014
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    • "Globally, the impact of CA practices on soil organic carbon is inconclusive (cf.Govaerts et al., 2009). In Zimbabwe,Nyamangara et al. (2013a)rejected the hypothesis that CA increases soil organic carbon after a study based on soil samples collected from about 450 farms in 15 districts across the country (Table 2). "
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    ABSTRACT: Conservation agriculture (CA) is increasingly promoted in southern Africa as a strategy to improve food security and reverse soil degradation in the face of climate change. However, the performance of CA under different environments and its ability to improve ecosystem services is still unclear. The effects of the CA options; direct seeding, rip-line seeding, and seeding into planting basins on maize grain yield, soil health and profitability across agro-ecological regions in Zimbabwe were evaluated through a review of literature in combination with meta-analysis. Overall, CA improved maize yield over conventional agriculture. Compared to conventional agriculture, direct seeding, rip-line seeding, and seeding into planting basins increased yield by 445, 258 and 241 kg ha−1, respectively. However, there was an initial yield decline in the first two years. CA practices reduced soil erosion and bulk density, and increased soil water content in most studies. Under high levels of residue retention (6 Mg ha−1), CA systems exhibited greater macro fauna abundance and diversity than conventional agriculture, particularly termites. Weed pressure tended to increase labour requirement for hand-hoe weeding under CA compared to conventional agriculture. However, the use of herbicides reduced weeding labour demand during the early season. The benefits of CA are tied to the farmers’ management intensity including: time of planting, weeding, fertiliser and herbicide application, and adequate training on equipment use. Economic analysis results showed that on average, a farmer incurs losses for switching from conventional agriculture to CA in the main maize growing regions of Zimbabwe. Based on the six seasons’ data, the losses were least with the ripper in drier areas and worst with the direct seeder in wetter areas. Incorporation of chemical herbicides worsens the economic returns of CA tillage options in all the agro-ecological zones. Overall, the study showed that the rip-line seeding is more attractive in the drier areas than direct seeding. Although not costed in this study, critical is the cumulative reversal of soil degradation associated with consistent CA practice which can sustain agriculture. Results from this review suggest that the benefits of CA depend largely on the type and context of CA being practised. It is thus imperative to profile the technology, the farmer socio-economic circumstances and the bio-physical environment in which the farmer operates for proper geographical and beneficiary targeting to achieve greater impact. More longer-term studies are required to fully elucidate the benefits and context of CA options and practice.
    Full-text · Article · Mar 2016 · Agriculture Ecosystems & Environment
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    • "A reason for the absence of a consistent and significant increase in C stocks across the sites investigated in the present study might be the relatively young age of the investigated validation-trials (2– 7 years) and the variability of the studied systems. Although results are not consistent, increases in soil C stocks have been shown in many instances if fields have been under minimum tillage for a longer period than the ones investigated in the present study (Govaerts et al., 2009). On-station research in Zimbabwe (Thierfelder and Wall, 2012) and Zambia (Thierfelder et al., 2012a) found increases in 0–30 cm soil depth C stocks under CA as compared with CP after only four years of CA practice. "
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    ABSTRACT: A B S T R A C T In view of the importance of soil carbon (C) and the scarce data on how conservation agriculture might influence its accumulation in Southern Africa this study presents data from 125 on-farm validation trials across 23 sites in Malawi, Mozambique, Zambia and Zimbabwe. These validation trials are paired plot comparisons of conventional agricultural practice and conservation agriculture that had been established between 2004 and 2009. Traditional cropping systems vary across the study area although they all are tillage based and maize is the main crop grown. The treatments proposed on the validations trials reflect this variability in conventional practice and propose an adapted conservation agriculture option. The sites are thus grouped into four specific treatment comparisons. Bulk density and soil C concentrations were measured from samples collected at four depth layers (0–10 cm, 10–20 cm, 20–30 cm and 30– 60 cm), thereafter C stocks were calculated. On the basis of the stover biomass harvest C inputs were assessed. No consistent differences in bulk density and soil C concentrations were found. Carbon stocks were found to be positively influenced by conservation agriculture only when a mouldboard ploughed maize-legume rotation was compared to a direct seeded maize legume rotation (with residue retention). Even when increases were significantly greater under conservation agriculture the order of magnitude was small ($2 Mg ha À1). Limited C inputs, ranging between 0.1 and 1 g C kg À1 soil yr À1 , are likely to be the major bottleneck for C increase. These results, based on on-farm validation trials indicate that there is a limited potential for conservation agriculture to significantly increase soil C stocks after up to 7 years of conservation agriculture practices, in the studied systems.
    Full-text · Article · Mar 2016 · Soil and Tillage Research
    • "of practices leading to SOC increases and implications for climate change mitigation Even where genuine increases in SOC from CA can be demonstrated in experiments, and where they can correctly be interpreted as delivering climate change mitigation through soil C sequestration, there are often significant technical, infrastructural, social or policy barriers to the adoption of the new practices by smallholder farmers in regions such as SSA and IGP. This topic has been discussed in detail elsewhere (Gowing and Palmer, 2008;Govaerts et al., 2009;Giller et al., 2011;Andersson and D'souza, 2013;Thierfelder et al., 2014;Corbeels et al., 2014;Mason et al., 2015) and there are some examples of these barriers being overcome (e.g.Thierfelder et al., 2014;Tittonell et al., 2012). The important point is that simply because SOC benefits are demonstrated under experimental conditions, the large scale adoption of CA in regions dominated by resource-poor smallholder farmers may be extremely slow: it is therefore unwise to rely on adoption of CA as a major strategy to mitigate climate change. "
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    ABSTRACT: Conservation agriculture (CA), comprising minimum soil disturbance, retention of crop residues and crop diversification, is widely promoted for reducing soil degradation and improving agricultural sustainability. It is also claimed to mitigate climate change through soil carbon sequestration: we conducted a meta-analysis of soil organic carbon (SOC) stock changes under CA practices in two tropical regions, the Indo-Gangetic Plains (IGP) and Sub-Saharan Africa (SSA), to quantify this. In IGP annual increases in SOC stock compared to conventional practice were between 0.16 and 0.49 Mg C ha−1 yr−1. In SSA increases were between 0.28 and 0.96 Mg C ha−1 yr−1, but with much greater variation and a significant number of cases with no measurable increase. Most reported SOC stock increases under CA are overestimates because of errors introduced by inappropriate soil sampling methodology. SOC increases require careful interpretation to assess whether or not they represent genuine climate change mitigation as opposed to redistribution of organic C within the landscape or soil profile. In smallholder farming in tropical regions social and economic barriers can greatly limit adoption of CA, further decreasing realistic mitigation potential. Comparison with the decreases in greenhouse gas emissions possible through improved management of nitrogen (N) fertilizer in regions such as IGP where N use is already high, suggests that this is a more effective and sustainable means of mitigating climate change. However the mitigation potential, and other benefits, from crop diversification are frequently overlooked when considering CA and warrant greater attention. Increases in SOC concentration (as opposed to stock) in near-surface soil from CA cause improvements in soil physical conditions; these are expected to contribute to increased sustainability and climate change adaptation, though not necessarily leading to consistently increased crop yields. CA should be promoted on the basis of these factors and any climate change mitigation regarded as an additional benefit, not a major policy driver for its adoption.
    No preview · Article · Mar 2016 · Agriculture Ecosystems & Environment
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