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


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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    • "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.
    Soil and Tillage Research 03/2016; 156:99–109. DOI:10.1016/j.still.2015.09.018 · 2.62 Impact Factor
    • "Besides, this SOC increase will influence other factors, such as infiltration, water content, porosity and soil aggregation (Veum et al., 2011), improving soil quality (Govaerts et al., 2009). In addition, higher ACF in depositional topsoil samples than in eroded ones suggested that, in agreement with Carter (1992), the more bioreactive carbon fraction was more sensitive to soil management than was stable carbon. "
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    ABSTRACT: Mediterranean mountain agroecosystems are sensitive areas to soil degradation due to climatic conditions and anthropogenic pressure from agriculture and grazing practices. Soil redistribution processes affect the spatial variability of soil properties and nutrients, as soil organic carbon and nitrogen (SOC and SON) are linked to soil quality and soil functions. A representative cereal field of rain-fed agroecosystems in a Mediterranean mountain environment where conventional and conservation agricultural practices have been implemented was selected on the northern border of the Ebro valley (NE Spain) to gain a better understanding of the effect of soil redistribution processes on the spatial pattern of soil properties at field scale. One hundred and fifty-six bulk (30-50cm) and 156 topsoil samples (5cm) were collected on a 10×10m grid. Furthermore, 21 soil samples (40cm) were collected at a nearby undisturbed reference site. 137Cs massic activity was measured on bulk and reference soil samples, and SOC and SON were measured on all soil samples. The characterisation of SOC pools into active (ACF) and stable carbon fraction (SCF) was carried out to study SOC pool dynamics. The fallout 137Cs was used in order to identify areas of 137Cs loss and gain and that of soil within the study field. Similar spatial patterns and direct and significant correlations (p<0.01) were found between 137Cs and soil nutrients. As much as 70% of the total study field had lower values of 137Cs inventory than the reference for the area. In addition, there was a generalised loss of soil nutrients. SOC gain was found in less than 1% of the study field and there was a large loss of SON compared to the undisturbed reference site. Higher and significant (p<0.01) contents of soil nutrients were found in topsoil than in bulk samples. Furthermore, there was an enrichment of the relative contribution of ACF to total SOC in sampling points where there was a 137Cs gain in both bulk and topsoil samples as a consequence of the soil redistribution particles by tillage and runoff. Understanding patterns of soil nutrients can be useful for developing or implementing land management strategies to preserve soil quality in Mediterranean agricultural areas.
    Soil and Tillage Research 09/2015; 155:318-328. DOI:10.1016/j.still.2015.09.007 · 2.62 Impact Factor
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    • "No-Till (NT) is defined as disturbing the soil as little as possible – only up to 20%–25% (Govaerts et al. 2009:98) – by using tine planters or combination tine and disc planters. Govaerts et al. (2006:99) described the conversion from conventional tillage to NT and/or CA as a gradual or step-wise process with minimum tillage (MT), NT and CACHEMICAL+ (High External Inputs) and CA (Low External Inputs) as phases in the process. "
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