Soil Organic Carbon Accumulation and Greenhouse Gas Emission Reductions from Conservation Agriculture: A literature review.

This study aims at developing a clear understanding of the impacts and benefits of the two most common types of agriculture, traditional tillage agriculture (TA) and Conservation Agriculture (CA), a no-till system, with respect to their effects on soil carbon pools. It is based on a meta analysis of scientificliterature, attempting to reduce the existing uncertainty about the impact of soil management practices on soil carbon pools.The results from literature review on carbon sequestration in TA are compared with CA, a broader agro-ecosystem management concept thatrequires compliance with three interrelated criteria, namely minimum or no mechanical soil disturbance, permanent organic soil cover, and diversifiedcrop associations and rotations. The review shows that CA permits higherrates of carbon sequestration in the soil compared with TA. When no carbon sequestration or carbon loss is reported in agricultural systems, this is most frequently associated with any one or a combination of the following reasons:i) soil disturbance, ii) monocropping, iii) specific crop rotations, iv) poormanagement of crop residues, and v) soil sampling extended deeper than 30 cm.Most of the world’s agricultural soils have become depleted in organic matter and soil health over the years under TA, compared with their state under natural vegetation. This degradation process has proved to be reversible and the main ways to increase soil organic matter content and improve soil health seem to be: i) keeping the disturbance impact and interactions betweenmechanical implements and soil to an absolute minimum, ii) using effective crop rotations and associations, and iii) leaving crop residues as carbon source on the soil surface. The implementation of these practices can help restore a degraded agro-ecosystem to a sustainable and productive state. However, soil organic carbon (SOC) sequestration is generally non-linear over time and theeffectiveness of conversion of a farming system from TA to CA depends on a number of variables: for example, soil carbon sink strength increases most rapidly soon after a carbon-enhancing change in land management has beenimplemented, and reduces with time as the stable SOC stock approaches a new equilibrium which in agricultural soils in Europe for example can takeapproximately 100 years after a carbon-enhancing land use change has been introduced. Even though some authors report significant increase in microbial activity soon after transition to CA, fuller advantages of CA in terms of soil health and its productive capacity can usually be observed only in the mediumtolonger-term, when CA practices and soil biological processes become well established within the farming system. The study discusses the effectiveness of using average rates of soil carbon content for estimating sequestration at the global level. In reality, thereare different carbon pools in the soil undergoing transformation from theundecomposed form to decomposing unstable form to decomposed stable form.The carbon sequestration potential of any soil, for the carbon pool considered,depends on the vegetation it supports (which influences the amount and chemical omposition of organic matter being added), soil moisture availability, soil mineralogical composition and texture, depth, porosity and temperature. Therefore, when addressing carbon sequestration, rates should always be referred to specific carbon pools, as each carbon category has highly differentturnover rates. Another aspect of CA in relation to carbon budgets are the reduced powerand energy requirements as a result of not tilling the soil. This translates intoless fuel consumption, lower working time and slower depreciation rates ofequipment per unit area per unit of output, all leading to emission redu[...]