Increasing soil organic carbon by applying farm-made organic fertilizers as a function of soil depth: A global synthesis.

Management practices that increase soil organic carbon (SOC) have productive and environmental benefits. Application of organic fertilizers increases SOC in the topsoil (≤30 cm) but its impact at the profile level is poorly known. The objective was to quantify SOC changes in surface and deep soil (30-150 cm) through the use farm-made organic fertilizers. Data were collected from 434 field experiments and meta-analysis techniques were used. Significant increases in SOC stock were detected up to 60 cm depth in treatments that received organic fertilizers, ranging between 28 and 30 %. SOC increases were greater in the topsoil layer in soils with low initial SOC, coarse texture, low water availability, and in grain and vegetable crops compared to forage crops. Changes in topsoil SOC stocks were linearly related to the cumulative carbon input from fertilizers. Carbon retention efficiency (CRE) was highest for low carbon inputs and decreased exponentially as carbon input increased, stabilizing at approximately 8 % with applications >200 Mg C ha. Using data from 42 experiments, it was calculated that the increase in SOC stocks was 41 % higher considering both the topsoil and subsoil (30-60 cm) compared to the topsoil alone, which would increase the CRE over 11 %, a value similar to the CRE of crop residues reported in the literature. These results show that the effect of organic fertilizers on SOC stocks should also be measured in the subsoil and that the increases in SOC are greater than those reported in previous synthesis studies that considered only the topsoil.