Technologically achievable soil organic carbon sequestration in world croplands and grasslands

Abstract

Reported potentials for sequestration of carbon in soils of agricultural lands are overly optimistic because they assume that all degraded cropland and grassland can be subjected to best management practices. Two approaches for estimating this potential are presented. Method 1 (M1) considers literature-derived best estimates for annual soil organic carbon (SOC) gains (Mg C ha−1) by bioclimatic zone; Method 2 (M2) assumes an annual C increase of 3 to 5 promille with respect to present SOC mass (similar to the French ‘4 pour mille’ initiative). Four management scenarios are considered, capturing the varying level of plausibility of meeting the full technological potential. According to M1, achievable gains range from 0.05–0.12 Pg C yr−1 to 0.14–0.37 Pg C yr−1, with a technological potential of 0.32–0.86 Pg C yr−1. For M2, these are 0.07–0.12 Pg C yr−1, 0.21–0.35 Pg C yr−1, and 0.60–1.01 Pg C yr−1. Consideration of the technological potential only and use of a proportional annual increase in SOC (M2), rather than using best estimates for soil carbon gains by bioclimatic zone (M1), will provide too ‘bright a picture’ in the context of rehabilitating degraded lands and mitigating/adapting to climate change. Further, M2 assumes that possible C gains will be greatest where present SOC stocks are highest, which is counter-intuitive. Although all measures aimed at increasing SOC content should be encouraged due to the creation of win-win situations, it is important to create a realistic picture of the amount of SOC gains that are feasible based on bioclimatic and management implementation constraints.