Carbon Sequestration in Cropland Soils

Humans began to cultivate land thousands of years ago, for growing crops after clearing the previous vegetation cover and plowing the soil. The soil disturbance altered, in particular, soil C dynamics which has recently been exacerbated by the increase in crop intensification (i.e., fertilization, irrigation, liming, mechanization). This has resulted in major losses of terrestrial carbon (C); i.e., global emissions by the expansion of agriculture for croplands were estimated at 98.4 Pg C for the period 1850–2015. Cropland management can alter soil inorganic carbon (SIC) stocks which, in arid and semiarid climates, can be similar to or more than the amount of soil organic carbon (SOC) stock. However, whether a specific practice leads to a net sequestration of SIC in cropland soils needs additional research. Otherwise, the conversion of land to cropland in temperate regions may release up to 36% of the SOC stock to 27 cm depth, and up to 30% of the SOC stock to 48 cm depth in tropical regions. In 2000, about 12% of Earth’s ice-free land surface or 15 million km2 was used for croplands. Climate, geology, and land and crop management practices control the magnitude of the cropland SOC stock. A major fraction (25–70%) of the carbon dioxide (CO2) fixed during crop photosynthesis by gross primary production (GPP) is respired autotrophically (Ra) back to the atmosphere. Estimates of global cropland GPP vary between 8.2 and 20.0 Pg C yr−1 (1 Pg = 1015 g). The remaining net primary production (NPP = GPP − Ra) is the main C input into cropland soils. Cropland NPP includes the production of biomass in foliage, shoots and roots, weed and seed production, root exudation, the carbon (C) transfer to microorganisms that are symbiotically associated with roots, and the volatile organic carbon (VOC) emissions that are lost from leaves to the atmosphere. However, not all components of cropland NPP have yet been measured in a single study. NPP enters soil by rhizodeposition and decomposition of plant litter, but the major fraction is heterotrophically converted back to CO2 by soil respiration and some is lost as methane (CH4). Aside decomposition, C losses from croplands soils occur also by fire, erosion, leaching, and harvest. Thus, a small amount of fixed C remains in cropland soils and accumulates as SOC due to a combination of short- and long-term stabilization processes. Stabilization processes include physical protection of organic matter (OM) against decomposers and their enzymes, stabilization by organo-mineral complexes and organo-metal interactions, and some as biochemically recalcitrant black carbon (BC). Soil aggregation, in particular, may be the most important stabilization process in the surface layers of cropland soils. Site-specific factors including climate, physicochemical characteristics, soil and vegetation management determine the balance between C input and losses. Cropland soils can be recarbonized through the adoption of recommended management practices (RMPs) such as conservation tillage, residue mulching and use of cover crops, practices which all contribute to SOC accumulation and sequestration by an additional transfer of C from the atmosphere to the soil. Translating science of cropland soil management into a global restorative program is a high priority for feeding the world, mitigating climate change and improving the environment.