Cropland soil carbon dynamics

Abstract

Humans began thousands of years ago to cultivate land for growing crops after clearing the previous vegetation cover and plowing the soil. The soil disturbance altered soil carbon (C) dynamics which has been recently exacerbated by the increase in crop intensification (i.e., fertilization, irrigation, mechanization). For example, conversion to croplands may release up tO36% of soil organic carbon (SOC) to 27-cm depth in temperate regions, and up tO30% of SOC to 48-cm depth in tropical regions. In 2000, about 12% of Earth’s ice-free land surface or 15 million km2 were covered by croplands. Climate, geology and land and crop management practices control the size of the cropland soil C pool. A major fraction (25–70%) of the carbon dioxide (CO2) fixed during plant photosynthesis in croplands by gross primary production (GPP) is respired autotrophically (R a) back to the atmosphere. Globally, cropland GPP is about 14.8 Pg C year-1 (1 Pg = 1015 g). The remaining net primary production (NPP = GPP-R a) is the main natural C input into cropland soils aside addition of manure and organic residues. Cropland NPP includes the production of biomass in foliage, shoots and roots, weed and seed production, root exudation, the 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. 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 lost as methane (CH4). Aside decomposition, C losses from croplands occur also by fire, erosion, leaching, and most importantly harvest removing about 2.2 Pg C year-1 in the 1990s. Thus, a small amount of fixed C remains in cropland soils and accumulates in the SOC pool 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 organomineral 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 cropland topsoils. Site-specific factors including climate, physicochemical characteristics, soil and vegetation management determine the balance between C input and losses. Cropland soils can be recarbonized to some extent through adoption of recommended management practices (RMPs) such as conservation tillage, residue mulching and use of cover crops, practices which all contribute to soil C accumulation and sequestration by an additional transfer of C from the atmosphere to the soil. Whether cultivation of SOC-accreting crops can also contribute to the recarbonization of cropland soils needs additional research.