Soil Carbon Stock

Total soil carbon (C) stock comprises of the soil organic C (SOC) and the soil inorganic C (SIC) components. The global SOC stock of ice-free land contains about 1,325–1,500 Pg (1 Pg = 1015 g) C in the top 1 m, 2,300 Pg C in the top 3 m, and 3,000 Pg C in the soil profiles. Up to 716 Pg SOC may be stored to 1 m depth in cropland, temperate grassland/shrubland, and tropical grassland/savannah. However, estimates of global terrestrial inventories have large uncertainties because of the fewer studies and lack of credible estimates of the SOC stocks in permafrost, peatlands, and subsoil horizons. The SIC stock, primarily occurring in soils of the arid regions, is estimated at 700–1,700 Pg C in the top 1-m of soil. The SIC stocks, probably more in soils of the temperate regions and in deeper layers, are not widely studied. The SIC consists of lithogenic inorganic C (LIC) or primary carbonates derived from soil parent material, and pedogenic inorganic C (PIC) or secondary carbonates formed through pedogenic processes. Climate, geology, and land management practices are principal controls of the magnitude of the soil C stock as they are determinants of the soil and vegetation type. The SIC stock can be sink, source, or neither relative to the atmospheric carbon dioxide (CO2). For example, an increase in SIC stock occurs after weathering of soluble Ca/Mg-bearing silicates followed by the precipitation of pedogenic carbonates. Ecosystems with annual net Ca-carbonate (calcite) dissolution are local geochemical sinks of atmospheric CO2 as bicarbonates move into the groundwater. With the mean residence time (MRT) of PIC at 85,000 years, it is much less dynamic than the SOC stock with MRT or mean turnover time of about 35 years. However, SIC and SOC interact with each other but underlying mechanisms are less well known. Carbon is sequestered in the SOC stock via the C inputs from photosynthetic fixation of atmospheric CO2 by vegetation, deposition, and the accumulation of stabilized SOC fractions, and the input of black carbon (BC) with charred biomass. The main C input into the soil is net primary production (NPP) as a major fraction of the CO2 fixed during plant photosynthesis by gross primary production (GPP) is respired autotrophically and returned back to the atmosphere. NPP enters soil by rhizodeposition and decomposition of plant litter, and a large fraction is converted back to CO2 by soil respiration and some lost as methane (CH4). Aside microbial decomposition, C losses from soils of agroecosystems are also associated with fire, erosion, leaching, and harvest. Thus, a small amount of fixed C remains in the soil and accumulates in the SOC stock through a combination of short- and long-term stabilization processes. Important among 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 BC. Soil aggregation and formation of organo-mineral complexes may be the most important stabilization process in topsoils of agroecosystems. Site-specific factors including climate, physicochemical characteristics, soil and vegetation management determine the balance between C input and losses. However, it is unclear whether and how SOC saturation may occur in soil profiles of agroecosystems. This chapter begins with a discussion about SIC dynamics and its sequestration in soils of agroecosystems. Then, the effects of agricultural practices on SIC are compared, and indirect effects of carbonates on soil C sequestration are discussed. The second subsection of this chapter discusses the SOC stock and its dynamics. First, photosynthesis and SOC input processes are discussed followed by a comparison of processes contributing to SOC loss from agricultural soils. The dynamics of SOC in the mineral soil, and stabilization and decomposition/destabilization processes are presented in detail. The importance of BC is discussed in the chapter about biochar. The concluding section discusses the importance of SOC to soil quality, ecosystem services, and food security. Research needs and some pertinent questions are summarized at the end of the chapter.