Carbon Sequestration in Grassland Soils

Abstract

Grasslands, including rangelands, shrublands, pastureland, and cropland sown with pasture and fodder crops, cover 35 million km² or 26% of the global ice-free land area. Grasslands support the livelihoods of 1 billion people with pastoralism (rising of livestock) being the most widespread human land-use system globally with 20 million km² of grassland used for livestock feed production. The global is under pasture has, particularly, strongly increased since prehistoric times. Many grasslands have suffered losses of soil organic carbon (SOC) because of soil disturbance, vegetation degradation, fire , erosion , nutrient shortage, and water deficit. The nature, frequency, and intensity of disturbance may play a key role in the SOC balance of grassland. Less well known are the effects of disturbance processes on the soil inorganic carbon (SIC) stock. However, bedrock, irrigation practices, soil acidification, liming, and grazing management potentially affect the SIC stock of grassland. In comparison, better studied are the SOC dynamics in grassland. Grasslands have a high inherent SOC stock with up to 343 Pg SOC stored to 1 m depth with a sequestration rate of 0.5 Pg C yr⁻¹. Grasslands sequester large amounts of SOC because of a high belowground C allocation, root turnover, and rhizodeposition . Grassland gross primary production (GPP) is the major natural soil C input and has been estimated at 31.3 Pg C yr⁻¹ for tropical savannas and grasslands, and to 8.5 Pg C yr⁻¹ for temperate grasslands and shrublands, respectively. The net primary production (NPP) of grassland denotes C assimilation by plants before losses caused by grazing, harvest, herbivory, mowing, and other processes. However, the numerous processes of NPP loss are among the reasons why direct measurements of NPP of grassland are challenging because not all of the biomass produced remains within the ecosystem. Further, all components of NPP of grassland must be measured in a single study. Additional research is also needed about the quantitative contribution of the major sources of SOC, and roots may play a critical role in maintaining SOC stocks in the future. Possible inputs of belowground C include: (i) incorporated surface plant residues, (ii) plant root litter and rhizodeposition , (iii) dung and urine of grazing animals, and (iv) black carbon (BC) in fire -affected grasslands. Grazing management must be targeted toward SOC sequestration due to the large global grazing land area and potential for considerable rates of increase in SOC stock.