Greenhouse-gas exchange of croplands worldwide: a process-based model simulation

Croplands cover about 15% of the land surface, and play unique roles in global biogeochemical cycles. Especially, greenhouse gas budget of croplands is important for climate projection in the future and for mitigation toward climate stabilization. Sustainable cropland is carbon-neutral (i.e., neither a sink nor a source of CO2 for a long time), but those in developed countries consume fossil fuels for agricultural operations and releases CO2 as revealed by LCAs. Paddy field is one of the substantial sources of CH4, and cropland may be the largest anthropogenic source of N2O. However, these features have not been evaluated and discussed using a spatial-explicit comprehensive framework at the global scale. This study applies a process-based terrestrial ecosystem model (VISIT) to worldwide croplands. Exchange of CO2 is simulated as a difference between photosynthesis and respiration, each of which is calculated in a biogeochemical carbon cycle scheme. Net carbon budget accounts for carbon flows by planting, compost input, and harvest. Exchange of CH4 is simulated as a difference between oxidation by aerobic soils and production by anaerobic soils, each of which is calculated using mechanistic schemes. Emission of N2O from nitrification and denitrification is simulated with a semi-mechanistic scheme on the basis of leaky-pipe concept. We are also validating the model through comparison with chamber and tower flux measurements. Global simulations were conducted during a period from 1901 to 2100 on the basis of historical and projected climate and land-use conditions, at a spatial resolution of 0.5 x 0.5 degree. Cropland type and distribution was derived from SAGE-HYDE dataset and country-base fertilizer input was obtained from FAOSTAT. Our preliminary simulation for the 1990s estimated that croplands are a net sink of CO2 by 1.1 Gt C/yr; this sink is offset by emission by food consumption. Paddy fields are estimated to release CH4 by 46 Tg CH4/yr, and croplands worldwide release N2O by 5.9 Tg N2O/yr. Because of high Global Warming Potential of CH4 (25 for 100-yr) and N2O (298), these results imply that agriculture is a net source of radiative forcing for the atmosphere. Additionally, recent studies show that N2O is the most important substance for stratospheric ozone depletion. Therefore, further studies are needed to improve quantification of greenhouse gas budget in croplands and to design mitigation strategy.