Greenhouse gas budgets of crop production and the mitigation potential of nutrient management

Agriculture is the main source of non-CO2 anthropogenic greenhouse gas (GHG) emissions, contributing an estimated 3,300 Mt (million tonnes) CO2-eq/yr as methane (CH4) and 2,800 Mt CO2-eq/yr as nitrous oxide (N2O) emissions, which amounts to 10-12% of total global GHG emissions. Within this, N2O from soils, mainly associated with fertiliser and manure applications, and CH4 from enteric fermentation, make up around 70% of non-CO2 emissions from the sector. Biomass burning, rice cultivation, and manure management account for the remainder. The relative magnitude of emissions and sources vary greatly between different regions of the world; emissions from rice cultivation and biomass burning are heavily concentrated in developing countries, while manure management is the only source where resulting GHG emissions are higher in developed regions. Large exchanges of carbon dioxide (CO2) occur between the atmosphere and agricultural ecosystems, but emissions are thought to be roughly balanced by uptake, giving a net flux of only around 40 Mt CO2/yr, less than 1% of global anthropogenic CO2 emissions. Many uncertainties remain, not least because emissions can vary widely, both spatially and over time, but strong relationships do exist. Direct N2O emissions from soils are related to N inputs meaning they are very low in some African countries, where crops are generally under-fertilised, but very significant in parts of Asia, where fertilisers are often applied far in excess of crop demand. As the world's largest consumer of mineral N fertilisers, China is estimated to emit roughly 0.3% of total global GHG emissions as direct N2O emissions from N inputs alone. As a result, there is significant scope for mitigating agricultural GHG emissions by improving nutrient management, through the wider use of strategies such as fertiliser Best Management Practices. Making the best use of nutrient resources allows increases in crop yields while minimising the N inputs required to achieve these and the resulting GHG emissions. Global agricultural production needs to increase to meet the demands of a rising population and increasing yields from existing cropland minimises the requirement for more land to be brought under cultivation. This is important because land use change is still an area of considerable uncertainty but may contribute as much as 17% of current total global GHG emissions.