Changing views of nitrous oxide emissions from agricultural soil: key controlling processes and assessment at different spatial scales

Nitrous oxide, N2O, is the third most important of the long‐lived greenhouse gases, in terms of its contribution to global warming, and is expected to be the dominant cause of stratospheric ozone depletion this century. The concentration of N2O in the atmosphere was fairly constant until the beginning of the industrial age, but has gone up by 20% since. This is because of increased anthropogenic emissions, of which about 60% come from agricultural soil. The cause is the increased use of synthetic fertilizer nitrogen globally to meet the demands for increased production of food and biofuels. This review examines the isotopic evidence for this role of fertilizer N, the main mechanisms for microbial production of N2O in soil, the key soil physical and other variables that greatly affect the magnitude of emissions, and the spatial and temporal variation in emissions and the associated problems of measurement. The review also considers the methodology devised by the Intergovernmental Panel on Climate Change (IPCC) to enable countries to compile national inventories of their emissions, direct and indirect, that arise from anthropogenic activity related to agriculture and land use. This methodology also enables modellers to make ‘bottom‐up’ global estimates of N2O emissions. These estimates agree quite well at the global scale with ‘top‐down’ estimates based on the relation between reactive nitrogen newly introduced into agricultural ecosystems and increases in the global atmospheric concentration of N2O, but not with some recent regional‐scale emission measurements. The discrepancy may be related to problems of estimating indirect emissions. Top‐down estimates indicate that N2O emissions from crop‐based biofuels are too great to comply with national and international environmental regulations. Finally, possible future trends in emissions and possible mitigation measures are discussed. Highlights Evidence for increasing N2O in the atmosphere suggests it is caused by fertilizer N use. Agreements and inconsistencies in assessments of N2O emissions are considered. The main cause of increasing N2O is the use of synthetic N fertilizers on agricultural soil worldwide. Indirect emissions from waters receiving leached N may be a major cause of assessment uncertainties.