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

Abstract

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.

A Short‐term Study to Compare Field Strategies for Ammonia Emission Mitigation

“…The low SWC enhances the infiltration of slurry liquid, hence the mass transport of NH 4 + into the soil (Sommer and Jacobsen, 1999). Thus, slurry infiltration rates were favored, and microbial processes related to other N losses than NH 3 volatilization (e.g., denitrification) were constrained (Smith, 2017). …”

X‐ray computed tomography to predict soil N2O production via bacterial denitrification and N2O emission in contrasting bioenergy cropping systems

“…Their denitrifying communities were not likely affected by sampling or sample preprocessing; thus, they continued to actively function during the incubation experiment of this study. The potential importance of adaptation history in defining the ability of microbial communities to produce N 2 O has been expressed before (Jungkunst, Freibauer, Neufeldt, & Bareth, 2006;Krause et al, 2017;Lagomarsino et al, 2016), while the history of soil moisture conditions was found to play an unexpectedly greater role than current soil water levels for other soil processes, for example, soil respiration (Smith, 2017).…”

“…The S P of N 2 O produced by fungi has been shown to be~37‰ (Rohe et al, 2014;Sutka, Adams, Ostrom, & Ostrom, 2008), which is substantially greater than the values of −10 to 0‰ associated with production from bacterial denitrification (Frame & Casciotti, 2010;Sutka et al, 2006;Toyoda & Yoshida, 1999). Fungi have been demonstrated to denitrify at more oxic conditions than bacteria and to be a substantial contributor to N 2 O production, especially in woody soils, but also in grasslands and soils of agricultural systems (Chen , 2017;Chen, Mothapo, & Shi, 2014;Crenshaw, Lauber, Sinsabaugh, & Stavely, 2008;Laughlin & Stevens, 2002). Fungi were found to be in a greater abundance in poplar and early-successional, however also in switchgrass, systems of this study than in corn-based systems (Jesus et al, 2016).…”

Smith, K.A., Ball, T., Conen, F., Dobbie, K.E., Massheder, J. & Rey, A. 2003. Exchange of greenhouse gases between soil and atmosphere: interactions of soil physical factors and biological processes. European Journal of Soil Science, 54, 779–791.

“…The merit of the review by Smith et al (2003) lies again in the important emphasis of the link between microbial processes and physical factors in addition to other factors, such as substrate availability and chemical factors such as soil pH (e.g. There is still some controversy about which physical soil property is most useful for estimating N 2 O emissions; for example, the ratio of gas diffusivity within the soil to that in free air, the degree of aggregation and compaction, matric potential, WFPS and volumetric water content (for a discussion see Ball, 2013 andSmith, 2017). This link is crucial for an understanding and prognosis of N 2 O emissions.…”