Terrestrial denitrification and nitrous oxide emissions: global estimates and uncertainties

Synthetic N fertilizer use has grown exponentially causing severe environmental consequences. Most reactive nitrogen (Nr) is ultimately removed by denitrification, however estimates of denitrification are highly uncertain. Quantifying denitrification is key for understanding Nr sinks and emissions of nitrous oxide (N2O). Unfortunately, denitrification from upland soils is not well quantified, with the magnitude of total denitrification losses (N2 + N2O) virtually unknown for the vast majority of agroecosystems. To narrow the uncertainty, we compared global terrestrial denitrification rates estimated using measured N2:N2O product ratios with rates estimated using an N mass balance and with previous published studies.

The agricultural revolution was primarily fueled by the use of synthetic nitrogen (N) fertilizers and increased cultivation of N-fixing crops (Battye et al., 2017), both of which have increased the amount of reactive N (Nr) in the biosphere. Accumulation of Nr has caused alterations to the global N cycle (Steffen et al., 2015) causing severe environmental problems including biodiversity loss, eutrophication, human health problems, and increased radiative forcing (Galloway et al., 2003). The primary sink for Nr is denitrification, where nitrate can eventually be reduced to dinitrogen (N2). However, due to the large atmospheric N2 background, N2 fluxes are difficult to measure causing high uncertainty when calculating global N budgets.