Estimating global terrestrial denitrification from measured N${}_2$O:(N${}_2$O + N${}_2$) product ratios

The use of nitrogen (N) fertilizers and cultivation of N-fixing crops has grown exponentially over the last century, with severe environmental consequences. Most of the anthropogenic reactive nitrogen will ultimately be returned by denitrification to the atmosphere as inert N${}_2$, but the magnitude of denitrification and the ratio of N${}_2$O to (N${}_2$O + N${}_2$) emitted (R${}_{N_{2}O}$) is unknown for the vast majority of terrestrial ecosystems. This paper provides estimates of terrestrial denitrification and R${}_{N_{2}O}$ by reviewing existing literature and compiling a N budget for the global land surface. We estimate that terrestrial denitrification has doubled from 80 Tg-N year${}^{-1}$ in pre-industrial times to 160 Tg-N year${}^{-1}$ in 2005 with a mean R${}_{N_{2}O}$ of approximately 0.08. We conclude that upscaling of R${}_{N_{2}O}$ can provide spatial estimates of terrestrial denitrification when data from acetylene inhibition methods are excluded. Recent advances in methodologies to measure N${}_2$ emissions and R${}_{N_{2}O}$ under field conditions could open the way for more effective management of terrestrial N flows.