Identifying the agricultural imprint on the global N2O budget using stable isotopes

Agricultural soils are the most important anthropogenic source of nitrous oxide to the atmosphere. We observed large shifts with time in the emission rate (from 170 to 16 ng N cm−2 h−1) and in δ15N of N2O emitted (from −46‰ to +5‰ relative to atmospheric N2) from a urea-fertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling period, which suggest that the microbial N2O production shifts from nitrification (week 1) to denitrification (week 2). Isotopic signatures of N2O emissions were not always in accord with other proxies (such as NO/N2O emission ratio or water-filled pore space) used to estimate the relative importance of nitrification and denitrification as N2O sources. These observations strongly suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N2O production in this system. Further clues as to the source of N2O come from the positional dependence of 15N in the emitted N2O, reported here for the first time in soil emissions. Enrichment at the central N position increased relative to the terminal N position by 9.3‰ during the first 4 days after irrigation, implying that nitrification preferentially enriches the central N position compared to denitrification. The overall δ15N signature we measured for N2O emitted from Nfertilized agricultural systems is more depleted than observed δ15N values for N2O emitted from more Nlimited forest soils. Assuming that one half of the total agricultural N2O emissions associated with the global increase in soil nitrogen fertilizer use have an isotopic composition comparable to those of the agricultural fields reported here, we predict a decline in the isotopic signature of tropospheric N2O during this century of as much as 3‰ for 15N. Although many uncertainties remain, we suggest that measurements of δ15N-N2O in firn air will provide constraints on how the N2O budget has changed during the past century.