Identifying the agricultural imprint on the global N2O budget using stableisotopes

Agricultural soils are the most important anthropogenic source of nitrous o xide 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 delta N-15 of N2O emitted (f rom -46 parts per thousand to +5 parts per thousand relative to atmospheric N-2) from a urea-fertilized and irrigated agricultural field in Mexico. We calculated overall instantaneous enrichment factors for the sampling perio d, which suggest that the microbial N2O production shifts from nitrificatio n (week 1)to denitrification (week 2). Isotopic signatures of N2O emissions were not always in accord with other proxies (such as NO/N2O emission rati o or water-filled pore space) used to estimate the relative importance of n itrification and denitrification as N2O sources. These observations strongl y suggest that the soil surface emissions integrate processes occurring at different depths in the soil and a decoupling of NO and N2O production in t his system. Further clues as to the source of N2O come from the positional dependence of N-15 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 parts per thousand during the first 4 days a fter irrigation, implying that nitrification preferentially enriches the ce ntral N position compared to denitrification. The overall delta N-15 signat ure we measured for N2O emitted from N-fertilized agricultural systems is m ore depleted than observed delta N-15 values for N2O emitted from more N-li mited forest soils. Assuming that one half of the total agricultural N2O em issions associated with the global increase in soil nitrogen fertilizer use have an isotopic composition comparable to those of the agricultural field s reported here, we predict a decline in the isotopic signature of troposph eric N2O during this century of as much as 3 parts per thousand for N-15. A lthough many uncertainties remain, we suggest that measurements of delta N- 15-N2O in fun air will provide constraints on how the N2O budget has change d during the past century.