Vertical gradients of δ15N and δ18O in soil atmospheric N2O-temporal dynamics in a sandy soil

The greenhouse gas nitrous oxide (N2O) can be both formed and consumed by microbial processes in the soil. As these processes fractionate strongly in favour of N-14 and O-16, delta(15)N and delta(18)O gradients of N2O in the soil profile may elucidate patterns of N2O formation, consumption or emission to the atmosphere. We present the first in situ data of such gradients over time for a mesic typic Haplaquod seeded with potatoes (Solanum tuberosum L.). On two adjacent fields in 2002 and 2003, topsoil N2O fluxes were measured and the soil atmosphere was regularly sampled for N2O concentrations, delta(15)N and delta(18)O signatures of N2O at depths of 18, 48 and 90 cm during similar to 400 days. During the entire sampling period, the N2O concentrations were the highest and the delta(15)N signatures the lowest in the subsoil (48 or 90 cm depth) as compared with the topsoil, indicating production of N2O in the subsoil. For delta(15)N, differences greater than 30%. between topsoil and subsoil on the same date were regularly observed. The highest N2O concentration of 100385 mu L m(-3) at 90 cm depth on 1 July 2003, was preceded by the lowest delta(15)N value of -43.5%, one week earlier. This was followed by a 150-day general decrease of N2O concentrations at 90 cm depth to 1723 mu L m(-3) and a simultaneous enrichment of delta(15)N to +7.1 parts per thousand, mostly without a significant topsoil flux. There was a negative logarithmic relationship between N2O concentration at 90 cm depth and its delta(15)N signature. This relationship indicated a delta(15)N signature of -40 to -45 parts per thousand during the production of N2O in the subsoil, and a subsequent enrichment during the consumption of N2O. We conclude that the isotopic signature of the N2O topsoil flux is the result of various processes of consumption and production at different depths in the soil profile. It is therefore not a reliable estimator for the overall delta(15)N signature of N2O in the soil atmosphere, nor for indirect losses of N2O to the environment. Therefore, these findings will pose a further challenge to ongoing efforts to draw up a global isotopic budget for N2O. Copyright (c) 2005 John Wiley & Sons, Ltd.