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

Abstract

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 ¹⁴N and ¹⁶O, δ¹⁵N and δ¹⁸O 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, δ¹⁵N and δ¹⁸O signatures of N₂O at depths of 18, 48 and 90 cm during ∼400 days. During the entire sampling period, the N₂O concentrations were the highest and the δ¹⁵N signatures the lowest in the subsoil (48 or 90 cm depth) as compared with the topsoil, indicating production of N₂O in the subsoil. For δ¹⁵N, differences greater than 30‰ between topsoil and subsoil on the same date were regularly observed. The highest N₂O concentration of 100385 μL m⁻³ at 90 cm depth on 1 July 2003, was preceded by the lowest δ¹⁵N 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 μL m⁻³ and a simultaneous enrichment of δ¹⁵N to +7.1‰, mostly without a significant topsoil flux. There was a negative logarithmic relationship between N₂O concentration at 90 cm depth and its δ¹⁵N signature. This relationship indicated a δ¹⁵N signature of −40 to −45‰ 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 δ¹⁵N 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 © 2005 John Wiley & Sons, Ltd.