Isotopic signatures and concentration profiles of nitrous oxide in a rice‐based ecosystem during the drained crop‐growing season

Abstract

The stable isotopic composition of nitrous oxide (N2O) in agricultural soils can improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N2O formation in soils, and can provide constraints for the atmospheric N2O budget. Here, we present soil profiles featuring N2O concentrations and δ15N and δ18O values in N2O over time, which permit the in situ identification of processes and sites of N2O production in a rice‐based ecosystem seeded with winter wheat. Our δ15N and δ18O soil profile values support the conclusion that denitrification is the dominant process behind N2O production during the winter wheat season. The soil N2O gas concentrations are higher below 10 cm than above 10 cm, and more depleted in 15N‐N2O at 10–15 cm depth compared to other soil depths, which indicates that the N2O production zone is located at a depth of 10–15 cm within 0–20 cm. The δ15Nair and δ18OSMOW (δ18O of standard mean ocean water) values for soil gas N2O averaged over the entire wheat growing season are +0.90 ± 2.9 (n = 64) and +39.3 ± 3.1 (n = 64), respectively. Given that paddy source N2O is much heavier in both measured 15N and 18O compared to other fertilized soils, agricultural soils have been underestimated as a source of N2O since isotopic constraints from rice agriculture have not been taken into account.