N2O concentration and isotope signature along profiles provide deeper insight into the fate of N2O in soils†

Abstract

Nitrous oxide is an important greenhouse gas and its origin and fate are thus of broad interest. Most studies on emissions of nitrous oxide from soils focused on fluxes between soil and atmosphere and hence represent an integration of physical and biological processes at different depths of a soil profile. Analysis of N2O concentration and isotope signature along soil profiles was suggested to improve the localisation of sources and sinks in soils as well as underlying processes and could therefore extend our knowledge on processes affecting surface N2O fluxes. Such a mechanistic understanding would be desirable to improve N2O mitigation strategies and global N2O budgets. To investigate N2O dynamics within soil profiles of two contrasting (semi)natural ecosystem types (a temperate acidic fen and a Norway spruce forest), soil gas samplers were constructed to meet the different requirements of a water-saturated and an unsaturated soil, respectively. The samplers were installed in three replicates and allowed soil gas sampling from six different soil depths. We analysed soil air for N2O concentration and isotope composition and calculated N2O net turnover using a mass balance approach and considering diffusive fluxes. At the fen site, N2O was mainly produced in 30–50 cm soil depth. Diffusion to adjacent layers above and below indicated N2O consumption. Values of δ15N and δ18O of N2O in the fen soil were always linearly correlated and their qualitative changes within the profile corresponded with the calculated turnover processes, suggesting further reduction of N2O. In the spruce forest, highest N2O production occurred in the topsoil, but there was also notable production occurring in the subsoil at a depth of 70 cm. Changes in N2O isotope composition as to be expected from local production and consumption processes within the soil profile did hardly occur, though. This was presumably caused by high diffusive fluxes and comparatively low net turnover, as isotope signatures approached values measured for ambient N2O towards the topsoil. Our results demonstrate a highly variable influence of diffusive versus production/consumption processes on N2O concentration and isotope composition, depending on the type of ecosystem. This finding indicates the necessity of further N2O concentration and isotope profile investigations in different types of natural and anthropogenic ecosystems in order to generalise our mechanistic understanding of N2O exchange between soil and atmosphere. Revised version of a paper presented at the 30th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 8–10 October 2007, Bayreuth, Germany.