Improving process-based estimates of N2O emissions from soil using temporally extensive chamber techniques and stable isotopes

Abstract

Nitrous oxide (N2O) is an important greenhouse gas that is emitted from soil, but obtain- ing precise N2O source and sink strength estimates has been difficult due to high spatial and temporal flux variability and a poor understanding of the mechanisms controlling fluxes. Tools that improve our ability to quantify trace gas fluxes from soil and constrain annual budgets are therefore needed. Here we describe an improved chamber-based sampling system that continuously traps evolving soil gases onto molecular sieve thereby obtaining a single sample that integrates fluxes over extended periods (several weeks or more) and the use of stable isotopic methods to study microbial origins of N2O. We demonstrate that N2O can be trapped on molecular sieve within our chamber system with near 100% recovery and without isotopic fractionation. In field trials the site preference of N2O (the difference in d 15 N between the central and outer N atoms) varied between -6 and 14.4%, indicating that the majority of flux was derived from bacterial denitrification. Further development with automation would improve flux estimates by providing a system capable of capturing episodic flux events owing to long-term deployment. Further, an automated trapping chamber approach will also provide process-based understand- ing of N2O dynamics via stable isotopes and a new and affordable tool for evaluating the response of trace gas fluxes to land management practices.