Isotopic composition of tropospheric and soil N2O from successive depths of agricultural plots with contrasting crops and nitrogen amendments

Abstract

Agricultural soils are the dominant contributor to increases in atmospheric nitrous oxide (N2O). Few studies have investigated the natural N and O isotopic composition of soil N2O. We collected soil gas samples using horizontal sampling tubes installed at successive depths under five contrasting agricultural crops (e.g., unamended alfalfa, fertilized cereal), and tropospheric air samples. Mean δ15N and δ18O values of soil N2O ranged from -28.0 to +8.9%o, and ftorn +29.0 to +53.6%o. The mean δ15N and δ 18O values of tropospheric N2O were +4.6 ± 0.7%o and +48.3 ± 0.2%o, respectively. In general, δ values were lowest at depth, they were negatively correlated to soil [N2O], and δ 15N was positively correlated to δ18Q for every treatment on all sampling dates. N2O from the different agricultural treatments had distinct δ15N and δ18O values that varied among sampling dates. Fertilized treatments bad soil N2O with low δ values, but the unamended alfalfa yielded N2O with the lowest δ values. Diffusion was not the predominant process controlling N2O concentration profiles. Based on isotopic and concentration data, it appears that soil N2O was consumed, as it moved from deeper to shallower soil layers. To better assess the main process(es) controlling N2O within a soil profile, we propose a conceptual model that integrates data on net N2O production or consumption and isotopic data. The direct local impact of agricultural N2O on the isotopic composition of tropospheric N2O was recorded by a shift toward lower δ values Of locally measured tropospheric N2O on a day with very high soil N2O emissions. Copyright 2007 by the American Geophysical Union.