Isotopic composition of tropospheric and soil N 2 O from successive depths of agricultural plots with contrasting crops and nitrogen amendments

Agricultural soils are the dominant contributor to increases in atmospheric nitrous oxide (N 2 O). Few studies have investigated the natural N and O isotopic composition of soil N 2 O. 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 δ 15 N and δ 18 O values of soil N 2 O ranged from −28.0 to +8.9‰, and from +29.0 to +53.6‰. The mean δ 15 N and δ 18 O values of tropospheric N 2 O were +4.6 ± 0.7‰ and +48.3 ± 0.2‰, respectively. In general, δ values were lowest at depth, they were negatively correlated to soil [N 2 O], and δ 15 N was positively correlated to δ 18 O for every treatment on all sampling dates. N 2 O from the different agricultural treatments had distinct δ 15 N and δ 18 O values that varied among sampling dates. Fertilized treatments had soil N 2 O with low δ values, but the unamended alfalfa yielded N 2 O with the lowest δ values. Diffusion was not the predominant process controlling N 2 O concentration profiles. Based on isotopic and concentration data, it appears that soil N 2 O was consumed, as it moved from deeper to shallower soil layers. To better assess the main process(es) controlling N 2 O within a soil profile, we propose a conceptual model that integrates data on net N 2 O production or consumption and isotopic data. The direct local impact of agricultural N 2 O on the isotopic composition of tropospheric N 2 O was recorded by a shift toward lower δ values of locally measured tropospheric N 2 O on a day with very high soil N 2 O emissions.