Spatial Variations of Soil N2 and N2O Emissions from a Temperate Forest: Quantified by the In Situ 15N Labeling Method

Emissions of dinitrogen (N2) and nitrous oxide (N2O) from soil are important components of the global nitrogen cycle. Soil N2O emissions from terrestrial ecosystems have been well studied. However, patterns and mechanisms of N2 emissions remain unclear due to the technical difficulty in measuring N2 production. In this study, an in situ 15N labeling method was employed to determine soil N2 and N2O emission rates from the lower, middle, and upper slopes, which correspond to different moisture conditions, in a temperate forest in Northeast China. We found that N2 emissions varied from 85 to 3442 μg N m−2 h−1 across the slopes and were dominated by denitrification. The emissions of bulk N2O (22 to 258 μg N m−2 h−1) and denitrification-derived N2O (14 to 246 μg N m−2 h−1) were significantly lower than N2 emissions from their corresponding slope positions. Both N2 and N2O emissions significantly increased when soils become wetter. The ratios of N2O/(N2O + N2) were significantly higher at the upper and middle slopes (0.22 and 0.20, respectively) compared with those at the lower slope (0.08 ± 0.01). At the catchment scale, N2 accounted for 85% of the total gaseous N losses (N2O + N2). Our study shows that soil moisture drives the patterns of N2 and N2O emissions and field quantification of N2O/(N2O + N2) ratio should further consider the effect of slope position of forest ecosystems to estimate total soil gaseous N losses.