Spatial and temporal variability of soil nitric oxide emissions in N-saturated subtropical forest

Abstract

Acidic subtropical forest soils, receiving high atmospheric nitrogen (N) deposition, are characterized by fast N turnover, making them potential “hot spots” for nitrous oxide (N2O) and nitric oxide (NO) emissions. Production of NO in soils is mediated by microbial nitrification and denitrification and/or chemical decomposition of microbially produced nitrite (NO2 −). In monsoonal forests, soil microbial processes are exposed to strong soil moisture and temperature fluctuations, known to cause pronounced variability in NO emission. We quantified in situ fluxes of NO throughout two summers in the N-saturated TieShanPing forest, Chongqing, SW China. Additional N2O measurements were carried out in the second summer to infer likely source processes. To address spatial variability, fluxes were measured along two transects, one covering well-drained Acrisols on a forested hill slope, and the other hydrologically connected terraces with poorly drained Cambisols situated in a groundwater discharge zone. Hill slope sites emitted on average 48.4 ± 60.6 µg NO–N m−2 h−1 in the relatively wet summer of 2015 and 88.4 ± 94.6 µg NO–N m−2 h−1 in the dry summer of 2016, illustrating the strong effect of soil moisture regime. The NO emission in the groundwater discharge zone, measured in the dry summer of 2016 only, while it was flooded in the summer of 2015, was significantly smaller (26.4 ± 52.7 µg N m−2 h−1). The mean N2O-N emission on the hillslope (18.1 ± 13.9 µg N m−2 h−1) was smaller than NO-N flux, while it was larger (38.3 ± 39.3 µg N m−2 h−1) in the groundwater discharge zone. The temporal variability of the NO flux was largely explained by soil moisture fluctuations, whereas soil organic matter pool sizes contributed most to spatial variability. Hourly measurements showed that the NO flux doubled with 8 °C increase of soil temperature, indicating that soil temperature was another important driver for NO emissions. Our study indicates that acid N-saturated subtropical forest soils are a large source for NO, particularly during dry summers.