N₂O consumption by low-nitrogen soil and its regulation by water and oxygen

Soils can be a source and sink for atmospheric nitrous oxide (N₂O). Consumption of N₂O has been reported for anoxic soils and sediments rich in organic matter and depleted in nitrates (NO3-), and also for some dry, oxic soils. However, the mechanisms and controls of N₂O consumption in dry soil are not clear.Here, we report on a field study in China (Taihang mountain region, Shijiazhuang), in which N₂O uptake by a sandy loam soil was measured for the greater part of the season (from April to October in 2011), and on four incubation experiments, in which we tried to reveal the roles of water content and oxygen (O₂) concentrations on N₂O consumption. Flux measurements in the field were made bi-weekly on unfertilized cropped land with static flux chambers (5 replicates) for 6 months. The results show that N₂O-N fluxes ranged from -26.0 to -726.6 μg m^-2 h^-1. Consumption of N₂O was largest when the soil was dry (5-20% soil water filled pore space). In the incubation experiments, N₂O consumption and N₂ production were measured in (an)aerobic soil with soil moisture content ranging from 1% to 50% (wt/wt) and with N₂O addition, using a thermostatic, robotized incubation system. Under anaerobic conditions, N₂O was rapidly consumed at water content of >10% (wt/wt). However, a significant consumption also occurred at 1% soil moisture. Under aerobic conditions, N₂O consumption increased with increasing soil moisture content, but significant consumption was still measured at 2% moisture. Sterilization of oxic soil completely blocked N₂O consumption, suggesting that the consumption had a biological nature. In conclusion, the steady N₂O consumption measured in the field was confirmed by the laboratory experiments, but the relationship with soil moisture content was reversed. Further studies are required to understand this apparent anomaly.