Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink

Based on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N2O on annual scales. To investigate the influence of drying/rewetting events on N2O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N2O sink. This was accompanied by subambient N2O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N2O sink strength. Rewetting quickly turned the soil into a source for atmospheric N2O again, but it took almost 4 months to turn the cumulative soil N2O fluxes from negative (sink) to positive (source) values. N2O concentration and isotope analyses along soil profiles revealed that N2O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N2O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N2O sinks. Accumulation of more field-scale data on soil N2O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N2O budget.