Drought turns a Central European Norway spruce forest soil from an NO source to a transient NO sink

摘要

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 NO on annual scales. To investigate the influence of drying/rewetting events on NO 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 NO sink. This was accompanied by subambient NO 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 NO sink strength. Rewetting quickly turned the soil into a source for atmospheric NO again, but it took almost 4 months to turn the cumulative soil NO fluxes from negative (sink) to positive (source) values. NO concentration and isotope analyses along soil profiles revealed that NO 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 NO fluxes from soils to the atmosphere or may even turn forest soils temporarily to NO sinks. Accumulation of more field-scale data on soil NO uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global NO budget.