Vortrag, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 25.02.2007 - 28.02.2007
Abstract:
Global change scenarios predict especially for higher elevation areas in Central Europe an increased probability of summer drought periods followed by heavy rainfall events. The current knowledge about the effects of these events on soil processes is scarce. Mainly based on data from laboratory experiments it is expected that these drying/rewetting events induce an increased matter cycling in soils, due to accumulation of dead biomass and thereby nutrients, which may become easily accessible during rewetting. We hypothesize that event–based increased matter fluxes in the gas and solute phase considerably contribute to the respective fluxes on annual scales. In order to test the influence of drying/rewetting events on soil processes, a field experiment in a mature Norway spruce forest on Haplic Podzol was initiated in the Fichtelgebirge area (NE Bavaria, Germany). Roofs were installed on three 400 m2 plots to exclude rainfall and thus induce soil drought. Three non-manipulated plots served as controls. A summer drought of 46 days was induced in the first year of this experiment, followed by a rewetting event of 67 mm in two days. During the drought period soil water content in the organic layer of the drought plots decreased continuously from 25 to 10% (Oh) and 15 to 5% (Of), respectively, and thus, were about 5% lower than the respective soil water contents in the control plots. The rewetting event increased the water content of the organic layer to the control level. The soil matrix potential at 90 cm depth of the drought plots decreased from about -100 hPa to minimum values of ca. -700 hPa during the drying period and increased to the initial values after rewetting. The drying period induced a tendency towards lower N mineralization and lower nitrate concentrations at 20 cm soil depth. Both of these observations continued on the drought plots even after the rewetting event. Fine root decomposition on the drought plots was lower than on the control plots. Soil CO2 emissions decreased from 3.4 to 1.1 mmol m-2 h-1 during the drying period and recovered after rewetting to the control level. Due to the low CO2 fluxes during the drying period, cumulative CO2 emissions on the treatment plots were reduced by about 10%. A similar effect of the drying/rewetting events was found for the cumulative N2O fluxes. The soil of the drought plots even served as a transient N2O sink during the drying period. After rewetting the soil N2O emissions quickly recovered to the control level (0.5 µmol m-2 d-1). NO emissions gradually decreased to zero during the drying period and recovered to fluxes slightly above control level after rewetting. In general, the NO emissions from this soil were up to 100-fold higher than the N2O emissions. In contrast to our hypothesis, drying periods severely reduced the soil trace gas emissions such that the cumulative emissions even after rewetting remained lower than in the control plots.