Experimental summer drought reduces soil CO2 effluxes and DOC leaching in Swiss grassland soils along an elevational gradient

Soil moisture affects belowground activity in grasslands, but the effects of summer drought on different soil C fluxes is uncertain. Soil respiration (SR), dissolved organic carbon (DOC) leaching and their components may all respond differently and drought effects will interact with other factors such as temperature, making a priori predictions of soil C balances difficult. In this study, we used rain shelters to simulate summer droughts by reducing annual precipitation by around 30 % in three managed grassland sites at 400, 1,000 and 2,000 m a.s.l. in Switzerland covering a gradient in mean annual temperatures of 7.5 °C. During the growing season, we quantified the impacts of drought on SR, DOC leaching, litter decomposition and the contribution of ¹³C-depleted litter to DOC fluxes. Along the elevational gradient, SR rates did not decrease with increasing altitude. Thus, SR was higher at a given temperature at higher altitudes, which probably reflects more labile soil C and hence greater substrate availability in a colder climate. Fluxes of DOC at 5 cm depth were a magnitude smaller than SR and did not show a pattern with elevation. At all altitudes, the experimental summer drought significantly reduced SR rates by 25–57 % and DOC leaching by 80–100 %, with a declining contribution of ¹³C-depleted litter-DOC. The remaining litter mass after drought was two to seven times larger as compared to the control. We did not observe a strong C release upon rewetting and hence, there was no compensation for the reduced soil C fluxes during drought. The more sensitive drought response in the litter layer than in the deeper soil and the declining DOC fluxes indicate an altered soil C balance with a C preservation in the topsoil, but ongoing losses of probably ‘older’ C in subsoils under drought.