Decreased summer drought affects plant productivity and soil carbon dynamics in a Mediterranean woodland

Precipitation patterns are expected to change in the Mediterranean region within the next decades, with projected decreases in total rainfall and increases in extreme events. We manipulated precipitation patterns in a Mediterranean woodland, dominated by Arbutus unedo L., to study the effects of changing precipitation regimes on above-ground net primary production (ANPP) and soil C dynamics, specifically plant-derived C input to soil and soil respiration (SR). Experimental plots were exposed to either a 20% reduction of throughfall or to water addition targeted at maintaining soil water content above a minimum of 10% v/v. Treatments were compared to control plots which received ambient precipitation. Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, SR and net annual plant-derived C input to soil which on average increased by 130%, 26%, 58% and 220 %, respectively, as compared to the control. In contrast, the 20% reduction in throughfall (equivalent to 10% reduction in precipitation) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or SR. We conclude that minor changes (around 10% reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodlands. However, if summer rain increases, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term. More studies involving modelling of long-term C dynamics are needed to predict if the estimated increases in soil C input under wet conditions is going to be sustained and if labile C is being substituted to stable C, with a negative effect on long-term soil C stocks.