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 test the following hypotheses: 1) in an evergreen Mediterranean woodland, plant growth and therefore plant-derived C input to the soil, both from above and below-ground are controlled by soil moisture, so a decrease in precipitation throughout the year will result in significant reduction of soil C input. On the contrary, enhanced water availability during the summer will stimulate NPP and strongly increase soil C input; 2) similarly, soil respiration in Mediterranean woodland is controlled directly and indirectly (i.e. via increased plant-derived C input) by the available soil water, thus a decrease in precipitation throughout the year will suppress soil respiration. Soil respiration will instead be enhanced if soil water increases during summer months. Experimental plots were exposed to either a 20% reduction of throughfall or to a water addition by targeted irrigation during summer months, to maintain soils above a water content of 10% v/v. Treatments were compared to control plots which received ambient rain. In each plot, we measured net annual C input to soil (i.e. litterfall), stem growth and continuous soil respiration and quantified, by the use of stable C isotopes, changes in plant-derived below-ground C input as affected by the water manipulation. Additionally, we discuss the overall implication of the observed treatment effects on soil C fluxes for soil C stores.
Results/Conclusions
Enhanced soil moisture during summer months highly stimulated annual stem primary production, litter fall, soil respiration and net annual plant-derived C input to soil which on average increased by 130%, 26%, 50% and 220%, respectively, as compared to control. On the contrary, the 20% reduction in throughfall (equivalent to 10% reduction of rainfall) did not significantly change soil moisture at the site, and therefore did not significantly affect ANPP or soil respiration. We conclude that minor changes (around 10% reduction) in precipitation amount are not likely to significantly affect ANPP or soil C dynamics in Mediterranean woodland. However, if summer rain will increase, C cycling will significantly accelerate but soil C stocks are not likely to be changed in the short-term.