Increasing drought decreases phosphorus availability in an evergreen Mediterranean forest

Mediterranean ecosystems are water-limited and frequently also nutrient-limited. We aimed to investigate the effects of increasing drought, as predicted by GCM and eco-physiological models for the next decades. on the cycle and P plant availability in a Mediterranean forest. We conducted a field experiment in a mature ever-green oak forest, establishing four drought-treatment plots and four control plots (150 m(2) each). After three years. the runoff and rainfall exclusion reduced an overall 22% the soil moisture, and the runoff exclusion alone reduced it 10%. The reduction of 22% in soil moisture produced a decrease of 40% of the accumulated above-ground plant P content, above all because there was a smaller increase in aerial biomass. The plant leaf P content increased by 100+/-40 mg m(-2) in the control plots, whereas it decreased by 40+/-40 mg m(-2) in the drought plots. The soil Po-NaHCO3 (organic labile-P fraction) increased by 25% in consonance with the increase in litterfall. while the inorganic labile-P fraction decreased in relation to the organic labile-P fraction up to 48%, indicating a decrease in microbial activity. Thus, after just three years of slight drought. a clear trend towards an accumulation of P in the soil and towards a decrease of P in the stand biomass was observed. The P accumulation in the soil in the drought plots was mainly in forms that were not directly available to plants. These indirect effects of drought including the decrease in plant P availability, may become a serious constraint for plant growth and therefore may have a serious effect on ecosystem performance.