Experimental drought reduced acid and alkaline phosphatase activity and increased organic extractable P in soil in a Quercus ilex Mediterranean forest

A six-year (1999-2005) experiment of drought manipulation was conducted in a Quercus ilex Mediterranean forest (Southern Catalonia) to simulate predicted climatic conditions projected for the decades to come. The aim was to investigate the direct and indirect effects of drought conditions on acid and alkaline phosphatase activity in soil and on P concentrations in soil, leaves and litter throughout the year. Soil acid phosphatase activity was higher than soil alkaline phosphatase activity. Drought reduced acid phosphatase activity in soil in all seasons, including summer and winter, the seasons with less biological activity due to water and cold stress. Reductions of soil water content between 13 and 29% reduced soil acid phosphatase activity between 22 and 27% depending on the season. Drought reduced alkaline phosphatase activity (by 28%) only in winter. Soil acid and alkaline phosphatase activities were positively correlated with soil water content in all seasons. In contrast short-term available-P which increased under drought in several seasons was weakly correlated with soil phosphatase activities. As a result, immediately/short-term available-P concentration ratios decreased in all the seasons (between 10 and 71%). Drought increased foliar P concentration and reduced the C/P concentration ratio in litter fall of the dominant tree Q. ilex. Drought also decreased the ratio between organic C and short-term available-P in soil. The results show that soil phosphatase activity is more directly dependent on changes in water availability than on changes in its substrate, short-term available-P. These effects of drought have several implications: the accumulation in the soil of labile P not directly available to plants, the increase in potential P losses from leaching and erosion during the torrential rainfalls typical of the Mediterranean climate, and changes in plant, litter and soil C:P stoichiometry that may lead to changes in soil trophic chains.