Simulated climate change and seasonal drought increase carbon and phosphorus demand in Mediterranean forest soils

A better understanding of soil biogeochemical responses to the increasing drought predicted in many regions for the next decades by climate models is needed. Extracellular enzyme (EE) stoichiometry provides integrative information about the soil community metabolism and resource availability that is crucial to understand soil microbial and plant strategies in response to those projected changes in soil moisture and resource availability. The aim of this study was to investigate the responses of enzyme allocation to seasonal and experimental drought conditions. We measured soil extractable organic carbon (EOC), extractable total nitrogen (ETN), extractable total phosphorus (ETP) and its organic and inorganic fractions (Po and Pi), together with soil extracellular enzyme activity (EEA) across four seasons, and tested for association between variables using pairwise resource and enzyme ratios, and vector length and angle enzyme integrated indexes in a Mediterranean holm oak forest subjected to the drought projected for coming decades (15% less soil water availability). Seasonal and experimental drought consistently increased levels of EOC and ETN, while decreases in ETP and Pi in autumn amplified the imbalance between soil extractable P with C and N, increasing the probability of P limitation throughout the year. Overall EE stoichiometry and vector angle indicated a higher relative demand for P than C and N. EE stoichiometry and vector length showed that both seasonal and experimental drought increased the relative demand for C than N or P by soil microbial communities, as supported by the observed rise in EOC and ETN. Vector angle showed that the transient seasonal drought increased the relative demand for P over N by soil microorganisms, especially in control plots, possibly due to the lower mobility of P in soil than N. The results show the important role of phosphatases in the recycling of organic P in this ecosystem. Our results indicated that prolonged drought decreased average decomposition of soil organic matter (SOM), due to the negative effect on individual enzyme activities, but accelerated SOM decomposition response to rewetting due to the accumulation of C and N in soil and the relatively higher presence of C-enzymes than those of other EE. Lower levels of P and N enzymatic activities than those of C under drought infer a reduction in nutrient release that may then negatively affect plant and microbial nutrition.