Climate change unbalances biogeochemical cycles of C, N and P in Mediterranean forests

Mediterranean forests are considered to be highly vulnerable ecosystems to climate change because they are water- and nutrient-limited. However, experimental evidence of the combined effects of increasing drought and warming on biogeochemical cycles in these ecosystems is still extremely scarce. To fill this gap, we analyzed during four consecutive years the impacts of rainfall reduction (RE; ambient vs. ~30% reduction in rainfall), soil warming (W; ambient vs. ~ 1 °C increase) and their interaction on biogeochemical cycling of C, N and P in Mediterranean forests. Rainfall exclusion translated into quick significant reductions of soil organic matter (SOM), enzymatic activity (β-glucosidase, urease and acid phosphatase activities) and nutrient availability (ammonium, nitrate and phosphorus) one year after the application of the treatments. These effects were consistent over time. Warming acted synergistically with rainfall reduction to further decrease C-related variables (SOM and β-glucosidase). SOM reduction in forest soils might be the result of delayed leaf senescence as a drought tolerant trait in the forest trees. Warming also had direct positive effects on N- and P- related variables that partially counteracted the negative effects of rainfall reduction on these variables. Overall, our results showed that the different components of climate change (drought and warming) have complex direct and interactive effects on biogeochemical cycles of Mediterranean forests that differ among soil nutrients (C, N, P). Consequently, drought and warming might cause an unbalance in natural biogeochemical cycles of Mediterranean forests, with important consequences for ecosystem functioning.