Divergent effects of drought and nitrogen deposition on microbial and arthropod soil communities in a Mediterranean forest

Abstract

Drivers of global change such as warming and increased nitrogen (N) availability are altering soil communities worldwide, with unknown consequences for the functioning of the terrestrial ecosystems. Higher N availability may stimulate soil communities and boost nutrient cycling, but the expected increase in aridity may nullify this effect. Additionally, shifts in microbial communities may lead to trophic cascades throughout soil food webs, affecting key ecosystem properties associated with the turnover of carbon and nutrients. Whether the responses to these different drivers of global change are similar between the main guilds of soil organisms, however, is unclear. We analyzed soil phospholipid fatty acids and extracted soil fauna to determine the simultaneous responses of microbial and arthropod soil communities to experimental drought and increased N availability in a Mediterranean forest. The experimental drought decreased community sizes across the entire soil food web, and N addition increased the biomass of all microbial groups except fungi, thereby increasing the relative dominance of bacteria. Likewise, the abundance of oribatid mites strongly increased after the addition of N, whereas the population of springtails quickly decreased, leading to a shift in the structure of the arthropod community. An increase in soil N availability promoted the energy channel dominated by bacteria, triggering a bottom-up trophic cascade affecting the arthropod mesofauna, thus propagating the effects of N inputs throughout the soil food web. The strong constraint of drought on soil microbes, however, may neutralize the effect of an increase in N availability. Complex interactions among factors of global change may therefore hinder our ability to predict their effects on soil biodiversity and associated consequences for ecosystem functioning. Forecasts of responses of soil biodiversity to global change should not only consider different drivers and their potential interactions, but also the likely contrasting effects among different groups of soil organisms.