The soil microbial community response when plants are subjected to water stress and defoliation disturbance

All communities and ecosystems are subjected to a range of extrinsic factors that can be broadly classified as stress factors (i.e. those that remain constantly limiting for organism growth and do not change markedly over time) and disturbance factors (i.e. those that involve rapid changes and often involve destruction of organism biomass). Although the interactive effects between stress and disturbance have been well explored for plant communities, little is known about how these interactions influence soil communities. We set up a 14-month glasshouse experiment, which consisted of varying levels of a disturbance (plant defoliation) and a stress (gradient of water limitation to plants), in a full factorial design. Response variables consisted of plant biomass and productivity, soil microbial biomass and activity (measured using multiple substrate utilization profiling), and soil microbial community structure (measured using phospholipid fatty acid analysis). Although the microbial community responded to both the stress and disturbance factors, these responses did not closely track plant responses to the same factors, pointing to a degree of decoupling between plant and microbial communities. Further, although stress and disturbance factors both limited some measures of microbial biomass and activity, the microbial community responded to these two factors in quite different ways. There were no interactive effects of stress and disturbance on any microbial response variables, with the exception of one variable that involved total microbial community substrate utilization. Although the ability of microbes to exploit water resources that are less available to plants may buffer the soil microbial community from stress, the general lack of interactions of stress and disturbance on the microbial community contrasts with what has been shown for plant communities and proposed for fungal communities. Our results suggest that while stress and disturbance are both strong drivers of the microbial community, the effects of these factors largely operate independently from one another.