Changing precipitation regimes, along with other global change factors, can alter the above- and below-ground processes that control carbon, water, and nutrient cycles in terrestrial ecosystems. The effects of climate change on the relationship between plants and microbes may be as important as climate impacts on either individually in determining the responses of ecosystem processes. However, uncertainty about how climate change will impact these interactions is a major barrier to predicting ecosystem responses and climate feedbacks. Understanding the conditions under which soil and plant processes are independent or linked is critical to mitigate the impacts of climate change on ecosystem function. We utilized experimental manipulations in a controlled greenhouse environment, and in three perennial grasslands, to investigate local and broad-scale soil and plant responses to precipitation. We predicted that soil microbes and plants would be more strongly impacted by reduced rainfall at sites with lower water availability. We estimated changes in the degree of interaction between soil and plant processes using a mutual information statistic computed from the canonical correlation coefficients for sets of soil and plant responses.
In the greenhouse experiment the precipitation treatments affected few soil and plant responses. However, bootstrap estimates of mutual information indicated more interdependence between soil and plant processes under low rainfall. Conversely, while many of the measured soil and plant processes were strongly affected by soil type, the mutual information statistic indicated that the degree of interaction did not vary between soil types. Intermittent dry periods had larger impacts on soil and plant processes than seasonal rainfall totals, but did not affect ecosystem carbon and water fluxes. Across the three field experiments, the relationships between plant ANPP and fungal abundance responses to precipitation varied significantly. At the driest site, soil hyphal abundance and plant ANPP declined markedly under drought. At the site with intermediate average rainfall, precipitation did not affect plant ANPP, but fungal hyphae increased dramatically as rainfall increased. At the wettest site, precipitation had minimal impacts on hyphal density while plant ANPP increased significantly with increasing rainfall. Our results suggest that soil and plant responses to climate change may be more tightly linked under dry conditions, and more independent as water availability increases. Thus, historical climate may inform the development of effective strategies for mitigating the impacts of climate change on soil and plant processes to minimize the loss of ecosystem function.