Microbial community, fungal hyphae, and plant productivity responses to altered rainfall in Western US grasslands

Background/Question/Methods

Climate change is rapidly altering weather patterns around the world, and most ecosystems are predicted to continue experiencing changing rainfall regimes in the future. Precipitation is a key mediator of mechanistic links between the below- and above-ground processes underlying ecosystem functions. Yet the joint responses of soil microbes and plant processes to changes in rainfall patterns are not well understood. We utilized rainfall manipulation experiments in three perennial grasslands that differ in long-term average annual rainfall: Jornada Experimental Range (230 mm), Colorado Plains Experimental Range (340 mm), and Ft. Hays College Pasture (580 mm). Local and broad-scale microbial community, fungal hyphae, and plant responses to three rainfall regimes (drought, control, and water addition) were investigated at each site. We predicted that soil microbes and plant productivity would have larger responses to reduced rainfall at sites with lower average annual rainfall.

Results/Conclusions

We found correlated responses of fungal hyphal length and ANPP to drought at the driest, warmest site, to water addition at the wettest site, yet completely decoupled responses to rainfall at the coldest site. We also found that the two drier sites had a more similar microbial composition compared to the wetter site. Variance partitioning within sites showed that the rainfall manipulation treatment only had a significant impact on composition at the Colorado Plains Experimental Range. Unlike fungal hyphal lengths, Actinomycetales relative abundance was negatively correlated with annual rainfall and ANPP. At lower water potentials where fungal density decreased, this may have significant implications for litter decomposition as Actinomycetes are known to be significant producers of extracellular plant polymer depolymerizing enzymes including peroxidases. Both hyphal density and microbial community structure had the greatest responses to rainfall manipulation at Colorado Plains Experimental Range, where plant productivity did not respond. Changes in rainfall patterns that alter the degree of seasonal water stress may have the greatest impacts on microbial processes.