Microbial functional response to altered precipitation timing and duration – Implications for the soil carbon cycle

Background/Question/Methods

A significant amount of carbon (C) is processed and stored in prairie soils: grasslands cover 6.1-7.4% of the earth’s land surface and hold 7.3-11.4% of global soil C. Global change models predict that future precipitation patterns across the North American Great Plains will entail less frequent but larger rainfall events. The response of prairie soil microbial C processing and allocation to this scenario is not known, but will be a key determiner of the future capacity for C sequestration. To address this problem, we evaluated microbial function and structure before and after rainfall events in field soils with a legacy of ambient and experimentally modified precipitation regimes.

The Rainfall Manipulation Plots (RaMPs) at the Konza Prairie Long-Term Ecological Research (LTER) site in northeastern Kansas, USA is a replicated field manipulation of the timing and magnitude of precipitation that was established in 1998. This experiment imposes extended dry periods and larger, less frequent rainfall events without changing total precipitation.  We collected soil before, immediately after and five days after rainfall events in ambient and extended interval treatments and measured microbial growth, respiration and potential organic matter degradation responses, as well as gene and transcript indicators of microbial community structure and function. 

Results/Conclusions

Equivalent rainfall events caused equivalent microbial respiration responses in ambient and extended interval soils, but biomass increased after the rainfall in extended interval plots only.  This implies a greater potential for belowground C retention in extended interval soils.  Also, C:N ratio of biomass was increasingly high as soil water content decreased.  This implies a physiological and/or population-level shift in the microbiota at low soil water content.  Further, extracellular cellulose hydrolysis potential was lower 5 days after rainfall.  This implies a decreased heterotrophic dependence on soil organic matter degradation, with a lag period, after rainfall events.

Thus, predicted molecular indicators of microbial community shift include: (H1) Microbial taxa that respond quickly to increased water availability after drought are more active and/or abundant in soil with an altered precipitation regime history, and transcriptional activity related to growth will be more abundant in extended interval soils. (H2) In soils with low water contents, fungal cells will be more abundant, and transcriptional indicators of compatible solute production will be more abundant.  In addition, the links between plant and bulk soil microbial heterotroph responses to rainfall need to be considered for future work.