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

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 during moist conditions (June 2011) and drought conditions (September 2011) in ambient and extended interval treatments, and measured microbial growth, respiration and potential organic matter degradation responses.

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 microbial C use efficiency in extended interval soils.  Also, C:N ratio of biomass was increasingly high as soil water content decreased across all soils.  This implies a generalized physiological and/or population-level shift in the microbiota at low soil water content: fungal-to-bacterial ratios were also lower in dryer soils.  Further, extracellular cellulolytic potential decreased after rainfall during dry conditions, but increased during moist conditions.  This implies a lower heterotrophic investment toward soil organic matter degradation during drought periods.

Thus, event-driven variation in soil moisture, seasonal drought periods and a long-term history of extended intervals between rainfall events all affect microbial carbon cycling function.  Overall, dryer conditions at all temporal scales may decrease microbially mediated soil organic matter loss.  Ongoing work includes molecular microbial analysis to address hypotheses regarding the active microbial sub-populations and functional mechanisms expressed under variable soil moisture conditions.