ABSTRACT

This project will determine how soils and soil organisms respond to severe drought, a condition that will be more common in the future, and how this response will impact the global carbon cycle. The increasing frequency of drought due to changing rainfall patterns is threatening the world's ecosystems and food security. Rainfall frequency also can affect whether the immense amount of carbon present in soil stays there, or is emitted by soil microbes into the atmosphere as carbon dioxide (CO2). In order to accurately predict the impact of drought on soils, soil microbes and the carbon cycle, this project will use samples from the Drought-Net Research Coordination Network supported by NSF. Drought-Net consists of a series of simple, inexpensive experiments that control rainfall at over 150 plots of land worldwide using the same standard protocols. Soil samples from 39 Drought-Net sites that have been exposed to drought for four years will be manipulated in the laboratory to determine how they respond to different amounts of moisture. Soil microbial community changes and CO2 emission will be measured, and the results will be incorporated into computer models of global carbon cycling. These models can then predict how drought will cause changes and feedbacks in carbon cycling and, in turn, ecosystem function and stability. The studies will also involve training at of postdoctoral researchers, graduate and undergraduate students, and other educational and outreach activities. Successful completion of the goals and objectives of this project will help society understand how it needs to adapt and respond to global environmental change.

While the recent inclusion of microbial mechanisms in ecosystem models has improved our ability to predict soil carbon (C) cycling, even the most advanced of these models explains only 50% of the variation in current C pools, leaving little confidence in projections of future soil C stocks. Past efforts have mainly focused on microbial temperature responses, but moisture and drought may be an even more important constraint on microbial activity. Thus, an understanding of microbial moisture responses is required to improve ecosystem C models. This research addresses four key questions: (1) what drives differences in moisture response functions across ecosystems? (2) how does severe drought alter moisture response across ecosystems? (3) what microbial mechanisms influence differences in moisture response functions? (4) what are the implications of different moisture responses for C storage? In order to build a robust predictive framework for soil microbial moisture functions, this project will leverage the Drought-Net Research Coordination Network, which has implemented standardized, coordinated rainfall manipulation experiments across the world. Intact soil cores from ambient and drought treatments at 39 sites will be exposed to a range of soil water potentials to quantify how the moisture response of heterotrophic respiration depends on historical climate and soil factors. Underlying microbial mechanisms will be examined by characterizing physiological traits and tolerances at individual and community levels. Results will be scaled to the ecosystem level by first testing how aggregated community responses influence function in a trait-based model, and then by incorporating response functions into conventional and microbially-explicit ecosystem models of soil C cycling.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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