The influence of fire on root and microbial carbon cycling in deep soils in a pine-oak forest

Forest fires scorch trees, shrubs, and the soil surface, but what are their impacts on deeper soils and the microbes that live there? Tree roots can grow as deep as 5 meters (16 ft) or more, interacting with diverse bacteria and fungi in soils, and potentially controlling how much, and how quickly, carbon moves from the atmosphere to the soil and back. When trees die suddenly in a high-intensity fire, their roots are left behind to rot. This changes which microbes are present, how they interact, and how carbon moves through the soil. In this study, researchers will sample the entire rooting depth of both living and fire-killed ponderosa pine trees to understand how microbial communities change when a tree dies—and how those changes affect how much carbon the soil holds, and for how long. Using these data, the team will build models to predict how fire alters soil processes at the scale of forested landscapes and ecosystems, and how changes to the frequency and size of forest fires could affect these soils in the future. This project will support the next generation of scientists, providing opportunities for a graduate student and a postdoctoral researcher to lead field, laboratory, and modeling work. Undergraduate students will receive hands-on training in processing soil samples and analyzing data through research internships. The PI will also design a first-year seminar focused on microbial ecology, creating a Course-based Undergraduate Research Experience to help students build skills and confidence in science. This project will develop an eco-evolutionary understanding of how microbial traits affect carbon cycling, particularly in deep soil horizons, and how their interactions with tree roots may shift the functional traits of entire microbial communities as stand-replacing fire becomes more frequent. Working at an instrumented study site in the southern Sierra Nevada, the Soaproot Saddle National Ecological Observatory Network site, researchers will use a Geoprobe direct push sampler to extract intact soil samples from soil surface to hard bedrock. These samples will be subjected to soil physical and chemical analyses, metagenomics, metatranscriptomics, organic carbon fractionation, and radiocarbon analysis of those carbon fractions to test three central hypotheses: 1) That deep roots of living trees significantly enrich surrounding soil microbial communities, increasing microbial activity at depth, and that these plant-microbe interactions result in younger pools of soil organic carbon directly below living trees; 2) that sudden widespread fire mortality causes turnover and loss of function in the deep soil microbial community, with functional consequences for carbon cycling, and 3) these processes vary seasonally, requiring multiple timepoints to measure. Finally, this project will apply these links between roots, microbial traits, and carbon pools to build predictive models that mechanistically incorporate microbial traits and their seasonal variation to understand landscape- and ecosystem-scale carbon cycling in forests subject to fire. By focusing on understudied deep soil systems, this project will construct trait-based models of microbial contributions to carbon cycling that will be helpful to forest managers and policymakers. 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.