BRC-BIO: Unraveling biogeochemical controls on the soil carbon response to nitrogen fertilization across a northern deciduous forest

Soils contain the largest terrestrial carbon pool and hold more carbon than vegetation and the atmosphere combined. As a result, it is critical to understand the processes that govern soil carbon formation to accurately predict future atmospheric carbon dioxide concentrations. Human activities have increased the amount of biologically-active nitrogen in the environment, a nutrient that often constrains plant and microbial activity in terrestrial ecosystems, and can influence their ability to store soil carbon. This research will leverage a field experiment to determine how nitrogen enrichment influences the formation and stabilization of soil carbon, and how those processes are influenced by soil chemical properties such as pH. The researchers will track the movement of decomposing plant material into different soil carbon fractions. The results will expand scientific understanding of the biological and biogeochemical mechanisms underlying the soil carbon responses to nitrogen fertilization across different types of ecosystems. The project will provide hands-on research experience to undergraduate student teams. The researchers will also develop elementary education and public outreach curricula related to forests & global change to engage the local community in project objectives and results. The microbially-mediated process of plant litter decomposition sits at the interface between plant primary production and soil carbon formation, and therefore functions as a critical hinge for soil carbon cycling and accumulation. Nitrogen fertilization increases the initial rate of plant litter decomposition while reducing the rate and extent of longer-term decomposition, but the effect of anthropogenic nitrogen enrichment on soil carbon pool size and cycling is highly variable, often decoupled from plant production responses, and can diverge across functional soil carbon fractions. The proposed research will leverage a full-factorial nitrogen fertilization and plant litter substrate addition experiment replicated at two sites with differing soil chemistry and mineral parent material at Hopkins Memorial Forest in northwestern Massachusetts. Using a 13C-labeled litter tracing approach, the researchers will evaluate the effect of nitrogen and litter substrate inputs on soil chemistry, physical structure, and fraction distribution. The approach will enable the researchers to test the central hypothesis that nitrogen fertilization increases mineral-associated organic carbon in low pH soils, while nitrogen decreases mineral-associated organic carbon in higher pH soils in which mineral protection is dominated by base cation bridging. In contrast, changes in soil particulate organic carbon pools are likely to be defined by the soil aggregation response to nitrogen, mediated by changes in root and mycorrhizal production. 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.