NSFDEB-NERC: Mycorrhizal drivers of soil organic matter formation and decomposition

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Changes in land cover, such as the transition from grassland to forest, or vice-versa, have become a common occurrence. Changes in soil microorganisms, such as fungi associated with plant root structures, can accompany these vegetation changes. Because the formation and decomposition of soil organic matter is determined by plant-microbe-soil interactions, the land-cover change may result in a change in the accrual, storage and physical protection of soil organic matter. This US-UK collaborative project, using state-of-the-art methods, will investigate how grasslands and forests, with their different types of symbiotic root fungi, transform plant residue inputs and soil organic matter. Experiments will determine whether the transition from grassland to forests enhances or degrades soils. Additionally, the research will create new computational tools to model soils to better predict carbon and nitrogen dynamics after land cover change. This research will help managers identify sustainable land management decisions through more accurate forecasting of future terrestrial carbon cycling. In addition to international collaboration, this project will provide mentoring for post-doctoral scientists and undergraduate students. This research will (1) determine the mechanisms by which temperate grasses and coniferous trees in interaction with their associated mycorrhizae affect soil organic matter formation and decomposition; (2) integrate this mechanistic understanding in a new process based model; and (3) use existing chronosequences of land use change from grassland to forest to independently verify model projections of long-term changes in ecosystem C stores. The project consists in an integrated set of greenhouse, field and modelling studies, with different components carried out in the US and UK. Continuous 13C isotope-labeling of plant and mycorrhizal biomass will be used to accurately trace the fate of root exudates and mycorrhizal products as they form particulate or dissolved organic matter, and then stabilize via physical occlusion or mineral bonding. In the field, the effects of different mycorrhizal types on the decomposition of 13C-labeled grass and tree residues, CO2 production, and soil organic matter formation will be determined. Bayesian inference will be used to parameterize the new model using the experimental data, and model projections will be evaluated using data on carbon stock changes collected from a grassland-forest transition chronosequences. The degree of soil organic matter stabilization by physical occlusion or mineral-association will be used to evaluate the process-model based findings and inform on the reliability of the new model as a policy tool for C cycling projections and ecosystem management.