Quantitative, Trait-Based Microbial Ecology to Accurately Model the Impacts of Nitrogen Deposition on Soil Carbon Cycling in the Anthropocene

Authors

Edward Brzostek¹* (erbrzostek@mail.wvu.edu), Ember Morrissey¹, Zachary Freedman², Chansotheary Dang¹, Nanette Raczka¹, Juan Piñeiro^1,4, Jeth Walkup¹, Steve Blazewicz³, Peter Weber³

Institutions

¹West Virginia University–Morgantown, WV; ²University of Wisconsin–Madison, WI; ³Lawrence Livermore National Laboratory, Livermore, CA; ⁴University of Cádiz–Cádiz, Spain

Abstract

Nitrogen (N) deposition has enhanced carbon (C) storage in temperate forest soils. However, it remains unclear whether this soil C will persist as N deposition declines across the region. Given that this uncertainty directly impedes the ability of predictive models to project future soil C stocks, there is a critical need to determine how N-induced shifts in key microbial traits drive soil C stabilization. To address this uncertainty, the team’s objectives are to (1) quantify variations in taxon-specific and community-level microbial traits across gradients in microbial community composition, the distribution of ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) trees, and N availability; and (2) integrate this data into a novel predictive framework that enhances the ability to project the regional soil C consequences of N deposition in temperate forests.

Under ambient N, researchers found that decomposition pathways in AM soils have greater flexibility in which microbes are the active decomposers and what they produce than those in ECM soils. Under elevated N, researchers found evidence of a reduction in functional evenness for both mycorrhizal types with a narrowing of the distribution of active taxa taking up C and N. However, when researchers examined how N fertilization impacts the response of microbes to simulated root exudation, there were greater declines in function and growth in AM soils than ECM soils. The maintenance of function in ECM soils appeared to result from elevated N promoting the activity of certain bacterial phyla at the expense of others. These empirical results have been instrumental in improving the soil decomposition model that represents microbial groups based on substrate preference. The model was able to capture increases in soil C in response to N fertilization when the team reduced the competitive advantage of microbes that degrade complex C. Upon cessation of N fertilization, the model showed that the added soil C was highly susceptible to loss under increasing temperature. Coupled together, the experimental and model results highlight that integrating microbial traits into models alters predictions of the response of soil C in temperate forests to global change.