The research described here will characterize how soil organic matter (SOM) dynamics are being altered in an aggrading pine forest exposed to elevated CO₂ and nitrogen fertilizer, and the combination of these two treatments. These perturbations represent significant on-going environmental changes that impact forest productivity. Specifically, we will quantify the formation rates of SOM pools of varying recalcitrance, and determine what communities of soil microbial populations are responsible for major SOM transformations, using isotopic tracers to quantify communities’ degrees of substrate usage.

The study will utilize soils collected from the Duke Forest Free-Air Carbon Enrichment site in North Carolina.

Hypotheses to be tested include: 1) Microbial groups that metabolize relatively less labile substrate will increase their substrate usage with elevated CO₂; and 2) The rate of stable SOM formation is greater under conditions of elevated CO₂.

We will test these hypotheses via SOM fractionation (aggregate size and acid hydrolysis fractions), long- and short-term incubations with and without added 13C substrates, and phospholipid fatty acid (PLFA) extractions. All organic matter fractions as well as bulk soil will be assessed for C distribution using total C and 13C data, and total respiration will be calculated during the long-term incubations; these data will be used to calculate the rate of stable organic matter formation since elevated CO₂ treatments began in 1996, and how relatively recent N fertilization is impacting these processes. Soils will be sub-sampled during short-term incubations and analyzed for δ¹³CPLFA; these data will be used to calculate ¹³C substrate usage by different microbial groups, and how it may be altered with elevated CO₂ and N fertilization.

Expected accomplishments include a greater scientific understanding of how current environmental perturbations (CO₂ and N additions) can affect aggrading pine forests, which represent significant areas of land worldwide. Project deliverables will include progress reports, published manuscripts, presentations at annual NICCR meetings, and at national (or international) conferences. Results will be part of a larger body of literature that can be used by policy makers when assessing how anthropogenic activities influence critical ecosystem properties.

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