SYMP 23-5 - Nitrogen as a mediator of soil organic matter decomposition in a changing climate: Linking stable isotopes, organic geochemistry, and microbial ecology

Friday, August 12, 2011: 9:30 AM
Ballroom F, Austin Convention Center
Sharon A. Billings, Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, Susan Ziegler, Earth Science, Memorial University, St. John's, NF, Canada, Jianwei Li, Department of Agriculture and Environmental Sciences, Tennessee State University, Nashville, TN and Chad Lane, Department of Geography and Geology, University of North Carolina at Wilmington, Wilmington, NC
Background/Question/Methods

Climate change will promote shifts in ecosystem variables that may induce climate feedbacks.  One such variable is soil nitrogen (N) dynamics – the extent to which N is mineralized, the C:N ratio of the organic precursor to mineralized N, and rates of C flow through microbial communities and loss from soil profiles.  These shifts can result in changes in heterotrophic CO2 release.  We use a stable isotope approach with tools from organic geochemistry and microbial ecology to examine C flow through microbial communities as it relates to natural and human-induced variation in N status in a pine forest in North Carolina, and the C:N ratio of Oi material in two fir forests along a boreal climate transect.  We use these data to assess how CO2 release vs. soil C retention may be influenced by N dynamics.  In the fir forests, we also assess how such responses may influence the magnitude of soil responses to warming.

Results/Conclusions

Enhanced N cycling rates can shift the identity of dominant CO2 producers and the CO2’s organic precursors.  Reduced C:N of soil inputs in a pine forest can increase heterotrophs’ C-use efficiency, decrease C flow through actinomycetes, and enhance flow through Gram-negative bacteria.  Increases in N-acquisition enzymes are linked to microbial use of 13C-deplete substrates, a sign that substrate choice changes with N cycling rates.  These soils also exhibited a 2.75-fold increase in C acquisition and an eight-fold increase in N acquisition exo-enzyme activities across the same gradient in N availability, consistent with the idea that enhanced N status promotes more efficient use of C substrates.  We also observed effects of N dynamics on C fluxes in two fir forests where warming induces preferential access of slower-turnover substrates via increases in oxidative enzyme activities two times those of hydrolytic C-acquisition enzymes.  Reduced C:N of Oi material further augmented the warming effect on phenol oxidase by ~80-1500%, and at one site increased Oi flow into fungi at relatively cool temperatures.  Combined, these three forests provide evidence that enhanced N status can promote relatively greater C use efficiencies, and thus greater potential for soils to retain organic C.  However, the fir forest soils suggest that enhanced N status augments the positive effects of warming on slow-turnover compounds’ rates of decay, and that respiratory losses with warming outweigh enhanced C retention with substrate use efficiency gains.  Such effects could result in enhanced destabilization of currently slow-turnover soil organic C.

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