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

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.