| The burning of fossil fuels and subsequent atmospheric deposition of nitrate (NO3-) has increased the global input of nitrogen (N) to many terrestrial ecosystems. This has the potential to alter the cycling of carbon (C) in these ecosystems by reducing microbial-mediated decomposition. After assimilating anthropogenic N, the soil microbial community can release that N as ammonium (NH4+), which can inhibit the activity of lignin-degrading soil fungi. My primary objectives were to determine if increases in NO3- deposition have altered microbial community composition and function in upland temperate forests and if these changes have altered decomposition. I hypothesized that anthropogenic N deposition will fundamentally altered the flow of C in the microbial foodweb, which, in turn, will alter ecosystem-level patterns of C cycling. This idea was tested in four sugar maple-dominated northern hardwood ecosystems in Michigan which have received experimental N additions (30 kg NO3--N ha-1 y -1) since 1994. I determined microbial community composition by phospholipid fatty acid analysis and microbial function by measuring the activities of extracellular enzymes that decompose plant tissue. Measuring the flow of C through the soil microbial foodweb was achieved using 13C labeled compounds (cellobiose and vanillin) that are common products of lignocellulose decomposition. Increases in NO3- deposition significantly (p < 0.05) suppressed the activity of beta-glucosidase, peroxidase, and phenol oxidase by at least 20%, enzymes which are responsible for degrading plant cell walls. Experimental N additions also have significantly (p < 0.01) reduced the biomass of all microbial groups (-25% to -68%), not just the biomass of lignin-degrading microorganisms. The reduction in lignocellulose-degrading enzymes coupled with a significant 76% increase in soil organic carbon, and the decrease in microbial biomass indicates that NO3- deposition can potential reduce rates of decomposition. This may increase the capacity for terrestrial ecosystems to accumulate C through soil organic matter formation. Therefore, anthropogenic N deposition, by slowing the flow of C through the microbial foodweb, can be a potent modifier of ecosystem-level patterns of C cycling. |
