Nitrogen cycling, greenhouse gas emissions and land-use change in the southern Appalachian Mountains
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The nitrogen cycle is one of the most complex and spatially heterogeneous elemental cycles crucial for life on earth. The southern Appalachian Mountains are currently experiencing unprecedented increases in anthropogenic residential land use, which is projected to intensify over the next few decades. The future is also expected to bring greater nitrogen deposition and net primary productivity. Therefore, it is imperative that we develop a strong understanding of how ecosystems in this region will respond to projected environmental stressors. In order to address the high spatial variability of soil nitrogen cycling processes, I have developed a novel extrapolation approach based on geophysical tools to decrease the uncertainty around process estimates. Forest growth is mainly nitrogen limited in the temperate zone; however, the limitations of soil microbial activity and growth are far less clear. I found soil microbial respiration to be driven solely by carbon while nitrogen removal (i.e. denitrification) and retention was solely controlled by nitrate. These results suggest that a world of increased nitrogen and carbon availability will result in lower soil carbon sequestration and higher nitrogen removal (potentially in the form of the potent greenhouse gas N2O). In order to address the dominant pathways of nitrogen removal and retention, I assessed gross nitrogen cycling rates using 15N isotopic techniques. I showed nitrogen removal by nitrifier denitrification and denitrification to be prevalent in soils of all forest types. Nitrogen retention by dissimilatory nitrate reduction to ammonium (DRNA) is important in some forest soils. Riparian soils are crucial in mitigating terrestrial pollution from reaching the stream. Therefore, I assessed gross riparian nitrogen cycling processes and greenhouse gas emissions under the three dominant land use types in the region (i.e. forested, agricultural and residential). Soils in residential development had low rates of nitrogen cycling and high rates of CH4 emissions. This implies that future riparian zones will have lower nitrogen retention and removal capacity while reducing the riparian CH4 sink. In conclusion, southern Appalachian nitrogen cycling will likely be characterized by higher leaching into streams, greater gaseous efflux, and lower soil carbon sequestration.