Quantification of and Controls on Dinitrogen and Nitrous Oxide Fluxes from Terrestrial Ecosystems
The production of dinitrogen (N2) and emission to the atmosphere returns reactive nitrogen (N) species to the unreactive form, completing the N cycle. Soil N2 emissions are the most poorly understood portion of the terrestrial N cycle because soil N2 fluxes are difficult to measure against the high atmospheric background N2 concentration. For my dissertation, I developed two new methods for quantifying soil N2 production in soils and described a new pathway for N2 production in the terrestrial N cycling. I also explored controls on soil N cycling at a regional scale relevant for policymakers. In laboratory experiments, I demonstrated that the N2/Ar technique can be used to accurately measure surface soil N2 fluxes when solubility and water vapor fractionation effects on the N2/Ar ratio are taken into account. Currently the detection limit of the method is 100 mg N m2 d-1, but proposed modifications to the sampling and analytical protocols can lower the detection limit as well as increase sample throughput. I also developed the 15N-nitrous oxide (N2O) pool dilution technique in a peatland pasture and showed that it can be used to measure gross N2O production and consumption rates without changing soil N2O emissions (i.e., net N2O production). Vertical diffusion modeling suggested that the 15N2O tracer was able to diffuse through the vadose zone to the water table at 60 cm depth during the sampling period, but additional modeling is necessary to evaluate the impact of lateral tracer movement on vertical tracer penetration into the soil profile. Nitrous oxide emissions ranged from 0.05 to 77.4 mg N m-2 d-1 across four micro-topographic zones and the proportion of gross N2O production released to the atmosphere was 0.84 ± 0.03. Mineral N concentrations and denitrifying enzyme activity together best predicted gross N2O production rates (R2 = 0.73). When the four landforms were grouped by soil moisture content, soil oxygen concentrations explained over 64% of the variability in N2O consumption rates. Denitrification is currently thought to be the only N2 production pathway in terrestrial soils, but I showed that, in a humid tropical forest soil, iron reduction coupled to anaerobic ammonium (NH4+) oxidation (Feammox) can lead to N2 production both directly and indirectly (via denitrification of Feammox-generated nitrite). Anaerobic ammonium oxidation rates were comparable to denitrification rates for this forest, suggesting the potential importance of this pathway. Lastly, I found that a small suite of soil properties could reliably predict internal N cycling processes in soils from deserts, forests, grasslands, shrublands, and wetlands in California. Microbial biomass N, soil organic carbon (C) concentration, and soil NH4+ concentrations together explained 80 % of the variability in gross mineralization rates whereas soil nitrate (NO3-) concentration and soil C:N ratios together explained 59 % of the variability in gross nitrification rates. Dissimilatory NO3- reduction to NH4+ occurred in all soils and rates were best predicted by the combination of gross nitrification rates and soil NH4+ concentrations (R2 = 0.83). These results will support modeling efforts at the regional scale that are important for setting land use and N emissions policies. Overall, my dissertation has moved the field closer to understanding soil N2 emissions and suggests that the controls on soil N cycling can be modeled at a regional scale incorporating many different ecosystem types.