Sources And Controls Of Reactive Nitrogen Gas Emissions From A Mojave Desert Ecosystem
This body of work considerably expands our understanding of gaseous nitrogen (N) loss dynamics in arid ecosystems, a process that directly affects longterm N bioavailability and the release of chemically important gas species into the atmosphere. Chapter one focuses on identifying controls over biological sources of reactive N gas emissions, especially the pulse of NO and NH3 that occurs following precipitation. This research shows a two-step response of reactive N gas emissions to pulsed water additions. First, there is a large transient NH3 pulse. Second, biological activity is stimulated leading to NO production by nitrifying bacteria. Results indicate that biological sources of NO and NH3 efflux respond primarily to the addition of water; however, fluxes are modulated by temperature and nutrient constraints on microbial activity. Chapter two explores the role of non-biological processes in reactive N gas emissions from desert soils. Combination of laboratory and field measurements show that abiotic reactions are a key component of N loss from desert soils, both under dry conditions and during post-wetting periods. It is hypothesized that during the summer, extreme surface soil temperatures caused by direct exposure to incoming solar radiation result in thermal decomposition of N-containing compounds, yielding elevated rates of reactive N gas efflux. Chapter three focuses on the effects of elevated CO2 on soil fluxes of reactive N gases. Under soil conditions optimal for biological activity, long-term fumigation with elevated CO2 reduces reactive N gas losses in the islands of fertility created by the dominant shrub Larea tridentata. These results provide supporting evidence that elevated CO2 alters soil N dynamics in arid ecosystems, including increased N immobilization and decreased N mineralization and nitrification. Chapter four provides a robust estimate of annual reactive N gas emissions for a Mojave Desert ecosystem, accounting for daily, seasonal and yearly variation in environmental conditions. This annual estimate of 0.1 to 0.6 kg N ha-1 y-1 includes considerable variation, reflecting inter-annual variability in summer precipitation and growing season moisture conditions. These results demonstrate the strong impact that alterations in precipitation patterns under future climate scenarios will have on N dynamics in water limited ecosystems.
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