Woody encroachment, an increase in abundance and density of woody plants, currently affects millions of hectares of grassland, savanna, desert and other dryland systems globally. By altering the structure and function of vegetative communities, the process significantly affects human land use, biodiversity, carbon storage and biogeochemical cycling. In the tropics and subtropics, encroachment is often driven by leguminous trees. These trees support increased rates of nitrogen (N) fixation, significantly altering ecosystem N cycling, accumulation and loss. The genus Prosopis is one of the most common encroaching species throughout the tropics, and the N-fixing tree Prosopis glandulosa (honey mesquite) affects more than 40 million hectares across the southern US alone. This thesis investigates the effects of Prosopis encroachment on ecosystem N cycling in a subtropical, semi-arid savanna in south Texas. Previous studies have identified increases in soil total N storage, inorganic N concentrations, and soil N cycling rates in Prosopis-encroached soils compared with adjacent remnant grasslands. This research characterizes the magnitude and dynamics of three key processes- symbiotic N fixation, N accrual in soil and biomass, and emissions of N trace gases and N2. Chapter 1 applies a new methodological approach comparing N isotopic composition ([delta]15N) of foliage, xylem sap and plant-available soil N to investigate patterns of N fixation along a space-for-time chronosequence of encroachment. Data supports increased rates of N fixation with tree age, and seasonal variability. This approach also demonstrates that the foliar [delta]15N method typically applied to estimate rates of N fixation in ecosystems such as this is not reliable. Chapter 2 summarizes two years of field measurements of a complete suite of N trace gases (ammonia, nitrous oxide, nitric oxide and other oxidized N compounds) from encroached and unencroached soils. Measurements revealed no effect of encroachment on upland N trace gas emissions. Rather, abiotic temperature and soil wetting dynamics (including re-wetting interval) were much stronger controllers of total N flux. Chapter 3 used excised-core incubation experiments to determine whether denitrification to N2 gas might be a significant but unappreciated gas loss pathway. The outcome of these incubations showed that under fieldrealistic abiotic conditions, N2 flux is likely to be low or absent. Chapter 4 uses the chronosequence to infer rates of N fixation over time by constructing a N mass balance for individual Prosopis clusters. These calculations combine measured rates of soil N accrual with modeled or estimated rates of biomass N accrual, N deposition and N gas loss to estimate a rate for symbiotic N fixation of 0.15 kg of N per individual tree per year of growth. Scaled to landscape level, this corresponds to fixation inputs of 9.2 kg N per hectare per year. Taken together, these results suggest that this ecosystem is still in a stage of N accumulation during ongoing Prosopis encroachment, experiencing net N storage in soil and biomass that outweighs gaseous losses. Given the large scale of Prosopis encroachment in subtropical North America, this finding implies significant and continuing increases in soil N stocks may occur throughout this region.