Effects of woody vegetation encroachment on soil nitrogen oxide emissions in a temperate savanna

Abstract

Woody vegetation has encroached into areas once dominated by herbaceous land cover in arid and semiarid regions of the southwestern United States and around the world, resulting in documented changes to the biophysical and biogeochemical structure of these ecosystems during the past century. In North Texas rangelands, encroaching mesquite (Prosopis glandulosa var. glandulosa), a known nitrogen (N)-fixing species, has caused changes in aboveground biomass, which, in turn, have influenced carbon (C) and N storage in surface soils. However, the effect on N oxide (nitric-NO and nitrous-N2O oxide) emissions from the soils was unknown. We examined biotic (vegetation type and soil organic and inorganic N dynamics) and abiotic (soil moisture, temperature, and soil texture) controls over soil NO and N2O emissions across a gradient of aboveground Prosopis biomass growing on two soil types. Soil N oxide fluxes were dominated by NO emissions produced during nitrification. Aboveground biomass was the best spatial predictor of NO emissions, with values increasing 20-fold (0.04-0.78 mg NO-N·m-2·d -1) across a 70-fold biomass gradient (5-350 g/m2). Emissions also covaried with soil pH and clay content. Microsite position, under or between the mesquite canopies, did not influence NO emission rates. NO fluxes were four times higher from clay loam than from shallow clay soils; however, soil N properties (total organic N and extractable inorganic N) and cycling rates (mineralization and nitrification) did not differ significantly across the sites. Temporally, NO emissions and nitrification potential were positively correlated with temperature, with precipitation events elevating NO emissions fourfold over a 24-h period and producing small amounts of N 2O. We conclude that mesquite encroachment in these grasslands increases NO emissions in a spatially explicit manner influenced by the aboveground biomass and soil type, which is then temporally mediated primarily by temperature and secondarily by precipitation.