Background/Question/Methods
Nitrogen dynamics in desert ecosystems are closely linked to seasonal patterns in temperature and soil moisture. Soil nitrogen pools respond quickly to plant or microbial immobilization or mineralization following rainfall events. These interactions are regulated by soil hydrology and infiltration rates for a particular location. Moreover, desert systems are sensitive to anthropogenic inputs of nitrogen and various disturbances which can alter vegetative cover. This research is focused on understanding mechanisms which regulate microbial and nitrogen dynamics associated with a low-elevation banded grassland in the Chihuahuan Desert at Big Bend National Park. The landscape of this desert grassland is structured by vegetation bands dominated by Tobosa grass, Hilaria mutica, with intermittent shrubs primarily Creosote bush, Larrea divaricata, alternating with bare areas. In this study, microbial community structure (FAME) and nitrogen pools have been evaluated to a depth of 135cm from vegetated and non-vegetated bands over the course of 6 months beginning in May 2008. The cores were divided up into 15cm sections to obtain FAME and nitrogen profiles with depth. Cores were taken in the middle of the bands to minimize edge effects and within 15m of the core taken previously to help minimize spatial heterogeneity across the landscape.
Results/Conclusions
There was a statistical difference in microbial community structure with a majority of the observed FAME signatures being fungal. There was also a statistical difference in FAME levels associated with depth, as well as cover type. Fungal FAME signatures increased with depth while bacterial signatures were highest in the top 45cm depending upon sample date. There were no differences in fungal FAME levels between the vegetated and bare areas. No depth effect was observed for soil nitrate levels within the grass bands irrespective of sample date. The greatest difference in nitrate levels between the bands was observed between 15 to 90cm depths. Vegetated soils had a maximum nitrate value of 20ppm, while bare soils reached values of 260ppm at 60cm. Extractable ammonium levels were routinely between 1-5ppm regardless of depth or band type. The banded vegetation of this low elevation grassland in the Chihuahuan Desert appears to resemble other banded vegetations patterns found in arid regions around the world. These vegetation patterns are considered to result from water limitations where the runoff of the bare areas is captured in the vegetated areas. However, in our system subsequent nitrate build-up leading to negative reinforcement of this pattern, may prevent revegetation.