Nitrogen Dynamics in an Annual Grassland: Oak Canopy, Climate, and Microbial Population Effects

Hardwood rangelands in California, which are commonly used for cattle production, are characterized by an oak overstory and annual grass understory. Our goal was to evaluate how canopies influence nitrogen-cycling processes and how tree removal affected these processes. We measured standing ammonium and nitrate pools, gross production and consumption of NH4 and NO3, nitrification potentials, and NOx and N2O emissions in soils under oak canopies and in open grassy areas over a two-year period in areas within the same watershed that were grazed by cattle or fenced to exclude cattle. We also performed a subset of these measurements in open areas and in plots near stumps of oaks in areas that had been clear-cut seven and nine years earlier. Nitrogen turnover was generally higher in canopy area soils than in open area soils, and higher in the grazed area than in the fenced area. Ammonium production was slow following first autumn wet-up, but achieved higher rates during the growing season. By contrast, NO3 production showed no discernible seasonal pattern. Inorganic N consumption generally balanced production, such that inorganic N only accumulated over summer. N gas emissions were generally <1 ng N·cm−2·h−1, except for one measurement period during an autumn rain, when total NOx plus N2O flux was 13 ng N·cm−2·h−1. In the clear-cut areas, properties of canopy-removed soils were converging with open area soil properties, with declining N capitals and nitrification potentials. The results demonstrate active N turnover in soils associated with oak trees, with tree removal appearing to reduce both the soil nitrogen capital and N-cycling rates. Corresponding Editor: R. J. Scholes