THE PINE CANYON WATERSHED PROGRAM: SOIL NITROGEN DYNAMICS AND PATTERNS OF ANTHROPOGENIC INPUTS

Extractable NH₄-N and extractable NO₃-N levels have been monitored twice yearly (January and August) since 1996 in the five vegetation zones that occur along the Pine Canyon Watershed in Big Bend National Park. The National Park provides data on wet deposition of nitrogen through NADP and on dry deposition through the CASTNET program. Extracble NH₄-N levels are highest in the forested sites along the elevational gradient and least at the low desert sites. In contrast, levels of extractable NO₃-N are highest in the low desert locations and decline as you increase in elevation. NO₃-N levels are highest in the summer and decline in winter. For NH₄-N, peaks in amounts can occur in either season for any location, and may reflect the yearly variation in soil moisture that is necessary for mineralization to occur. Although atmospheric inputs of NO₃-N and NH₄-N are low compared to other locations across the United States, chronic inputs of nitrogen to desert systems can have significant negative impacts on ecosystem processes. The peaks in NO₃-N that are associated in the summer at the low desert locations correspond to seasonal increases in atmospheric wet deposition also at this time. When wet depositional data is analyzed across time, while there has been no significant increase in NH₄-N over the last six years, the levels of NO₃-N that are being deposited along the watershed have continued to increase over the last six years. Levels of soil NH₄-N and NO₃-N have been found to either have no affect on the microbial biomass associated with soils along the watershed or have a negative impact depending upon vegetation type, and yearly level of soil NO₃-N. Changes in precipitation patterns could have a major impact on the desert ecosystems at Big Bend by increasing seasonal inputs of soil nitrogen.