Evaluation of microbial activity and nitrogen dynamics along selected national park watershed sites

In recent years, many of the United States National Parks have experienced severe impacts and disturbances on natural resources from atmospheric chemical inputs and climate change. The Pine Canyon Watershed at Big Bend National Park is part of a network of watershed sites at five National Parks that is evaluating the effects of nitrogen inputs and climate change on soil processes and watershed dynamics. In conjunction with the Pine Canyon Watershed Program the objectives of the current study were to determine: (1) Patterns of microbial biomass (2) Seasonal and yearly patterns in extractable soil NH4-N and NO3-N concentrations (3) Extent of nitrogen mineralization, and (4) The relationships of microbial activity at Big Bend to other National Park Watershed sites. Samples for microbial and nutrient concentrations were collected biannually in January and August from 1996 to 2001.

There was a significant interaction between site and sampling time for microbial biomass carbon, extractable NH4-N, NO3-N and soil organic matter along the Pine Canyon watershed. Overall, the highest microbial biomass and NH4-N values were associated with the Oak Forest site. Importantly, all locations showed a steady increase in extractable NH4-N beginning in August 1998. The Lowland Scrub site illustrated the most variation in extractable NO3-N and also the highest levels observed throughout the study. NO3-N levels exhibited pulses in August of 1998, 1999 and 2000 when compared to January levels in most locations along the watershed. The Oak-Pine Forest site at the highest elevation showed only slight seasonal patterns in NO3-N. In general, NH4-N levels increased up the watershed with NO3-N levels increasing down the watershed. There was a significant interaction between site and sampling time on nitrogen mineralization and nitrification rates. The highest rates of nitrogen mineralization occurred in the Sotol-Grassland site, and the lowest ratios were observed in the Oak Forest site. The high elevation Oak-Pine Forest site had the greatest amount of nitrification during the fall with a dramatic drop in nitrification in all three vegetation types in mid-winter.

The form of the relationships between NH4-N, NO3-N levels and microbial biomass carbon differed among sites. At the Lowland Scrub, Creosotebush, and Sotol-Grassland sites microbial biomass carbon decreased as NH4-N levels increased. For both forested sites, the relationship between microbial biomass carbon and NH4-N levels was significant and positive. Across all vegetation zones, the r^ values never accounted for more than 15% of the variation in the relationship between NH4-N and microbial biomass. For NO3-N levels, microbial biomass carbon increased with increasing levels of NO3-N within all vegetation zones except the high elevation Oak-Pine Forest. The variation in microbial biomass that could be explained by the variation in NO3-N levels was never greater than 9%.

Seasonal patterns in soil moisture levels were found to be more critical in regulating microbial biomass carbon dynamics and soil nitrogen pool sizes in the low desert sites. The importance of seasonal patterns in soil moisture as the main regulator of microbial biomass and soil nitrogen pool sizes decreased with elevation. The higher elevation forested sites maintain a more favorable soil moisture level for longer periods of time, resulting in a greater importance of plant uptake and immobilization in regulating microbial biomass. Changes in microbial biomass carbon amounts in the forested sites along the gradient reflect seasonal differences in microbial turnover rates, while moisture availability controls microbial biomass production in the low desert sites.

Yearly total atmospheric inputs of NH4-N, NO3-N and SO4 were relatively constant for the period 1995 to 2000 at Big Bend National Park. The highest inputs of nitrogen and sulfate normally occurred between January and August. Inputs of NH4-N and NO3-N as dry deposition, from 1995 to 1999 were consistent within and between years.