Effects of simulated precipitation on nitrogen cycling and microbial processes in a grassland ecosystem at Big Bend National Park, Texas



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Texas Tech University


Variations in the timing and magnitude of precipitation events have the potential to influence microbial dynamics and subsequent ecosystem level processes on a variety of scales leading to irreversible changes in vegetation structure and composition. According to the Hadley Climate Change Model #2, the Big Bend region of far west Texas is expected to receive 25% more precipitation in both the winter and summer months.

The effects of changes in precipitation amounts and timing on nitrogen dynamics and microbial processes in the Sotol-grasslands at Big Bend National Park were studied. The objectives for this thesis were to: (1) monitor changes in soil nitrogen dynamics in response to variations in precipitation timing and amounts, (2) evaluate impacts of increased precipitation on soil microbial dynamics, and (3) examine, using a greenhouse experiment, the impacts of rainfall pH on soil nitrogen dynamics and microbial biomass production.

The field experimental site was in the Sotol-grasslands along the Pine Canyon Watershed at Big Bend National Park, Dominant species plots, containing either Side-Oats grama, Sotol, or Brownspined Prickly-Pear cacti, and also community plots containing all three plant species, were established to examine the impacts of additional precipitation on soil microbial and nitrogen dynamics. Four water treatments (control, summer water only, winter water only, and summer and winter water) were applied beginning in January 2002. Winter water treatments were applied once during the winter season and summer watering took place over three different watering events. Additional water amounts were determined during the first year of the experiment by adding an additional 25% of the past 100 years precipitation averages. During the second year of the experiment, additional moisture was an additional 25%. of the previous three months' precipitation. This approach accounted for yearly precipitation variation.

Plant type and season of water addition influenced the rate of N-mineralization, N-mineralization rates in the dominant species plots were significantly different among dates during the winter and spring, but not during the summer and warmer periods of the year. Plots with Side-Oats grama experienced the highest average rate of N-mineralization across date while Sotol plots had the lowest average rate of mineralization. Across the two years, ammonification rates were positive during the ApriI-June (2002) sample dates in all treatments and after the summer and winter watering events. For the remainder of the sampling periods, ammonification rates were negative. The negative rates are indicative of NH4+-N loss through either plant uptake and/or the process of nitrification. An increase in nitrification across plant types was found during the spring months and also early autumn months. The increase in nitrification rates does not appear to be directly related to water addition, but is a seasonal interaction for both the single species and community plots.

Microbial biomass carbon production was directly related to soil moisture in that, for both plot types, plots watered in both the summer and winter contained the largest amounts of microbial biomass on average over the duration of the experiment. In the dominant species plots, vegetation type had a direct impact on the amounts of microbial biomass carbon and the levels of extractable NH4+-N and extractable NO3- -N contained within the plots.

In the greenhouse experiment using soil from the Sotol-grasslands treated with water of varying pH's to simulate acid rain, it was found that water addition initially stimulated the release of immobilized or otherwise unavailable nitrogen, thereby allowing that nitrogen to become available for N-mineralization, It was also found that, like the field study at the Sotol-grasslands, nitrification is the primary contributor in N-mineralization, Microbial biomass carbon amounts were found to increase when treated with water pH 4,5, but levels of microbial biomass carbon declined initially when treated with water at pH at 3.5 and 5, The increase may be attributed to an increase in bacterial and fungal activity resulting from the changes in soil conditions, namely nitrogen and water availability, in response to acidic precipitation.

Together, the field and greenhouse results suggest that alteration in precipitation patterns and amounts coupled with changes in rainfall chemistry as a result of pollution will have significant impacts on soil microbial activity and subsequently nitrogen dynamics in a complex manner within the Sotol grasslands at Big Bend National Park,.