PARENT SESSION
Oral Session 96: Forest Ecology V: Soils and Nutrient Cycling.
Presiding: G Newman and S Bigelow
Thursday, August 5, 8:00 AM to 11:30 AM, Meeting Room B 114.
Controls on nitrogen retention and loss in tropical forest soils: Part 1. Patterns of nitrogen cycling.
Templer, Pamela*,1, Pett-Ridge, Jennifer1, Silver, Whendee1, Firestone, Mary1, 1 University of California, Berkeley, CA, USA
ABSTRACT- Tropical rainforests have increasingly been the subject of interest and environmental concern because of their large role in global nitrogen (N) biogeochemistry. We conducted a seven-day field experiment adding trace levels of 15NH4 and 15NO3 to tropical forest soils of the Luquillo Mountains, Puerto Rico to determine plant, microbial and soil N sinks. Our work demonstrates that a combination of plant and microbial processes interact to conserve N within these soils. Plants take up the product of microbial dissimilatory nitrate reduction to ammonium (DNRA) in significant amounts, leading to relatively smaller amounts of NH4 remaining in the soil to be nitrified, leached or lost as gas. Plant N uptake was inversely correlated with soil NH4, NO3, %N, % carbon (C) and C:N ratios. In contrast, there was no relationship between microbial N uptake and any of these soil properties. Gross rates of mineralization and nitrification were both strongly correlated with soil moisture, although rates of DNRA showed no soil moisture response. Nitrous oxide gas flux from the soil was only related to soil NO3 pools. These results demonstrate that key differences among forest sites in standing N and C pools explain differences in plant assimilation and thereby tropical forest N retention.
Key words: nitrogen transformations, stable isotopes, forest biogeochemistry, plant-microbe competition