PARENT SESSION
Oral Session # 38: Forest Ecology II: Soils, Carbon, and Nutrient Cycling.
Presiding: B Clinton
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 100.
Factors controlling nitrification in nitrogen-fertilized tropical forest soils.
Hall, Sharon *,1, 1 The Colorado College, Colorado Springs, CO
ABSTRACT- Atmospheric N deposition has been shown to alter biogeochemical processes in terrestrial ecosystems. Rates of nitrification in soil are known to increase following long-term N additions in many temperate systems, but little is known about the response of soil microbial communities and nitrification in tropical forests. I measured laboratory rates of nitrification (potential and net/gross nitrification) in soils collected from N-fertilized, N-limited and P-limited tropical forests in the Hawaiian Islands. Nitrification was significantly higher from soils in the P-limited forest compared to the N-limited forest, and long-term N fertilization increased nitrification in soils from both sites. Although soil pH at both sites is naturally low in the field (pH 3-4), potential nitrification (shaken-slurry) was significantly inhibited at pH 4 compared to pH 7. Heterotrophic nitrification (measured w/acetylene) comprised approximately 25% of total nitrification at both sites (and in N-fertilized plots) when measured at pH 4 (close to field pH), but contributed less than 10% of nitrate produced at pH 7. Two-month laboratory incubations of soil with a range of N-additions (dose response w/ equivalent of 0-125 kg N/ha) did not significantly increase rates of nitrification in control or long-term N fertilized plots in the N-limited site. Based on prior dose-response experiments in the field, it is expected that nitrification in soils from the P-limited site (both control and N-fertilized) will show high sensitivity to increasing N additions. Preliminary molecular analyses of microbial community structure failed to locate nitrifying microorganisms in either the N-limited or P-limited forest soil using known DNA primers specific to nitrifiers and their enzymes.
Key words: nitrification, N additions, tropical forest, pH