NUTRIENT DYNAMICS AND NITROGEN TRACE GAS FLUX DURING ECOSYSTEM DEVELOPMENT IN MONTANE RAIN-FOREST

Patterns of nitrogen trace gas emissions, soil nitrogen flux, and nutrient availability were evaluated at five sites that form a chronosequence in Hawaiian montane rain forest. The estimated age of basaltic parent material from which soils developed at the Kilauea site was 200 yr, 6000 yr at the Puu Makaala site, 185 000 yr at the Kohala site, 1.65 x 10(6) yr at the Molokai site, and 4.5 X 10(6) yr at the Kauai site. Peak net N mineralization and nitrification values were found in soils from the 185 000-yr-old Kohala site. Nitrogen content of foliage and leaf litter was highest in the intermediate age sites (Puu Makaala and Kohala) and N and P retranslocation was lowest at the Puu Makaala site. Soil cores fertilized with nitrogen had significantly higher rates of root ingrowth than control cores at the two youngest sites (200 and 6000 yr old) but not in older sites (185 000 and 4.5 X 10(6)-yr-old sites) and total fine root growth into control cores was greatest at the Kohala site. The highest N2O emissions were found at the 185 000-yr-old Kohala site, while the highest combined flux of N2O + NO was observed at the 4.5 x 10(6)-yr-old Kauai site. While overall N2O emission rates were correlated with rates of N transformations, soil water content appeared to influence the magnitude of emissions of N2O and the ratios of emissions of NO vs. N2O. N2O emissions occurred when water-filled pore space (WFPS) values were >40%, with highest emissions in at least two sites observed at WFPS values of 75%. Among sites, high N2O emissions were associated with high soil N transformation rates. Large NO fluxes were observed only at the Kauai site when WFPS values were <60%.