EXOTIC GRASS INVASION ALTERS POTENTIAL RATES OF N FIXATION IN HAWAIIAN WOODLANDS

Exotic grass invasion promotes fire which drives the conversion of nat ive woodlands to exotic grasslands in the seasonally dry submontane fo rests of the island of Hawai'i. We compared potential rates of N fixat ion in an unburned forest site and a converted grassland site using th e acetylene reduction assay. In addition to measuring rates of N fixat ion on separate and mixed substrates in each site, we tested the effec t of abiotic factors on rates of N fixation of specific substrates. We hypothesized that rates of N fixation would be higher in the converte d grassland site. N fixation estimates were 4.9 kg N ha(-1) year(-1) f or the unburned forest, and 0.10 kg N ha(-1) year(-1) for the grasslan d site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter of native woody species. Thes e substrates are absent from the grassland site, except for wood debri s which was not consumed during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres o f shrubs or in soil. Although wood debris is not a significant contrib utor to the N fixed in the unburned forest, it contributes the majorit y of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature suggests that microcli matic differences between sites do not control differences in N fixati on activity; rather, these differences are due to the abundance of N-f ixing substrates. The substantial decrease in N fixation activity afte r the conversion from woodland to grassland implies that ecosystem-lev el rates of N accretion are decreased by fire in these sites so much t hat the N lost during volatilization due to fire is not replenished ov er the long term by N fixation.