Nitrogen-fixing trees alter the composition and function of soil microbial communities in young Hawaiian ecosystems

Human activities are rapidly altering the distribution of plant species across the globe. The presence or absence of specific plant species can influence many ecosystem factors, including the composition and function of soil microbial communities. Symbiotic nitrogen-fixing plants have been shown to have a particularly dramatic impact on ecosystem function. The goal of this dissertation was to investigate how N-fixing tree species affect the composition of soil microbial communities, as well as their decomposition and nitrogen cycling functions, and how these effects vary on substrates of differing ages. I also investigated the mechanisms by which these effects take place.; I examined the impacts of three different N-fixing tree species, one native and two invasive, in young forest ecosystems on the island of Hawaii. I found that all three N-fixers can alter the composition of soil microbial communities, as measured by phospholipid acid analysis. However, these effects decline with increasing substrate age, and disappear entirely on older substrates at some sites. The presence of these N-fixing trees increases the net nitrification rates and nitrification potential of microbial communities, and these changes are also greater in younger soils than in older ones.; In order to determine the mechanism(s) by which these trees alter soil microbial communities, I carried out a laboratory incubation experiment, using soil from sites invaded by the N-fixing tree species Falcataria moluccana , and from nearby uninvaded sites. This experiment allowed me to independently measure the effects of N-fixer invasion, substrate age, litter inputs, and nitrogen availability, and the interactions between them. I determined that increased N availability can explain the influence of this N-fixing tree on microbial community composition and litter decomposition rates, as measured both by litter mass loss and microbial respiration. However, differences in nitrification rates and nitrous oxide production between microbial communities from the invaded site and the uninvaded site on the youngest substrate were not eliminated by increased N availability within the 17-week timeframe of this incubation.