Isotopic evidence for the climate dependence of nitrogen cycles across old tropical rainforests, Mt. Haleakala, Hawaii.

摘要

This dissertation explores the origin of the climate-dependence of nitrogen (N) cycles across old tropical rainforests. Here, I adopt a model system approach to this problem, by taking advantage of six sites positioned within the influence of sharp rain shadow on Mt. Haleakala, Hawaii, which roughly span the rainfall regimes observed for tropical forests worldwide. I mix empirical measures with quantitative modeling to deconvolve the mechanisms that underpin a monotonic decrease in the N isotope abundance (15N/14N) of forested ecosystems with increasing rainfall. In chapter 1, I provide a brief introduction to the questions addressed and give an overview of the dissertation chapters. In chapter 2, I report on 15N/ 14N input-output budgets across the Hawaiian rainfall gradient. Results of this work highlight the importance of gaseous N losses in controlling ecosystem 15N-signatures, the amount of N available for plant growth, and the scale-dependence of isotopic expression of N cycling reactions within the rainforests. In chapter 3, I examine strategies of plant N acquisition across the rainforests, by exploiting natural variations in the 15N/ 14N of foliage and ecosystem N pools. Here, I report that plants rely on nitrate for growth in dry forests, but ammonium in the wettest ones, and that this transition is strongly influenced by microbial N consumption in the soil. Finally, in chapter 4, I explore how spatial complexity influences the vertical distribution of N in soil subsystems, and provide strong isotopic and modeling evidence that the N cycle is depth-stratified within the sites. Most broadly, my findings bring much needed resolution on controls of the climate-dependence of ecosystem 15N-signatures across tropical rainforests, with implications for understanding N balances, N limitation, and organismal responses to shifts in forest N availability across tropical rainfall climates.