| Nitrogen (N) availability is rapidly increasing. Because N is required by the enzymes that fix and decompose carbon (C), N availability moderates C cycling in biological systems. To predict global C budgets, it is important to understand the impact of N enrichment on C dynamics. Nitrogen generally stimulates plant growth, but global patterns of plant response to N enrichment have not previously been synthesized. Furthermore, N frequently has negative effects on biodiversity, but the relationships between fungal diversity and decomposition of leaf litter components have not been tested. In addition, I evaluated grassland C pools that are insensitive to five years of an experimental N addition gradient.;My first study investigated global trends in plant growth response to N. I conducted a meta-analysis found that N limitation of NPP is globally distributed, varies in strength among biomes, and depends on climate. Because N reduces fungal diversity, my second study investigated the effects of biodiversity on decomposition. I manipulated the number of fungi growing on litter and carbon dioxide release and enzyme production. Total decomposition and cellulose degrading enzyme activity increased with species number, but activity of lignin degrading enzyme activity was invariant. My final study measured carbon pools in a California grassland exposed to an experimental gradient of N availability. I measured plant and microbial biomass, non-living organic matter pools, and enzyme activities. I found no effect of N on C pools other than an increase in aboveground NPP at the lowest rate of N availability. At higher levels of N addition, biomass returned to control levels. This unexpected response was linked to gopher herbivory and a change in plant community composition. The lack of any change in soil carbon pools, microbial biomass, or enzyme activities suggests that a resource other than N limits decomposer biomass and decomposition, and long term C storage has not been affected by N over the first five years of the study.;Collectively, this research demonstrates biotic and abiotic controls that will affect the response of ecosystem carbon balance to present and future increases in N availability. Predicting future C balance requires a better understanding of the controls on N inputs and losses to ecosystems, and I have quantified some of these controls. |
