Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology & Evolutionary Biology

First Advisor

Alan R. Townsend

Second Advisor

Cory C. Cleveland

Third Advisor

William D. Bowman

Fourth Advisor

Noah Fierer

Fifth Advisor

Jason C. Neff

Abstract

This dissertation explores how physical and biological processes organize the interaction of carbon and nutrient cycles, which underpin life on Earth. In Chapter 2, I establish that ecosystem nitrate accrual exhibits consistent and negative nonlinear correlations with organic carbon availability along a hydrologic continuum from soils, through fresh- water systems and coastal margins, to the open ocean. Across this diversity of environments, we find evidence that resource stoichiometry strongly influences nitrate accumulation by regulating a suite of microbial processes that couple dissolved organic carbon and nitrate cycling.

In Chapter 3, I address the climate sensitivity of carbon cycling in old-growth tropical rainforests, which are among Earth’s most carbon-rich and productive ecosystems. Collectively they exchange more CO2 with the atmosphere than any other terrestrial biome – annually, about 16 times more C than the change in atmospheric CO2 concentration resulting from fossil fuel use – thus small imbalances between rainforest carbon uptake and release can influence atmospheric CO2 concentrations. Here, I use meta-analysis of field data to examine the long-term climate sensitivity of rainforest CO2 exchange and storage. I found that net primary productivity and biomass carbon peaks in warm, lowland rainforests peaks at the highest rainfall levels, contrasting with a saturating response expected from previous studies in montanae forests. The pattern results from interactions between climatic, edaphic, geographic and biotic controls over carbon accumulations.

In Chapter 4, I found that seasonal water availability plays an important role in structuring the tropical nitrogen cycle. Counter to current paradigms that expect tropical lowland rainforests to freely leach bioavailable N, I discovered very low export of bioavailable N from an old-growth tropical watershed. Nitrate loss was closely tied to organic carbon availability for heterotrophic microbes. PON export constituted the largest hydrologic loss pathway for N and was regulated by episodes of intense rainfall that caused surges of sediment yield from erosion. The magnitude of PON loss is larger than measured N inputs, and may constrain N accumulation within the ecosystem over long timescales.

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