Interactions of climate, carbon, and nutrient cycling in wet tropical forest. Thesis
PhD: University of Colorado Boulder, 2011.
Tropical forests play a substantial role in the global carbon (C) cycle and are projected to experience significant environmental change, highlighting the importance of understanding the factors that control C and N cycling in this biome. Yet interactions between biogeochemical and abiotic variables, notably species diversity and precipitation, remain poorly resolved in the tropics. In a wet lowland tropical forest in southwest Costa Rica I examined how: i) biologically generated chemical diversity affected soil C mineralization; ii) abiotic and chemical factors control litter decomposition; and iii) potential changes in precipitation and labile C inputs may alter nitrous oxide (N2O) emissions.
I investigated how inter-specific chemical variation in soluble dissolved organic matter (DOM) affected heterotrophic soil respiration in a series of laboratory incubations. Following DOM additions, soil respiration rates varied by as much as an order of magnitude; largely driven by variation in the C: phosphorus (P) ratio of DOM. These results suggest that tropical tree species composition may influence soil C storage and mineralization via variation in plant litter chemistry.
Subsequently, I assessed how variation in precipitation and litter chemistry affect rates of leaf litter decay. Despite exceptionally high rainfall, simulated throughfall reductions consistently slowed rates of litter decomposition. Overall, variation between species was greater than variation induced by throughfall reductions, and was best explained by initial litter solubility and lignin: P ratios. These results support a model of litter decomposition where mass loss rates are positively correlated with rainfall, and reemphasize the importance of litter solubility and P availability in controlling decomposition rates.
Finally, I examined how changes in precipitation and labile C availability may affect soil N2O production. Experimentally reducing throughfall significantly increased emissions of N2O; at least part of this response was driven by an increase in the concentration of litter-to-soil inputs of dissolved organic C under the drier conditions, as well as a positive relationship between leaf litter inputs and soil N2O production. These results highlight the importance of understanding the potential direct effects of changing precipitation on soil biogeochemistry (e.g., altered soil redox conditions), and also the indirect effects resulting from interactions between hydrologic, C, and N cycles.