Carbon and nitrogen dynamics in fluvial systems across biomes

Carbon (C) and nitrogen (N) cycles are tightly coupled in ecosystems as many N transformations rely on carbon as the energy source. In aquatic ecosystems this coupling between C and N has been studied via assessment of dissolve organic carbon (DOC) and inorganic-N concentration or whole stream manipulations of C and/or N to determine the influence of C on N processing. However, the majority of these studies have been performed in temperate systems and whether these relationships hold across other biomes, like artic and tropics, is unclear. In addition, how the more N-rich organic matter or dissolved organic nitrogen (DON) interacts with inorganic-N has also received little attention. Overall, the research presented here aims to better understand the how C and N relate in streams and what is the role of C in N transformations across different biomes. In the chapters presented here, I use a variety of approaches to better understand C and N coupling in streams such as assessing long-term trends in DOC and DON concentrations across biomes to whole stream manipulations in boreal forests of the Central Siberian Plateau (CSP) and the Luquillo Mountains of Puerto Rico. Time series analyses on dissolve organic matter (DOM, composed of DOC and DON) have demonstrated that DOC and DON concentrations are changing (positive and negative trends) in streams regardless of biome. The potential drivers of changes in DOM are watershed specific suggesting that DOM is controlled by local and regional-scale factors. Experimental work has focused on the coupled biogeochemistry of DOM and nitrate, including how variation in DOC concentration and DOM composition affect nitrate and ammonium uptake. I performed whole stream manipulations of NO3 and NH4 in Siberia to study watershed resiliency and recovery from major disturbances including permafrost thaw and wildfires. DOC concentrations returned to pre-fire conditions after 50 years while recovery for NO3 was 10 years. Uptake rates of NO3 and NH4 declined with increasing NO3 and NH4 concentrations, DOC:DIN molar ratios, and relative abundance polyphenols; as the frequency of fire or the rate of permafrost thaw continue to increase in high latitude regions, streams could become major exporters of inorganic nutrients to receiving bodies including the major Arctic rivers and the Arctic Ocean. I performed similar stream manipulations but of NO3 and C sources in Luquillo which revealed that the demand for NO3 is energy-limited and is enhanced with the availability of DOM. In addition, these streams are carbon limited and have high demand of added C sources. Collectively, these dissertation chapters demonstrate that across biomes DOM availability can be an important factor in nitrogen processing and the quantity and composition of DOM is tightly linked to the landscape