Disentangling The Separate And Intersecting Pathways Of Carbon And Nitrogen Response To Overlapping Drivers

Anthropogenic activities have drastically altered atmospheric composition — leading to unprecedented disturbances that might push ecosystems across thresholds where important ecosystem services, such as clean water and healthy soils are at risk. Such disturbances include increased heavy precipitation, rain on snow events, and longer-term shifts in rain composition and precipitation amount. Catchment response to such perturbations is widely variable, indicating that specific catchment characteristics may govern the resistance and resilience of the system. Forested catchments in the northeastern, U.S. have reported increasing dissolved organic carbon (DOC) concentrations in streams, and links to shifts in drivers—such as precipitation chemistry, season, and event hydrology—have been proposed. While DOC response to overlapping disturbances is well-studied, changes in dissolved nitrogen (N) species and shifts in stoichiometry have not been investigated as thoroughly, presenting an important knowledge gap. My objective was to investigate the connection between superimposed disturbances, catchment dynamics, and differential stream response of carbon (C) and N in acid impacted soils. I used Sleepers River Research Watershed (SRRW) as a testbed because it has experienced significant shifts in precipitation dynamics and acid deposition, and long-term stream discharge and chemistry records are available. To investigate the connection between overlapping disturbances, catchment soil dynamics, and differential stream response, we combined analyses of these records to with newly collected data from soil core experiments.I used Seasonal Kendall tests to quantify C and N trends in long-term datasets and compared results to processes in soil core experiments. To investigate how shifts in solution chemistry impact the liberation of C and N, I simulated hydrologic flushing events on soils from SRRW using flushing treatments of varied pH and ionic strength—which represent acid-deposition and reduced-acid deposition conditions. I found significant seasonal variability in both concentration-discharge behavior and soil effluent, indicating that seasonal hydrologic conditions and biological activity are principal drivers of C and N mobility and liberation at catchment scale. DOC and the dissolved organic fraction of N (DON) were coupled by season and landscape position, whereas inorganic N (DIN) was largely decoupled. Changes in soil solution were significant for all species during the winter, highlighting the importance of snowpack for processing and mobilizing materials. This research highlights the complex, coupled, and intersecting pathways of C and N which influence catchment response to disturbance. With these results, I investigated the relevance with respect to ecosystem resistance and resilience, and their significance to the possible trajectory of these disturbances in the future. I conclude that specific catchment characteristics at SRRW such as naturally buffered soils, may make the watershed more resistant to climate extremes