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Through a combination of in-stream incubations, incubation of soil cores, and mesocosm experiments, this dissertation examines denitrification of woody debris in stream settings, and denitrification, soil N transformations and GHG generation of beaver pond sediments. In the first chapter we examined the effect of instream large wood on denitrification capacity in two contrasting, lower order streams – one that drains an agricultural watershed with no riparian forest and minimal stores of instream large wood and another that drains a forested watershed with an extensive riparian forest and abundant instream large wood. We incubated two types of wood substrates (fresh wood blocks and extant streambed wood) and an artificial stone substrate for nine weeks in each stream. After in situ incubation, we collected the substrates and their attached biofilms and established lab‐based mesocosm assays with stream water amended with 15 N-labeled nitrate-N. Wood substrates at the forested site had significantly higher denitrification than wood substrates from the agricultural site and artificial stone substrates from either site. Nitrate-N removal rates were markedly higher on woody substrates compared to artificial stones at both sites. We found nitrate-N removal rates were significantly correlated to biofilm biomass and denitrification capacity accounted for only a portion of nitrate-N removal observed within the mesocosms in both the wood controls and instream substrates. N2 accounted for 99.7% of total denitrification. In terms of management, restoration practices that generate large wood in streams should be encouraged for N removal and do not appear to generate high risks of instream N2O generation. In the second chapter we used 15 N tracer additions in soil core mesocosm incubations with a mass-balance approach to address the fate of nitrate in beaver ponds and understand the capacity of beaver ponds to serve as long-term watershed N sinks. We evaluated and quantified different nitrate transformation pathways, including: denitrification, assimilation into soil microbial biomass and organic N, and net generation of ammonium N. Denitrification constituted between 52 and 86 percent of total N transformations under enriched levels of nitrate; approximately 3 to 5 fold higher than the rates ascribed to nitrate assimilation in soil organic N, which constituted the next highest mechanism of nitrate transformation. On average, 0.2% of denitrification is being released as N2O under low nitrate-N concentrations in the three beaver ponds, while under N-enriched conditions, the average was 7%. Our data suggest that under enriched conditions beaver ponds have greater N2O production than streams, but are similar to wetland soils. We estimate that beaver pond denitrification can remove approximately 50 to 450 kg nitrate-N km -2 of catchment area, assuming 0.7 beaver ponds per km 2 of catchment area. Based on the beaver pond/watershed area ratios, and inter-pond variability in denitrification we estimate that beaver ponds in southern New England can remove 5-45% of watershed nitrate loading from rural watersheds with high N loading (i.e., 1000 kg km -2 ). Thus, beaver ponds represent a proportionally significant sink for watershed N if current beaver populations persist. In the third chapter we determined the diffusive flux of greenhouse gases (GHGs) — methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) — from the air-water interface of three beaver ponds in Rhode Island, USA. We launched five floating static gas chambers on each beaver pond during spring, summer, and fall seasons, and sampled at 15-minute intervals over one hour. Emission rates were derived for each gas from the linear regression of the change in concentration of the gas over time. Fall had significantly higher CO2 emission than other seasons, mean 9.298 g CO2 m -2 day -1 versus 3.305g CO2 m -2 day -1 in spring and 3.188g CO2 m -2 day -1 in summer. CH4 and N2O emissions did not show seasonal differences: annual means were 174 mg CH4m -2 day -1 and 1 mg N2Om -2 day -1 , respectively. When flux was expressed in CO2 global warming equivalents, CH4 emissions comprised the majority of the GHG emissions, at 67.5% across all sites and seasons. Significant correlation was found between CO2 emission rates and pond water DOC, while CH4 emissions were significantly correlated to air or water temperature. Our results show that beaver ponds generate high fluxes of CH4 and CO2 emissions per surface area of the pond. However, the relatively small areal footprint of beaver ponds at the watershed scale greatly diminishes their net effect. Thus, at a catchment scale we estimate that the global warming potential of the GHG emissions from the beaver ponds expressed as CO2 equivalents range from 3-26 Mg km -2 yr -1 . Assessment of the net effect of beaver ponds on the greenhouse gas budget of the Northeast U.S. must consider more than the GHG emissions from the ponded areas of the beaver ponds. Studies are warranted on the extent of changes in water tables, and associated changes in GHG emissions, in the lands surrounding the ponds and the fate of the organic soils in abandoned beaver

BIOGEOCHEMICAL HOTSPOTS IN FLUVIAL SYSTEMS: WOODY DEBRIS AND BEAVER PONDS