BIOGEOCHEMICAL HOTSPOTS IN FLUVIAL SYSTEMS: WOODY DEBRIS AND BEAVER PONDS

Author

Julia Grace Lazar, University of Rhode IslandFollow

Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Environmental Sciences

Specialization

Natural Resources Science

Department

Natural Resources Science

First Advisor

Arthur J. Gold

Abstract

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 ¹⁵N-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 ¹⁵N 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 km² 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 ponds.

Recommended Citation

Lazar, Julia Grace, "BIOGEOCHEMICAL HOTSPOTS IN FLUVIAL SYSTEMS: WOODY DEBRIS AND BEAVER PONDS" (2013). Open Access Dissertations. Paper 122.
https://digitalcommons.uri.edu/oa_diss/122