Beaver Pond Geomorphology Influences Pond Nitrogen Retention and Denitrification

Abstract

Beaver ponds are increasingly promoted as a strategy for physical stream restoration, and more recently recognized as natural solutions for excess nonpoint source pollutants, such as nitrogen (N). Beaver ponds facilitate N transformations on the streamscape by increasing sediment-water interactions and creating low and high redox conditions in close proximity. Because beaver ponds vary widely in their geomorphology, the spatial extent of morphological units should dictate the degree of biogeochemical processing. We explore the role of beaver pond geomorphic units in facilitating sediment N transformations using a sediment mass balance approach. We quantified input, output, and transformation of N species within the sediments and water of the riffle, backwater, and margin geomorphic units of a beaver pond in Northern Utah. Our interpretations were supplemented using sediment oxygen demand (SOD) and sediment C and N stable isotopes. The backwater performed most of the biogeochemical work, including increased sedimentation (2.1 g N m−2d−1), ammonification (1.3 g N m−2d−1), nitrification (0.07 g N m−2d−1), and denitrification (0.1 g N2 m−2 d−1), the latter facilitated by high sediment OM (14%) and SOD (−0.95 ± 0.2 g O2 m−2 d−1). Backwater sediment isotopes, supported by N flux measurements, indicated increased benthic microbial activity and denitrification. The margin and backwater surface sediments had low C:N ratios, indicating sediment input from autochthonous production. These findings suggest the representation of geomorphic units in a beaver pond may predict whether the pond can facilitate N removal through sediment sequestration and denitrification.