Effects of seasonal variation and land cover on riparian denitrification along a mid-sized river

Abstract

Urban areas contribute disproportional nitrogen (N) loads to downstream aquatic ecosystems resulting in potential hypoxic ‘dead’ zones. Riparian areas along streams and rivers reduce inorganic N concentrations through denitrification, an anaerobic microbial process. Our study objective was to investigate the denitrification potential of riparian areas with differing land cover composition along the Licking River in Kentucky, USA – a tributary of the Ohio River. For one year we collected monthly samples from four sites along a 60 km reach of the Licking River. We experienced substantial drought conditions in the first half of our study period followed by record precipitation and flooding in the second half of our study period. Land cover surrounding the sample sites was classified into distinct classes hypothesized to contribute to N loads. We found flooding increased denitrification potential, water nitrate concentration, and soil organic matter, while drought conditions increased soil ammonium concentrations. Our site with the greatest impervious surface had the highest denitrification potential, and soil and water ammonium concentrations. However, we determined that denitrification potential was mostly driven by soil organic matter content and only slightly by soil nitrate among all study sites. Our data demonstrate that riparian zones of mid-sized rivers in urban areas can be integral in removing excess N during flood events and can do so year-round provided sufficient N and carbon resources are present.