Denitrification Enzyme Activity as an Indicator of Nitrate Movement through a Diversion Wetland

Abstract

The Davis Pond freshwater diversion is intended to help restore Louisiana's coastal wetlands by reintroducing Mississippi River water to Barataria Basin. We hypothesized that the high NO3− concentration (2.0 mg NO3–N L−1) of the Mississippi River water would control the rate of denitrification in the receiving marsh given that the soils are saturated, anaerobic, and contain high C. Therefore, areas of high denitrification enzyme activity (DEA) in the marsh would represent soils exposed to river NO3− and actively involved in denitrification. Data from 88 soil samples (0–10 cm) collected throughout the marsh revealed significantly higher rates of DEA in a 715‐ha area adjacent to the diversion inflow. This area of generally high DEA contained >80% of all DEA observed while representing only 19% of the total marsh area at the low discharge rate of 39.5 m3 s−1 The area of high DEA coincided with the highest surface water NO3− and indicated that the marsh has a greater aerial capacity for NO3− removal than is utilized. A laboratory experiment suggested that soils loaded with external NO3− typically had higher DEA rates than soils receiving no added NO3− The DEA was strongly dependent on soil depth (92% of DEA occurred at 0–5 cm) and internal N cycling was substantial in this wetland soils. This study demonstrates the applicability of using soil DEA to map where denitrification activity is greatest, the aerial extent of soils involved in denitrification, and the general flow path of introduced nutrients in large wetlands where NO3− is the limiting factor for denitrification.