Fate of 15N-nitrate in unplanted, planted and harvested riparian wetland soil microcosms

Riparian wetlands are important for the protection of river water quality in agricultural landscapes by intercepting and removing nutrients, such as nitrate (NO3-), in runoff. However, limited information is available on the relative importance of biological NO3- removal processes in these ecosystems. In this study the fate of N-15-NO3- was investigated for 32 days in three types of wetland soil microcosm (unplanted, planted, and planted with shoot harvest) in order to identify the key processes responsible for NO3- removal, elucidate the role of the wetland plant and determine the effect of shoot harvest. N-15-NO3- solution (7.9 mg N l(-1), 99 at.% N-15) was added to each microcosm at a rate of 0.5 mug N g(-1) soil every 2 days. In both types of plant-inhabited microcosm, similar proportions of added N-15-NO3- were denitrified (61-63%), soil-immobilised (24-26%), plant-assimilated (11-15%) and reduced to ammonium (NH4+) (<1%). However, in unplanted microcosms, 49% was reduced to NH4+, 29% denitrified and 22% immobilised. Elevated denitrification in the presence of the plant, glaucous sweetgrass (Glyceria declinata), was attributed to a higher degree of soil oxidation, which-is considered to be the principal regulator of NO3- partitioning between denitrification and DNRA. Shoot harvest did not affect the fate of N-15-NO3-, but it decreased new shoot production (by a factor of 3.9), inhibited new root production, and increased the NO3- assimilation capacity of shoots (by a factor of 5.2). Although this study lasted for only 1 month, the results have important implications for riparian ecosystem management, restoration and design. The water quality protection afforded by riparian wetlands might be substantially enhanced by maximising vegetation cover with plants like G. declinata that promote high denitrification nitrogen (N) losses. In contrast, shoot harvest as a mechanism of permanent N loss may be much less important. (C) 2002 Elsevier Science B.V. All rights reserved.