Wetland denitrification: influence of site quality and relationships with wetland delineation protocols

Denitrification, the microbial conversion of NO3- to N gases, is important to wetland biogeochemistry and water quality maintenance functions. We measured denitrification along toposequences from forested upland-wetland transition zones to red maple wetlands in Rhode Island. The sites were located on different soil parent materials derived from different geomorphic features including glacial outwash, granitic glacial till, basaltic glacial till, and alluvium. Each toposequence contained four or five different natural soil drainage classes. We assessed spatial and temporal variation in denitrification relative to site quality (parent material), water table dynamics, hydric soil and hydrophytic vegetation status, and a "growing season" defined as soil temperature 5 degrees C. Denitrification rate was measured by intensive sampling during a 2-yr period using an acetylene-based intact soil core method. Annual estimates of denitrification were produced by extrapolating measured rates over the intervals between sample dates. Soils that were very poorly drained or that had developed on nutrient-rich parent materials had higher denitrification than more well-drained and/or less fertile soils. Nitrogen inputs from adjacent ecosystems also appeared to be a key regulator of denitrification rates. Annual denitrification ranged from 1 to 130 kg N ha-1 yr-1 and was lower in Year 2 than in Year 1 in all soils. There were poor relationships between spatial and temporal patterns of denitrification and patterns of hydric soils and hydrophytic vegetation within the wetlands. Wetland delineation and classification protocols based on soils, vegetation, or hydrology do not appear to encompass the capacity of a wetland to support denitrification