Denitrification in Riparian Wetlands Receiving High and Low Groundwater Nitrate Inputs

Wetlands potentially remove a high percentage of the groundwater-borne nitrate (NO^- ₃ ) that moves from upland environments before it reaches streams. It is important to determine how much of the NO^- ₃ that enters wetlands is actually removed from the ecosystem by denitrification (conversion of NO^- ₃ into N₂ gas) rather than cycled between plants and soil. We measured denitrification in riparian forests with upland to wetland transition zones (moderately well drained and somewhat poorly drained soils) and red maple (Acer rubrum L.) swamps (poorly and very poorly drained soils) on two sides of a stream. Soils on the two sides were similar, but the upland land use on one side was a high density, unsewered residential development (enriched site), while the upland on the other side was undeveloped (control site). Denitrification was measured using an acetylene-based intact core (0-15 cm) technique under unamended, water amended, and water plus nitrate-amended conditions. Denitrification (both unamended and amended rates) and soil and groundwater NO^- ₃ levels were consistently higher in soils on the enriched site. Estimates of annual denitrifieation ranged from <5 kg N ha^-1 yr^-1 on the moderately well drained control site soil to nearly 40 kg N ha^-1 yr^-1 on the very poorly drained enriched site soil. Stimulation of surface soil denitrification by subsurface NO^- ₃ enrichment requires a complex interaction between hydrology, plant uptake of NO^- ₃ , and movement of plant N into soil NO^- ₃ pools through litterfall, mineralization, and nitrification. Comparison of measured denitrification rates with estimates of groundwater NO^- ₃ loading suggested that denitrification may have removed up to 50% of the groundwater NO^- ₃ that entered the enriched site.