Effects of headwater wetlands on dissolved nitrogen and dissolved organic carbon concentrations in a suburban New Hampshire watershed

Coupled processing of N and C in headwater wetlands can strongly affect downstream water quality. To understand how wetlands affect dissolved N and C concentrations in surface waters and the factors that influence changes in concentration, we measured landscape and physicochemical variables and dissolved N and C concentrations at the in- and outflows of 10 headwater wetlands in a suburban watershed (New Hampshire, USA) over 18 mo. We analyzed changes (Δ) in dissolved N and C concentrations (outflow - inflow) with linear mixed-effects models. A pulse in N₂O concentrations that exceeded mean values by 2 to 3 orders of magnitude was observed during a summer characterized by rapidly fluctuating water-table elevations. NO₃^- concentrations were significantly lower, and DOC and DON concentrations were significantly higher at outflows than inflows. ΔNO₃^- was predicted by inflow concentrations of NO₃^-, NH₄⁺, and DON, ΔDON, and season. ΔDON was predicted by inflow DON and DOC concentrations, ΔDOC, and dissolved O₃ (as the mean of in- and outflow concentrations). ΔTDN (total dissolved nitrogen) was predicted by inflow TDN and DOC concentrations, dissolved O₂, and season. ΔDOC was predicted by DON inflow concentration and ΔDON, season, and the ratio of study-wetland to subwatershed area. No significant predictors of ΔN₂O or ΔNH₄ + concentrations were found. In this suburban watershed, the passage of surface water through headwater wetlands is associated with decreased NO₃^- concentrations, increased DON and DOC concentrations, and seasonal shifts in TDN fractionation. Linear mixed-effects modeling was an effective approach to understanding coupled N and C dynamics in the study wetlands.