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

Abstract

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 O2 (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.