Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO 4 2− , Cl − , organic C, inorganic nitrogen and phosphorus), ecosystem CO 2 and CH 4 exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2-4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO 2 and CH 4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects.

Adsorption to soils is one of the dominant mechanisms of P storage in wetlands. We examined P sorption dynamics in soils collected at 12 sample points with diverse hydrology, geomorphic position, mineralogy, and plant communities in two riverine wetlands in northern Minnesota and Wisconsin. Phosphorus sorption parameters from these 12 sample points were correlated with corresponding biogeochemical variables and subsequently extrapolated across 157 sampling points in the two wetlands, based upon a large spatial dataset. We then used a series of single and stepwise regressions to determine the best set of predictive variables for surface water, soil, and plant P pools. Intrasite variation in P sorption dynamics was greater than intersite variation between the two wetlands and rivaled the variation found in the literature for both upland and wetland soils. An essentially constant final P concentration occurred at moderate P additions (≤32 μmol P L−1), indicating extreme soil buffering capacity of porewater P concentrations. Spatial variation in soil P pools across each wetland were predicted very well in stepwise regressions, particularly in the summer (R2 = 0.49–1.00). Variables that were important in explaining this variation included the amount of P sorbed at equilibrium, maximum P sorption capacity, percentage of P sorption sites occupied at equilibrium, organic matter content, bulk density, and oxalate‐extractable Fe and Al content. Phosphorus concentrations in surface water were predicted less well by stepwise regression (R2 = 0.04–0.46), suggesting only weak‐to‐moderate spatial coupling between soils and surface‐water P dynamics. Plant P pools were predicted poorly. Our results indicate the importance of geochemical sorption in controlling P dynamics in riverine soils. We suggest that nutrient studies in spatially diverse wetlands must be designed in a manner that adequately captures the rich spatial dynamics of the system.

Variation in water depth and soil properties associated with geomorphic structures can affect riverine wetland nutrient dynamics by altering biogeochemical processes. We examined the seasonal influence of soils and geomorphology on nutrient forms and concentrations in riverine wetlands in northeastern Minnesota (silty soils) and northwestern Wisconsin (clayey soils). Soil, water, and plant biogeochemistry were contrasted between and within the wetlands according to geomorphic features (riverbed, levee, and backwater zones). There were few inter‐wetland differences, and most were the result of differences in river water chemistry and levee elevation between the two sites. Levees were hot spots of NO3–N, with spring porewater NO3–N concentrations (340 μg L−1 at Fond du Lac, 44 μg L−1 at Pokegama) that were orders of magnitude higher than elsewhere in the wetlands. Summer denitrification potential was high in the levees (≈6 nmol N2O g−1 h−1) and in organic backwater zones (8.3 nmol N2O g−1 h−1 at Fond du Lac, 4.8 nmol N2O g−1 h−1 at Pokegama), but denitrification was consistently NO−3‐limited throughout both wetlands. Riverbeds were zones of highest P concentration in soil, vegetation, and summer surface water. Sedimentation rates were higher in riverbeds (289 g m−2 d−1 at Fond du Lac, 54 g m−2 d−1 at Pokegama) than in backwaters (80 g m−2 d−1 at Fond du Lac, 17 g m−2 d−1 at Pokegama). The two backwater zones had comparably low summer surface water concentrations of NO3–N (≈4 μg L−1), NH4–N (≈6 μg L−1), total P (TP) (≈80 μg L−1), total suspended solids (TSS) (≈6 mg L−1), and volatile suspended solids (VSS) (≈4 mg L−1). This seasonal convergence of surface water chemistry implies that biotic processes common to the two backwater areas override their substrate differences. Backwaters were hydrologically connected to the river mainstem via openings in discontinuous natural levees, but the different water chemistry of riverbed vs. backwater zones indicated minimal water exchange between them. This hydrologic zonation of riverine wetlands by geomorphic structures was the major source of intra‐wetland variability.

Nitrogen loading from developed watersheds to aquatic ecosystems can stimulate microbial denitrification, a process that reduces nitrate (NO { 3 ) to dinitrogen (N 2 ) or nitrous oxide (N 2 O), the latter a potent greenhouse gas. While aquatic ecosystems are a globally significant source of N 2 O to the atmosphere, the relationship between denitrification and N 2 O production is not well known. Until recently, this field of research has been limited by the technical challenges of simultaneously measuring denitrification and N 2 O production or consumption in situ at the ecosystem scale. Here we use membrane inlet mass spectrometry, an analytical method providing precise and accurate measurements of dissolved N 2 , and gas chromatography to directly measure N 2 and N 2 O concentrations in the hypolimnion of a stratified reservoir draining an agricultural watershed. Denitrification resulted in a consistent increase in dissolved N 2 and decrease in NO