The effects of increased nitrogen loading on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in marsh sediments were studied in permanently submerged subtidal creek sediments and on the tidally inundated vegetated marsh platform in Plum Island Sound estuary, Massachusetts. DNRA and denitrification in surface sediments were measured at all sites using whole-core incubations and the isotope pairing technique, which allows distinction between denitrification of water column nitrate and coupled nitrification-denitrification. On the marsh platform, denitrification was also measured at depth in the rhizosphere, using a new approach that combined the push-pull method and the isotope pairing technique. In tidal creek sediments, fertilization increased denitrification of water column nitrate by approximately one order of magnitude, and coupled nitrificationdenitrification threefold. Coupled nitrification-denitrification made a significant contribution to the total N 2 production in the unfertilized creek but was of minor importance in the fertilized creek due to increased rates of denitrification of water column nitrate. In the surface sediment of the marsh platform, fertilization increased denitrification of water column nitrate by an order of magnitude during inundation of the marsh platform, about 12% of the day. However, coupled nitrification-denitrification occurring at depth in the rhizosphere was the main denitrification pathway, accounting for more than 50% of the total N 2 production in the fertilized as well as in the reference marsh. DNRA was measured in the surface sediment only, where it was comparable in magnitude to denitrification in the fertilized as well as in the unfertilized marsh.

The effects of increased nitrogen loading on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in marsh sediments were studied in permanently submerged subtidal creek sediments and on the tidally inundated vegetated marsh platform in Plum Island Sound estuary, Massachusetts. DNRA and denitrification in surface sediments were measured at all sites using whole‐core incubations and the isotope pairing technique, which allows distinction between denitrification of water column nitrate and coupled nitrification‐denitrification. On the marsh platform, denitrification was also measured at depth in the rhizosphere, using a new approach that combined the push‐pull method and the isotope pairing technique. In tidal creek sediments, fertilization increased denitrification of water column nitrate by approximately one order of magnitude, and coupled nitrification‐denitrification threefold. Coupled nitrification‐denitrification made a significant contribution to the total N2 production in the unfertilized creek but was of minor importance in the fertilized creek due to increased rates of denitrification of water column nitrate. In the surface sediment of the marsh platform, fertilization increased denitrification of water column nitrate by an order of magnitude during inundation of the marsh platform, about 12% of the day. However, coupled nitrification‐denitrification occurring at depth in the rhizosphere was the main denitrification pathway, accounting for more than 50% of the total N2 production in the fertilized as well as in the reference marsh. DNRA was measured in the surface sediment only, where it was comparable in magnitude to denitrification in the fertilized as well as in the unfertilized marsh.

Anammox bacteria are widespread in the marine environment, but studies of anammox in marshes and other wetlands are still scarce. In this study, the role of anammox in nitrogen removal from marsh sediments was surveyed in four vegetation types characteristic of New England marshes and in unvegetated tidal creeks. The sites spanned a salinity gradient from 0 to 20 psu. The impact of nitrogen loading on the role of anammox in marsh sediments was studied in a marsh fertilization experiment and in marshes with high nitrogen loading entering through ground water. In all locations, nitrogen removal through anammox was low compared to denitrification, with anammox accounting for less than 3% of the total N 2 production. The highest relative importance of anammox was found in the sediments of freshwater-dominated marshes, where anammox approached 3%, whereas anammox was of lesser importance in saline marsh sediments. Increased nitrogen loading, in the form of nitrate from natural or artificial sources, did not impact the relative importance of anammox, which remained low in all the nitrogen enriched locations (<1%).

In many wetland plants, belowground transport of O2 via aerenchyma tissue and subsequent O2 loss across root surfaces generates small oxic root zones at depth in the rhizosphere with important consequences for carbon and nutrient cycling. This study demonstrates how roots of the intertidal salt-marsh plant Spartina anglica affect not only O2, but also pH and CO2 dynamics, resulting in distinct gradients of O2, pH, and CO2 in the rhizosphere. A novel planar optode system (VisiSens TD®, PreSens GmbH) was used for taking high-resolution 2D-images of the O2, pH, and CO2 distribution around roots during alternating light–dark cycles. Belowground sediment oxygenation was detected in the immediate vicinity of the roots, resulting in oxic root zones with a 1.7 mm radius from the root surface. CO2 accumulated around the roots, reaching a concentration up to threefold higher than the background concentration, and generally affected a larger area within a radius of 12.6 mm from the root surface. This contributed to a lowering of pH by 0.6 units around the roots. The O2, pH, and CO2 distribution was recorded on the same individual roots over diurnal light cycles in order to investigate the interlinkage between sediment oxygenation and CO2 and pH patterns. In the rhizosphere, oxic root zones showed higher oxygen concentrations during illumination of the aboveground biomass. In darkness, intraspecific differences were observed, where some plants maintained oxic root zones in darkness, while others did not. However, the temporal variation in sediment oxygenation was not reflected in the temporal variations of pH and CO2 around the roots, which were unaffected by changing light conditions at all times. This demonstrates that plant-mediated sediment oxygenation fueling microbial decomposition and chemical oxidation has limited impact on the dynamics of pH and CO2 in S. anglica rhizospheres, which may in turn be controlled by other processes such as root respiration and root exudation.

Belowground sediment oxygenation in rhizospheres of wetland plants promotes nutrient uptake, serve as protection against toxic reduced compounds and play an important role in wetland nutrient cycling. The presence of 1.5-mm-wide oxic zones around roots of the intertidal marsh grass Spartina anglica was demonstrated below the sediment surface using planar optode technology recording 2D images of the sediment oxygen distribution. Oxic root zones were restricted to the root tips stretching up to 16 mm along the roots with an oxygen concentration up to 85 μmol L −1 detected at the root surface. Radial oxygen loss across the root surface ranged from 250 to 300 nmol m −2 s −1 , which is comparable to other wetland plants. During air exposure of the aboveground biomass, atmospheric oxygen was the primary source for belowground oxygen transport, and light availability only had a minor effect on the belowground sediment oxygenation. During inundations completely submerging the aboveground biomass cutting off access to atmospheric oxygen, oxic root zones diminished significantly in the light and were completely eliminated in darkness. Within the time frame of a normal tidal inundation (~1.5 h), photosynthetic oxygen production maintained the presence of oxic root zones in light, whereas oxic root zones were eliminated within 1 h in darkness. The results show that the sediment oxygenation in Spartina anglica rhizospheres is temporally dynamic as well as spatially variable along the roots.

in the fertilized system but could not be accurately calculated in the reference system due to rapid (,4 h) NO 3 À turnover. Over the fiveday paired tracer addition, sediments sequestered a small fraction of incoming NO 3 À , although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 molÁha À1 Ád À1 , respectively. Macrophyte NO 3 À uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO 3 À by plants was lower in the reference system (1.75 mol NO 3) than the fertilized system (;10 mol NO 3 À Áha À1 Ád À1 ). Nitrogen processing efficiency decreased with NO 3 À load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

This study presents a new approach for measuring denitrification at depth in the marsh rhizosphere (below 5 cm). The method combines the push-pull technique and the

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