Salt marsh ecosystem biogeochemical responses to nutrient enrichment: a paired 15 N tracer study

Abstract

We compared processing and fate of dissolved NO 3 − in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of ∼1–4 μmol NO 3 − /L and the other receiving experimentally fertilized flood tides containing ∼70–100 μmol NO 3 − /L. We conducted simultaneous 15 NO 3 − (isotope) tracer additions from 23 to 28 July 2005 in the reference (8.4 ha) and fertilized (12.4 ha) systems to compare N dynamics and fate. Two full tidal cycles were intensively studied during the paired tracer additions. Resulting mass balances showed that essentially 100% (0.48–0.61 mol NO 3 ‐N·ha −1 ·h −1 ) of incoming NO 3 − was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO 3 − concentrations were 1.3–1.8 μmol/L. In contrast, only 50–60% of incoming NO 3 − was processed in the fertilized system when NO 3 − concentrations were 84–96 μmol/L; the remainder was exported in ebb tidewater. Gross NO 3 − processing was ∼40 times higher in the fertilized system at 19.34–24.67 mol NO 3 ‐N·ha −1 ·h −1 . Dissimilatory nitrate reduction to ammonium was evident in both systems during the first 48 h of the tracer additions but 15 NO 3 − was exported as 15 NH 4 + . Nitrification rates calculated by 15 NO 3 − dilution were 6.05 and 4.46 mol·ha −1 ·h −1 in the fertilized system but could not be accurately calculated in the reference system due to rapid ( 3 − turnover. Over the five‐day paired tracer addition, sediments sequestered a small fraction of incoming NO 3 − , 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 mol·ha −1 ·d −1 , respectively. Macrophyte NO 3 − 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 − by plants was lower in the reference system (1.75 mol NO 3 − ·ha −1 ·d −1 ) than the fertilized system (∼10 mol NO 3 − ·ha −1 ·d −1 ). Nitrogen processing efficiency decreased with NO 3 − load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

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