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

Posted on 2016-08-05 - 10:36

We compared processing and fate of dissolved NO3 in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of ∼1–4 μmol NO3/L and the other receiving experimentally fertilized flood tides containing ∼70–100 μmol NO3/L. We conducted simultaneous 15NO3 (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 NO3-N·ha−1·h−1) of incoming NO3 was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO3 concentrations were 1.3–1.8 μmol/L. In contrast, only 50–60% of incoming NO3 was processed in the fertilized system when NO3 concentrations were 84–96 μmol/L; the remainder was exported in ebb tidewater. Gross NO3 processing was ∼40 times higher in the fertilized system at 19.34–24.67 mol NO3-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 <1% of incoming 15NO3 was exported as 15NH4+. Nitrification rates calculated by 15NO3 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 (<4 h) NO3 turnover. Over the five-day paired tracer addition, sediments sequestered a small fraction of incoming NO3, 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 NO3 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 NO3 by plants was lower in the reference system (1.75 mol NO3·ha−1·d−1) than the fertilized system (∼10 mol NO3·ha−1·d−1). Nitrogen processing efficiency decreased with NO3 load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

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AUTHORS (9)

D. C. Drake
Bruce J. Peterson
Kari A. Galván
Linda A. Deegan
Charles Hopkinson
J. Michael Johnson
K. Koop-Jakobsen
Lynsey E. Lemay
Christian Picard
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