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Salt marsh ecosystem biogeochemical responses to nutrient enrichment: a paired N-15 tracer study
Drake, DC Peterson, BJ al, et Lemay, LE al, et
Drake, DC
Peterson, BJ
al, et
Lemay, LE
al, et
Abstract
We compared processing and fate of dissolved NO3- in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of similar to 1-4 mu mol NO3-/L and the other receiving experimentally fertilized flood tides containing similar to 70-100 mu mol NO3-/L. We conducted simultaneous (NO3-)-N-15 (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 mu mol/L. In contrast, only 50-60% of incoming NO3- was processed in the fertilized system when NO3- concentrations were 84-96 mu mol/ L; the remainder was exported in ebb tidewater. Gross NO3- processing was similar to 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 (NO3-)-N-15 was exported as (NH4+)-N-15. Nitrification rates calculated by (NO3-)-N-15 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 (similar to 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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2009-08-01
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Biological Sciences Peer-Reviewed Articles, biogeochemistry; eutrophication; New England, USA; nitrogen processing efficiency; salt marsh; stable isotopes
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Virginia Institute of Marine Science
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https://doi.org/10.1890/08-1051.1