, Volume 16, Issue 2, pp 295-311
Date: 10 Oct 2012

Groundwater denitrification capacity and nitrous oxide flux of former fringing salt marshes filled with human-transported materials

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Abstract

While former salt marsh sites filled with human transported material (HTM) have altered the surface marsh ecosystem, if artificial drainage is absent, subsurface conditions may continue favorable for denitrification, a microbial process reducing nitrogen (N) export to estuaries. We used piezometer networks to evaluate the in situ groundwater denitrification capacity and nitrous oxide (N2O) flux (with 15N-enriched nitrate-N via the push-pull method) in four former fringing salt marshes topped by HTM along the Rhode Island coast, U.S.A. Groundwater at these sites commonly interacted with the buried marsh horizon and the HTM. In situ groundwater denitrification capacity site means ranged from 15.2 to 71.7 μg N kg−1d−1 with no significant differences between sites due to high intrasite variability. The site with the highest and most consistent denitrification capacity also had HTM of the finest texture and highest soluble organic carbon. Three of four sites had minimal N2O flux [mean N2O:(N2O + N2) = 0.082] while the final site had N2O generation rates up to 52.5 μg N kg−1 d−1. The site with the highest N2O contributions also had the lowest ambient groundwater nitrate-N indicating lack of priming for N2O reduction to N2. Former salt marshes with HTM deposits may still have the capacity for substantial groundwater denitrification capacity, similar to that observed in undisturbed salt marshes, but may also contribute substantially to global N2O emissions. For both salt marsh restoration and greenhouse gas mitigation efforts, attention should be given to ensuring that a tidally-driven, fluctuating water table regularly intercepts the buried organic horizons of the filled salt marsh.