Abstract
In a whole-ecosystem, nutrient addition experiment in the Plum Island Sound Estuary (Massachusetts), we tested the effects of nitrogen enrichment on the carbon and nitrogen contents, respiration, and strength of marsh soils. We measured soil shear strength within and across vegetation zones. We found significantly higher soil percent organic matter, carbon, and nitrogen in the long-term enriched marshes and higher soil respiration rates with longer duration of enrichment. The soil strength was similar in magnitude across depths and vegetation zones in the reference creeks, but showed signs of significant nutrient-mediated alteration in enriched creeks where shear strength at rooting depths of the low marsh–high marsh interface zone was significantly lower than at the sub-rooting depths or in the creek bank vegetation zone. To more closely examine the soil strength of the rooting (10–30 cm) and sub-rooting (40–60 cm) depths in the interface and creek bank vegetation zones, we calculated a vertical shear strength differential between these depths. We found significantly lower differentials in shear strength (rooting depth < sub-rooting depths) in the enriched creeks and in the interface zones. The discontinuities in the vertical and horizontal shear strength across the enriched marshes may contribute to observed fracturing and slumping occurring in the marsh systems. Tide gauge data also showed a pattern of rapid sea level rise for the period of the study, and changes in plant distribution patterns were indicative of increased flooding. Longer exposure times to nutrient-enriched waters and increased hydraulic energy associated with sea level rise may exacerbate creek bank sloughing. Additional research is needed, however, to better understand the interactions of nutrient enrichment and sea level rise on soil shear strength and stability of tidal salt marshes.
Highlights
Population growth and human activities, especially along the coast, result in increasing loads of reactive nitrogen from land to coastal waters (e.g., Deegan 2002, Galloway et al 2004, Howarth and Marino 2006)
Marsh vegetation was characteristic of a typical New England salt marsh with a high marsh dominated by Spartina patens, Distichlis spicata, and stunted Spartina alterniflora and a low marsh dominated by tall S. alterniflora, primarily growing along the creek banks (Johnson et al 2016)
In both the enriched and reference creeks, there was a trend of lower soil shear strength in the low marsh zone compared with the interface and high marsh zones (v22 = 5.53, P = 0.06; Table 1; Fig. 1), significant at the 40–60 cm depth
Summary
Population growth and human activities, especially along the coast, result in increasing loads of reactive nitrogen from land to coastal waters (e.g., Deegan 2002, Galloway et al 2004, Howarth and Marino 2006). Sea level rise is reportedly accelerating and considered a key factor driving large-scale marsh loss in some parts of New England (Watson et al 2014, 2017, Weston 2014, Raposa et al 2017) Since both nutrient enrichment and rapid sea level rise can cause marsh loss (Kirwan and Megonigal 2013, Watson et al 2014, Wong et al 2015), in the present study we consider the possible effects of increased inundation associated with accelerated sea level rise to the creek systems in addition to nutrient treatments in a whole-ecosystem enrichment experiment
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