Beneath the Salt Marsh Canopy: Loss of Soil Strength with Increasing Nutrient Loads
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Although the broadly observed increase in nutrient loading rates to coastal waters in the last 100 years may increase aboveground biomass, it also tends to increase soil metabolism and lower root and rhizome biomass—responses that can compromise soil strength. Fourteen different multiyear field combinations of nutrient amendments to salt marshes were made to determine the relationship between soil strength and various nitrogen, phosphorus, and nitrogen+phosphorus loadings. There was a proportional decline in soil strength that reached 35% in the 60- to 100-cm soil layer at the highest loadings and did not level off. These loading rates are equivalent to those in the flow path of the Caernarvon river diversion, a major wetland restoration project near New Orleans; 12% of the wetlands in the flow path were converted to open water in 2005. The increased nutrient loading from the Mississippi River watershed this century has also driven the formation of the low oxygen zone (the “Dead Zone”) that forms off the Louisiana–Texas shelf each summer. These results suggest that improving water quality in the watershed will aid the restoration of both offshore waters and coastal wetland ecosystems.
KeywordsWetland Soil strength Eutrophication Sustainability Louisiana
Support was provided by the NOAA Coastal Ocean Program MULTISTRESS Award No. NA16OP2670 to Louisiana State University and from the Coastal Restoration and Enhancement through Science and Technology program. I thank J. Tripp, Environmental Defense Fund, for asking the pregnant question about the relationships between nutrient loading and soil quality, C. Swarzenski for comments on an earlier draft, and D. Daigle and C. Milan for their editorial review. The many, many constructive suggestions made by two anonymous reviewers are appreciated.
Conflict of Interest
I have no financial relationship with the organization that sponsored the research. I have full control of all primary data and agree to allow the journal to review the data if requested. There are no potential conflicts in the use of the data or financial conflicts.
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