The Use of Sediment Removal to Reduce Phosphorus Levels in Wetland Soils

Excess nutrients in run-off, such as phosphorus (P), from cultivated fields enter wetlands as agricultural pollutants/contaminants with sediment or in surface run-off (Neely and Baker 1989). Increased nutrients can decrease the quality of a wetland by causing changes in plant community composition. In the Florida Everglades, for example, the spread of Typha domingensis into conservation areas is believed to be a result of an increase in agricultural run-off (Mitsch and Gosselink 2007). In small depressional wetlands, sediment from cultivated fields, even in small amounts, will impact function and vegetation communities (Richardson et al. 2001).

In North Dakota, the hybrid cattail (Typha × glauca) is an invasive species which forms monotypic stands due to the species' inherited traits and clonal nature along with its ability to rapidly uptake nutrients (Woo and Zelder 2002). Monotypic stands of clonal species such as Typha degrade the quality of habitat for waterfowl and inhibit other types of vegetation (Mitsch and Gosselink 2007). In Nebraska, removal of sediment from "cattail-choked" wetlands has resulted in an increase of waterfowl use and improvement of multiple wetland functions (LaGrange et al. 2011). Changes in wetland ecosystem functions and structure within the first ten years after restoration may be greatest in shallow soil depths (0–10 cm) (Meyer et al. 2008), which would support sediment removal as a viable option for restoration of low-quality wetlands. The objective of this study was to determine if the removal of sediment in the shallow marsh zone of seasonal prairie pothole wetlands in North Dakota decreases the amount of plant-available P which in turn would reduce the possibility of a wetland being dominated by hybrid cattail.

Soil samples were collected in 2012 from three different wetland types: 1) Excavated, where 10 to 51 cm of sediment was removed from within the basin of the wetland; 2) Converted Cropland, which were unexcavated wetlands recovering from past cultivation; and 3) Reference wetlands, which were in a natural state having not been greatly disturbed by humans and occurring in native prairie. Smith (2011) found that for many of the same wetlands cattail density was low for Reference, higher for Excavated, and highest for Converted Cropland to the point of being cattail choked. All three wetland types were considered recharge wetlands located in the Prairie Pothole Region of North Dakota, USA. Using excavating equipment, sediment removal was performed on the Excavated wetlands, which generally had greater than 25 cm of sediment removed above the original A horizon prior to sediment accumulation (C. Dixon, United States Fish & Wildlife Service, North Dakota, pers. comm.). A total of 18 Excavated, 11 Converted Cropland and 9 Reference sites were sampled (Table 1). Because previous P data does not exist for these sites, Converted Cropland sites were included to represent wetland conditions prior to sediment removal. Sites from all three wetland types were arranged in clusters, according to geographical location, as determined by Smith (2011). All sites were in North Dakota. Cluster A wetlands were located in Benson County (north-central North Dakota). Sediment was removed in the fall of 2007 and the depth of sediment removal ranged from 20 and 30 cm. Cluster B wetlands were located in Towner County (north-central North Dakota). Sediment was removed in the fall of 2008 and depth of sediment removal ranged from 25 to 51 cm. Between 10 to 41 cm of sediment were removed in the fall of 2003 from Cluster C wetlands located in Wells and Eddy Counties (central North Dakota). Cluster C Reference wetlands for Wells and Eddy County were located on Camp Grafton South, National Guard Maneuver Training Site. At each site, three soil samples were randomly collected along a 10 m transect in the shallow marsh zone from two depths (0 to 15 cm and 15 to 30 cm) and were stored at 4°C until analysis. The Olsen P method and water extractable P (WEP) were determined from field-moist samples according to methods outlined by Frank et al. (1998) and Self-Davis et al. (2009), respectively. Two...