Water Resources Research Act Program

Details for Project ID 2011PA156B

Quantifying the Nitrogen Retention Capacity of Legacy Sediments and Hydric Soils Before and After Restoration

Institute: Pennsylvania
Year Established: 2011 Start Date: 2011-03-01 End Date: 2012-02-29
Total Federal Funds: $16,700 Total Non-Federal Funds: $33,403

Principal Investigators: Jason Kaye

Abstract: Eutrophication is the leading cause of impairment in surface waters in the United States (Carpenter et al., 1998). While eutrophication is often attributed to contemporary nutrient pollution, there is growing evidence that past practices also are important. After European settlement, upland soil erosion due to forest clearing and plowing increased sedimentation rates to the Chesapeake Bay watershed (Brush et al., 2008). Much of this sediment was captured behind mill dams. A major new development in eutrophication research recognizes that these legacy sediments have altered nutrient cycling at the land-streamwater interface (i.e. the riparian zone) (Walter and Merritts, 2008). Legacy sediments likely alter eutrophication processes in two ways. First, as abandoned dams breach, stream bank erosion of legacy sediments occurs and acts as a significant non-point source of suspended sediment (and nutrients entrained in the sediment). Second, legacy sediments that have not yet eroded affect the contemporary transfer of nutrients from uplands to streams. One approach to reducing the impacts of legacy sediments is through streambank restoration that removes legacy sediment. This is a proposal to increase the understanding of nitrogen (N) processing and movement in legacy sediment soil layers (1) before and after stream restoration, and (2) before and after soil drying. The integration of legacy sediment restoration and soil drying was motivated by recent analyses showing that even in the relatively wet environment of the mid-Atlantic, there is a pulse of nitrate (NO3-) in streams following drought. Our research will take place at Big Spring Run (BSR) in Lancaster, PA, where stream restoration is planned. The restoration efforts are based on the finding that the pre-settlement hydric soils beneath the legacy sediments formed in a fluvial wetland environment (Walter and Merritts, 2008). The BSR project will be used to establish a new Best Management Practice (BMP) for stream restoration in the region. Currently, there are 19 scientists from 10 institutions involved in research at BSR. The main objective of the proposed study is to determine how N is cycled through, and processed differently, along a legacy sediment strewn stream before and after restoration. We will test three hypotheses: 1) Prior to restoration, three soil layers that are typical of legacy sediments areas (surface legacy sediment enriched in organic matter, subsurface legacy sediment low in organic matter, and buried hydric soil) will differ in their ability to remove NO3- from soil solutions. 2) Drought followed by rewetting will cause variations in NO3- flushing from the different layers. 3) Following restoration, the previously buried hydric layer will increase its NO3- retention capacity (relative to the pre-restoration hydric layers). To test these hypotheses we will extract intact soil columns that extend from the surface soil into the legacy sediment and down to the basal gravels that exist below the buried hydric layer. We will engineer the replicated columns so that we can mimic flowpaths through the three soil layers that are the focus of Hypothesis 1. Flowpaths being tested are: 1) saturation from the surface, 2) rising water tables, 3) lateral flow through the hydric layer only, and 4) lateral flow through the hydric layer plus subsurface legacy sediment. In addition, we will remove legacy sediment from one replicate set of columns to mimic restoration. We will add isotopically labeled nitrate (15NO3-) to these columns to quantify NO3- retention along the different flowpaths. Following these experiments, we will allow the columns to dry and then rewet them with N-free water to quantify the drought-induced loss of 15NO3- from different soil layers. Quantifying changes in N along legacy sediment flowpaths will provide critical information for 1) assessing sources of N to streams, 2) improving the efficacy of riparian buffers on legacy sediments, and 3) understanding the effects of past land use on contemporary N flow from soils to streams. Restoration at BSR will provide valuable information to assess nutrient load reductions through the proposed BMP.