BioCycle October 2005, Vol. 40, No. 9, p. 40
Duke University researchers evaluate additions of compost to low organic matter soils, establishing plants in low marshes and high marshes.
Ariana E. Sutton-Grier, James Pahl, Mengchi Ho and Curtis Richardson
SUCCESSFUL restoration of riparian wetland habitat is often inhibited by the lack of organic matter in soils. To meet this challenge, organic matter amendments were planned for wetland cells within a stream and floodplain restoration of Little Sugar Creek in Mecklenburg County, North Carolina, to be called the Hidden Valley Ecological Garden. Researchers from the Duke University Wetland Center (DUWC) began working with the county’s Stormwater Services to develop a set of experimental soil organic matter amendments to test if adding compost would increase growth of planted vegetation and improve water quality. Three treatments were established: 1) Topsoil only; 2) Low compost at a ratio of 4:1 topsoil:compost; and 3) High compost at a ratio of 2:1 topsoil:compost. Following the end of restoration activities in July 2004, Duke personnel launched a five-year program of quarterly ambient stream water quality sampling and habitat development, and annual surveying of soils and vegetation in wetland cells.
Ambient water quality data, as measured in June and November 2004, showed limited reduction of nutrients. Decreases in P concentrations with movement downstream may be due to physiochemical retention as opposed to in-stream biological processing, although our sampling scheme is not structured to confirm that assertion. We measured high levels of fecal coliform bacteria at all sites, and there was little evidence that the stream processes were acting on those bacteria. There were no obvious trends in the other parameters examined.
Preliminary soils data show that there is a high degree of variability among sites in measured parameters, including soil organic matter, which is highly variable both among and between amendment treatments. The application of compost amendments to the soils was more haphazard than hoped. That, along with the reluctance of the compost supplier to provide background data on the compost product, will complicate subsequent efforts to determine the relative merits of augmenting soils with organic matter.
Plant growth in nine of the 11 cells was robust between July and September, particularly in the low marsh areas. By September, growth in the high marsh areas was also vigorous, although volunteers and undesirable invasive grasses dominated much of it. The first few months after conclusion of construction activities mainly provided background data, and we expect that real trends in the data will begin to become evident during the first full growing season after construction (in 2005).
The Hidden Valley Ecological Garden project includes 13 acres of “reclaimed” floodplain and 3,500 feet of stream. Stated goals of the project include: reducing flooding during rain events by reducing peak flows and flow duration; reconnecting the stream to the bank full floodplain; improving water quality; increasing ground water recharge; restoring terrestrial and aquatic habitat; stabilizing the embankment with native vegetation and restoring the “natural” channel dimensions and profile; eliminating fecal coliform; and decreasing stream temperature.
When 17 homes were built on the previous floodplain site between 1956-1968, six feet of fill material was added. Removing the fill material as part of the restoration exposed relatively undeveloped, clay-rich soil horizons. DUWC was asked to develop and test whether adding topsoil to the site would establish better soil conditions for the plants. We realized that soil organic matter levels were potentially going to be low in the newly established marshes due to the intense degree of disturbance to the site and suggested a soil amendment study to examine the effect of organic matter additions on the soil processes and plant growth and survival.
We are specifically testing the following hypotheses: organic matter additions will increase extractable N and P and soil moisture holding capacity, all of which are soil processes that are critical to plant survival and growth; organic matter additions will also increase microbial biomass; and improved plant growth and survival will be significantly correlated with soil physical and biogeochemical properties as well as biological processes.
Following the completion of construction activities, we conducted our first quarterly sample of ambient water quality on November 1, 2004. The remainder of quarterly sampling will occur in January, April, July and October through 2008.
EFFECTS OF COMPOST AMENDMENTS ON WETLAND CELLS
Initial analysis of the proposed compost to be used on the site showed a moisture content of 57.9 percent and an organic content of 63 percent. Thus, for every pound of compost, there was only about 0.25 pounds of dry organic matter. Although ideal treatment levels of compost amendment would have been to add compost at the 1:1 and 2:1 ratios, financial and logistical realities forced our next best scenario of a 2:1 and 4:1 topsoil:compost ratios for the High and Low Compost treatments, respectively. According to our calculations, this would raise the organic content to 10 percent (by mass) in the High Compost treatment and 7 percent (by mass) in the Low Compost mixture.
We determined that the topsoil itself already had on average about four percent organic matter. We predicted that ratios lower than these, such as 3:1 (high) and 6:1 (low) ratios would only raise the organic matter content of the low compost treatment by about one percent, which we predicted would not be enough compost to see any effects in plant growth or soil properties when compared to the topsoil only treatment. These were our best approximations based on the limited information available to use prior to the onset of construction activities and some generous assumptions about the characteristics of the materials with which we were working.
The most significant aspect of the soils data is the high degree of variability between cells, both within and among amendment treatment levels. This variability provides strong evidence that the establishment of the soil amendments was not as controlled a process as we would have hoped. This conclusion is bolstered by observations of the soils data from September 2004 suggesting that not only was the application of the compost to the soils highly haphazard, but that efforts to thoroughly mix that material into the topsoil were limited at best. Our initial observations and lab determinations suggest that in essence we have poorly mixed topsoil and compost sitting as a distinct layer atop the harder unmixed soils that were exposed after the preexisting soil was scraped from the surface during the restoration of floodplain elevations in the initial stages of construction.
Recent developments on the wetland studies will be reported by the author at the BioCycle Southeast Conference in a Monday, November 14, 2005 session on Storm Water Management, Compost Style. Other presenters at this session are Britt Faucette of Filtrexx International (Watershed Research Findings); Chuck Friedrich of the Carolina Stalite Company (Green Roof Growing Media); and Ann Gill of Mecklenburg County, NC Solid Waste Reduction (Compost Role in Local Ordinances.)
The authors are with the Duke University Wetland Center based in Durham, North Carolina.
October 25, 2005 | General
COMPOST USE IN URBAN RESTORED WETLANDS
BioCycle October 2005, Vol. 40, No. 9, p. 40