by Dave Wilson
An arm of the US Department of Agriculture, the service took nine years to complete its groundbreaking work which shows that native plant buffers retained or removed at least 60 percent of the nitrogen and 65 percent of the phosphorus that entered from adjacent manure/fertilizer application sites. This is the first time that a study of a restored riparian buffer has shown that the retention of phosphorus was as high or higher than nitrogen retention.
In their "Designing the Best Possible Conservation Buffers,"Agricultural Research Service scientists Richard Lowrance and engineer George Vellidis of the University of Georgia wanted to find out exactly what grass and forests buffers were capable of.
The buffer system used in the long-term study had three zones of grass, new forest, and old forest. To get a baseline of runoff amount and content researchers monitored both nutrient levels put on the field as well as nutrient levels leaving the field prior to buffer planting.
For the next nine years, they monitored amounts of water and concentrations of nutrients in water entering and leaving the riparian wetland. Generally, young forests have high nutrient uptake rates because the plants within them are growing more, while mature forests provide forest soils (leaf litter and shallow roots) which increase biomass on the forest floor. All these factors are keys to reducing the movement of nutrients to surface waters.
But when the scientists found they could reduce phosphorous flow by two thirds, they were shocked. "Most of our management recommendations... have focused on nitrogen,” Lowrance said in a USDA interview last week, “now we know that a restored riparian forest buffer can be just as effective for phosphorus removal.”
The study was in response to a request made in the late 1980s and early 1990s by USDA's Natural Resources Conservation Service and Forest Service to recommend riparian buffer specifications. At that time, the general recommendation was that natural mature buffers should be used, but USDA needed national specifications based on the best science.
To compliment this work, another study is now being conducted by soil scientist Robert Hubbard and animal scientist G.L. Newton of the University of Georgia to evaluate effectiveness of grass-forest buffers to filter nutrients from fields. They have so far found that nutrient uptake for the grasses is limited, with uptake of nitrogen at about 45 percent and phosphorus at around 20 percent. The research shows that grass buffers work better when combined with other buffer systems such as forest.
In another study headed by Lowrance and Vellidis, herbicides were examined in a grass-forest buffer system. During this 3-year study, they found that the grass filter strip buffers were especially effective at reducing the amounts of two herbicides, atrazine and alachlor, that entered the shallow groundwater and surface runoff. Atrazine has been pinpointed as a hormone disrupter and cause of malformations in frogs and salamanders. In contrast to the results with nitrogen and phosphorus, the grass filter strip received higher amounts of herbicides and provided a higher rate of removal.
The crux of their work concludes that a combination of forest and grass buffers is best for curbing both chemical and nutrient runoff. So far Worcester County farmers and property owners have gone to great lengths to utilize USDA buffer programs. This work shows that in addition to the several thousand more acres of wildlife habitat they have created, their work to protect water quality in the coastal bays is paying big dividends.
"Designing the Best Possible Conservation Buffers” was published in the Dec ember 2003 issue of Agricultural Research magazine.
