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Lori Sprague, the USGS hydrologist who was lead author of the study concluded that "The effects of conservation practices are not yet consistently detectable at a large watershed scale," noting that "current nutrient conditions in streams may still be reflecting agricultural practices that were in place prior to the implementation of the conservation practices."
The USGS press release summarizing the study describes the study as one that, "assessed conservation tillage and the Conservation Reserve Program, both designed to reduce soil runoff and nutrient loss from farmland. Conservation tillage, which limits soil plowing while retaining crop residue on the soil surface, is used on approximately 25 percent of the cropland in the U.S. Approximately eight percent of cropland was enrolled in the Conservation Reserve Program, through which environmentally sensitive farmland is restored to filter strips, grassed waterways, riparian buffers, and long-term vegetative covers, such as introduced or native grasses.
"The lack of detectable impact from conservation practices could also be due to an increase in dissolved nutrients from areas in conservation tillage, where fertilizer, manure, and crop residues are not fully incorporated into the soil. Other possible explanations include nutrient runoff from nearby cropland without conservation practices in place and an incomplete characterization of the location and spatial extent of conservation practices.
"If changes in nutrient loss from agricultural watersheds do lag implementation of conservation practices, nutrient levels in streams may be reduced in the years beyond the scope of this study, which includes USGS data from 1993 to 2001 paired with conservation data from that time period that has only recently become available. Long-term river monitoring at the large watershed scale can provide future accounting of any changes—lagged or otherwise—resulting from the implementation of conservation practices" (emphasis added).