Posted 11/7/2012
A U.S Geological Survey study found no consistently detectable reductions in nutrient pollution in streams across the country as a result of conservation practices.
The study analyzed 133 large agricultural watersheds around the country and found no significant improvements in water quality resulting from common conservation practices designed to reduce soil runoff and nutrient loss.
The study assessed the watersheds in relationship with conservation tillage and the Conservation Reserve Program, which are both designed to reduce soil runoff and nutrient loss from farmland.
“When you look at it on a large watershed scale, we clearly are not seeing the effects of conservation practices yet,” said Lori Sprague, the lead author of the study, which was recently published in the Journal of Environmental Quality.
Sprague said a possible explanation for the lack of improvement is that changes in water quality could lag significantly behind the implementation of conservation practices.
This lag time could mean conservation practices “don’t translate into improvements in stream quality in real time,” she said. “Current nutrient conditions in streams may still be reflecting agricultural practices that were in place prior to the implementation of the conservation practices.”
Sprague said lags may occur for several reasons, including the fact that nitrogen from ag land moves slowly to streams through groundwater, so it can take several years for reductions in nitrogen inputs to affect nitrogen levels in streams.
Phosphorus runoff to streams can continue to be an issue even after inputs are reduced because of past accumulation in soils, she said. And while nutrient pollution can be reduced when ag land is restored to natural vegetation, it takes time for those plants to reach their maximum ability to retain nutrients.
The study included USGS data from 1993 to 2001, paired with conservation data from that time period that has only recently become available, Sprague said.
If changes in stream quality do lag the implementation of conservation practices, nutrient levels in streams may be reduced in years beyond the scope of the study, she adds, and the agency plans to continue to monitor watersheds to see if that’s the case.
Kent Politsch, chief of public affairs for the Farm Service Agency, which administers the U.S. Department of Agriculture’s CRP program, spoke with Sprague and believes her explanation of improvements in water quality lagging behind the implementation of conservation practices accounts for the study’s results.
He said the FSA has evidence that shows CRP practices reduce sediment, nitrogen and phosphorus runoff.
“At the surface, we know we’ve succeeded,” he said. “We can extrapolate from that that eventually the evidence will show up at the (stream level). It does take time for that evidence to show up.”