10 years of management fails to reduce phosphorus in study area
Rate of growth slowed in some areas of Green Run on MD’s Eastern Shore.
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The flat fields that fill the small watershed of Green Run on Maryland’s Eastern Shore don’t look particularly threatening. But the findings of a recent study suggest that they will plague the tiny creek’s water quality for decades.
The results show that 10 years of management actions failed to reduce the amount of phosphorus that had built up in the soil during the previous four decades. They only slowed the rate at which phosphorus was accumulating.
Findings from the run, which is small enough to easily hop across and is located in the headwaters of the Pocomoke River, may be bad news for achieving water goals in areas with high phosphorus buildups. They suggest that it may take decades in those areas — primarily regions where animal farming operations prevail — to reduce phosphorus runoff, even if phosphorus applications were halted altogether.
“The reality is that when you have levels up this high, in 10 years you are not going to get them down,” said Kenneth Staver, an associate research scientist with the University of Maryland Wye Research and Education Center. “It took 40 years to get there. This is a long-haul kind of deal.”
Ironically, the findings come from a small watershed that had yielded good news concerning nitrogen. Green Run was the site of a study overseen by the Maryland Department of Natural Resources in the late 1990s and early 2000s. Farmers were paid to replace poultry litter with easier-to-manage chemical fertilizers and to plant cover crops on the 848 acres of crop lands in the watershed.
Monitoring showed that nitrogen levels in the stream fell 25 percent after the changes took place. The study is considered important because it is one of the few small watersheds in the Bay region to show a water quality change as the direct result of changes in farming practices. (See “Farm study yields benefits in water quality,” July-August 2003.)
But the study saw no change in stream phosphorus levels. To better understand what was happening with phosphorus, the University of Maryland’s Harry R. Hughes Center for Agro-Ecology funded Staver in recent years to revisit the fields.
Staver took fresh samples in 2010 at the exact same 285 locations where he had gathered soil data for part of the earlier study a decade before.
He found that phosphorus concentrations in the top 5 centimeters of soil ranged from less than 100 parts per million at a couple of sites, to 1,000 in others. The average in the top 20 centimeters of soil was a bit more than 400 ppm. The optimal range for crop production is generally considered to be 50–100 ppm.
Rather than decreasing, Staver found that phosphorus concentrations overall had slightly increased since the earlier sampling. “There were some fields that were down, but on average, it was up, not down,” he said.
But, he added, the rate of increase appeared to have slowed over what had taken place in the previous four decades. Green Run is in the heart of the intense poultry production on the lower Eastern Shore, and Staver estimated that about 40 pounds of phosphorus were being applied per acre annually during the four decades prior to 2000 — about twice what was being taken up by crops.
The heavy applications occurred in part because of the conventional wisdom that prevailed for years: Phosphorus typically binds with sediment, and therefore phosphorus runoff could be controlled by reducing erosion on fields. Flat land with low erosion rates like that around Green Run was not thought to be a major problem.
As a result, nutrient management efforts for years concentrated on nitrogen, which is more easily carried away by the rain. But research has shown that the ability of sediment to bind with phosphorus is limited by a number of factors, such as the amount of iron and aluminum in the soil.
On fields that get large amounts of fertilizer, especially animal manure, the soil’s capacity to hold phosphorus can be overwhelmed. That’s because manure — especially chicken litter — has a higher ratio of phosphorus to nitrogen. When enough manure is applied to meet the nitrogen needs of crops, phosphorus is often dramatically overapplied — which is what happened on the Green Run fields over the years. When soils became saturated with phosphorus, it is more easily carried off with rainwater, causing algae blooms and other downstream problems in the stream and the Bay.
Scientists have increasingly emphasized the need to balance the amount of phosphorus being placed on fields with the amount being removed by crops to prevent buildups in the soil.
That can be difficult, especially in areas with large livestock operations such as the lower Eastern Shore, where poultry operations generate huge amounts of manure. In those regions, farmers often have limited opportunities to get rid of manure other than applying it on the land.
The Green Run findings indicate how difficult management becomes for highly saturated lands.
Staver said that during roughly half of the years between 2000 and 2010, no poultry litter was applied to the land, only chemical fertilizers. During the other years, poultry litter was used on the land, but it would have needed to conform with phosphorus-based nutrient management requirements, which seek to better balance phosphorus applications with crop needs.
Those actions — although they should have reduced phosphorus inputs — were not enough to halt continued phosphorus buildups in the soil.
“What you can say is in that 10-year period, whatever they did, there was no net reduction overall,” said Russ Brinsfield, executive director of the Center for Agro-Ecology. “The bottom line is that things haven’t gotten better.”
Even if no phosphorus was applied in those areas, Staver said, it would take decades for crops to draw phosphorus levels down to desirable levels in the soil from a water quality perspective. As a result, phosphorus will continue to make its way into the stream, and the Bay, for decades.
Staver and Brinsfield said the study also points to the utility of using soil phosphorus information to more comprehensively verify whether phosphorus management actions are having an impact. While soil phosphorus concentrations are routinely measured to help develop nutrient management plans, they are not recorded in any central database where changes can be assessed over time or used to model watershed-scale phosphorus losses.
“This study confirms the need to have these types of data so we can know with some level of confidence where we are with phosphorus in the future,” Brinsfield said. “The only way you are going to know that is by knowing where you have been.”
If over time soil phosphorus levels are going down, it means the overall trend is in the right direction, even if improvement is not quickly seen in water monitoring data.
On the other hand, if soil phosphorus concentrations are increasing, it means existing management is inadequate. “There are some short-term things that we can do to reduce phosphorus runoff, but if soil phosphorus keeps going up, you are going to lose the battle in the long term,” Staver said. “You won’t get reductions in the water if the phosphorus levels in the soil keep going up.”
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