Tile drains a major path for phosphorus loss, studies find

October 10, 2014 - Madeline Fisher

It’s been largely ignored in the past as a route for phosphorus loss from farms, but the buried network of drainage pipes known as the tile system can carry away as much phosphorus as surface runoff.

That’s the conclusion of a pair of studies published in the Journal of Environmental Quality today (Oct. 3). In research in Ohio and Indiana led by USDA-ARS scientists, nearly 50% on average of both dissolved, “bioavailable” phosphorus and total phosphorus left fields via the tile system—a percentage much higher than previously thought.

While the findings are troubling, they also suggest that curbing subsurface phosphorus transport could reap huge water quality rewards for water bodies like Lake Erie beset by phosphorus pollution. At least 40% of farmlands are tiled today across the U.S. Midwest, where subsurface drainage remains a necessity, says Kevin King, a USDA-ARS researcher in Ohio who led one of the studies.

“If we don’t have tile drainage, we cannot farm in this landscape,” he says. “So what we have to do is figure out how to work with it.”

The research is especially relevant to Ohio, Indiana, and Michigan, where movement of dissolved phosphorus from farms to waterways has been helping feed nuisance algal blooms in Lake Erie. These include a record-breaking but mostly harmless bloom in 2011, and the smaller, toxic bloom that contaminated the city of Toledo’s drinking water supply in early August.

In response to the problems, a 2013 Ohio phosphorus task force recommended a nearly 40% reduction in phosphorus loadings for the Lake Erie Basin. It’s a worthy goal, but reaching it will be nigh impossible if only today’s conservation practices, such as reduced tillage, are used, says USDA-ARS scientist Douglas R. Smith, who led the other study. That’s because current practices aim mainly to limit soil erosion and runoff at the surface, in keeping with the prevailing view that most phosphorus in agricultural systems is lost there.

Of course, preventing surface losses is still crucial. But with tile drainage now looking like a major source of phosphorus, additional practices are needed. It’s just not clear yet what exactly those practices should be.

“We hope that more researchers will start looking at phosphorus in tile drainage, both the soluble and total phosphorus forms,” Smith says. “The more minds we can get [working] on the issue, the better off we’ll be in finding solutions.”

At the same time, caution the scientists, people shouldn’t be lulled into thinking that phosphorus transport in tile is the “smoking gun” in Lake Erie’s algae problems.

“What the new data emphasize is that a more holistic approach [taking both the surface and subsurface into account] is warranted to address phosphorus movement from agricultural lands to Lake Erie,” King says. In other words, fixing the problem will require a range of approaches.

ECONOMIC VERSUS ENVIRONMENTAL CONCERNS

Researchers and farmers have already been contending for decades with nitrogen transport in the tile system; as highly mobile nitrate, nitrogen leaches quickly through soil and into underground pipes. But for years, phosphorus in tile discharge was mostly dismissed, Smith says. The measured concentrations were so low—usually well below 1 ppm—people didn’t think they were a concern.

Today, the levels are still low; what’s changing is our understanding of their impact. In the study led by King of the Upper Big Walnut Creek watershed—part of Columbus, Ohio’s water supply—phosphorus concentrations in tile drains were less than 2% of the amount typically applied by farmers on fields. In monetary terms, that’s roughly $1 to $2 per acre.

Yet, more than 90% of these same concentrations exceeded 0.03 ppm, the recommended limit for curtailing blooms of toxic and nuisance algae. “So, from an agronomic standpoint, the farmer is doing great,” King says. “But from an environmental standpoint, [the loss] is very significant.”

“I think we haven’t connected agronomy with lake ecology,” agrees Smith, who conducted his work in an Indiana watershed that feeds into the Maumee River and Lake Erie. He puts things this way: While the amount of phosphorus that typically escapes from fields—about 1 pound per acre—means almost nothing to crop growth, it’s roughly this same amount that is driving the blooms. So what’s the solution? That’s the tricky part.

“Most people will think of a pound per acre, well, that’s good land stewardship if you can get that low in intensively managed croplands,” says eminent phosphorus expert, Andrew Sharpley, with the Division of Agriculture, University of Arkansas. But, he adds, the issue isn’t so much the magnitude of the losses or even the tile system itself; it’s the extent to which tile drainage has been implemented by farmers.

“When you consider how many miles of tiles have been installed and that these connect field drainage directly to a ditch or stream, you understand how we have increased the contributing source area greatly,” he says.

That is, water from countless fields now flows straight into streams that didn’t before, bypassing the soil’s natural ability to bind phosphorus. And even though each field contributes just a small amount of phosphorus, this quickly adds up to a large amount.

“So we are now asking farmers to think about managing water and nutrients in both surface water and leaching, and burdening them with difficult environmental tradeoffs,” Sharpley says. This means, he adds, “they need help based on sound science to deal with these new challenges.”

POTENTIAL SOLUTIONS

Many new studies are in fact underway, but in the meantime practices already exist which should help. One is drainage water management. In it, an inexpensive structure composed of removable stop logs, or weirs, is installed on a tile outlet. Farmers use the structure to drain the ground ahead of field operations that require dry conditions, and then hold more water—and, thus, nutrients—in fields during the off-season. The practice is already known to cut nitrate transport in tile drainage significantly, and King now has data indicating it can control phosphorus movement, too.

Mixing or injecting fertilizer into the soil is also critical. Many farmers in the Lake Erie Basin now practice conservation tillage or no-tillage management—part of a very successful effort to reduce erosion and loss of sediment-bound phosphorus. But these farmers also tend to broadcast-apply phosphorus fertilizer at the surface. And when phosphorus isn’t tilled in or otherwise incorporated, it builds up quickly in the top half-inch of soil, Sharpley says.

As a result, excess phosphorus not only runs off the surface more easily; it may also be more prone to enter the tile through “macropores” or other preferential flow paths, the scientists say. Reduced tillage, for example, encourages the development of macropores—large, beneficial pores that help aerate the soil, facilitate water flow, and provide habitat for soil microbes. But when phosphorus-laden water on the surface leaks into cracks and holes in the soil, macropores can also serve as “a direct conduit” to the tile, King says.

“So, I’m not advocating tillage,” he says, “but we’ve got to get the phosphorus incorporated in some way, even in a no-till framework.”

At the same time, King and others are also investigating whether using some type of minimum tillage to disrupt the macropores might help. For example, farmers might till directly above their tile lines only, or just where soil test phosphorus values are extremely high, such as in spots that have received long-term manure applications. “We think that might be a really good place to target deep tillage to break up macropore flow, as well as to mix these high soil test P soils with some subsoil,” Smith says.

But the first step is for people to realize they need to look below the surface of things when it comes to phosphorus—like Smith and his collaborators did in their research.

“After a few years of working in the watershed we decided that we really needed to study not only what’s running off the surface, but also what was moving through the tile,” Smith says. “We knew we had a missing link that we needed to capture.”

View the abstracts for the papers:

http://dx.doi.org/doi:10.2134/jeq2014.04.0149

http://dx.doi.org/doi:10.2134/jeq2014.04.0176