Volume 4, Number 1
Table of Contents
From Farm to Bay
Nitrogen's Underground Passage
By Erica Goldman
Ever since a severe thunderstorm felled the 450-year-old Wye Oak in June 2002, residents of Maryland's Eastern Shore have missed it sorely. Ken Staver's office in the Wye Research and Education Center is only few miles down the road from where the oak, 96 feet tall with a circumference of nearly 32 feet, once stood, and he has heard a lot of "bellyaching" from people who lament the tree's fate.
"People want the tree back now but they just can't have it," he says. "But we can get on track to have another Wye Oak someday."
Staver feels the same is true about nutrient reduction in the Chesapeake Bay. An ecohydrologist who studies the flow of nutrients through watersheds, Staver works to understand how farming affects the ecosystem and to develop strategies for change that are both ecologically and economically viable. Real reductions in nitrogen loads will not happen fast, he says. "We should be focusing on the long-term."
At the heart of the problem is that nitrogen can move slowly through groundwater, like an underground glacier. And since it can take time to wend its way down to the Bay — sometimes on the order of decades — the positive effects of nutrient reduction efforts will not be immediately obvious, explains Staver.
Nitrogen, along with phosphorus, is a prime culprit in excessive algae growth and oxygen depletion in the Chesapeake Bay. But it has only been over the past few years that scientists have understood the nuts and bolts of how nitrogen enters the watershed, what it does once it is there, and how it makes its way into the Bay.
From the Farm
The largest loads of nitrogen and phosphorus to the Bay come from agriculture, though specific inputs have changed as farming practices have evolved, says Staver. As recently as pre-World War II, farmers relied primarily on animal manures and naturally occurring microbes in the soil to make nitrogen available for growth of their crops. They also planted legumes for animal feed, like clover and alfalfa, which convert nitrogen from the atmosphere to a form that other plants, like corn and wheat, can use for growth. Their plows aerated the soil, stimulating the microbes that convert organic nitrogen to nitrate — the form useable by plants.
But nitrate can also be carried by water downward through the soil or leach. Less intensive agriculture in the pre-War era meant lower soil nitrate levels, with little excess nitrate leaching into the shallow groundwater that feeds streams.
With the advent of greater fertilizer and herbicide use on agricultural land, however, the amount of nitrate in the soil available for crops increased along with the rates of nitrate leaching. Readily available inorganic nitrogen fertilizer also helped to support feed crops for concentrated animal production, especially poultry. At the same time, farmers no longer had to depend on manure from their animals to fertilize crops.
An expanding poultry industry in the Chesapeake region also meant an increase in nutrients, since chicken manure is two to three times higher than cow manure in nitrogen and phosphorus. Since the early 1980s, the amount of nutrients from manure and poultry litter generated in the watershed has grown an estimated 17 percent. Together, fertilizer and manure make up 68 percent of the total nitrogen applied to the land in the watershed, according to data from the Chesapeake Bay Program.
Now superimpose on this increase in soil nutrient availability what Staver calls "Hydrology 101," the basics of how water moves across and through the landscape. For the Chesapeake watershed this hydrology means episodic direct runoff from the land and the chronic flow of groundwater into streams that empty into the Bay. Both flow paths carry water-soluble nutrients from cropland into the Bay, although over much different time frames. Both routes lead to an increase in nitrogen loads.
But how does nitrogen physically move from farms into groundwater? Pure supply and demand, says Staver. If nitrate remains in the soil after summer crops die, it can leach into the groundwater during the following winter. Most inorganic fertilizers supply nitrogen as nitrate or in a form that can be rapidly converted to nitrate.
Nitrogen in manure, on the other hand, takes longer to convert to a form usable by plants. Since it is difficult to predict exactly how much nitrogen per pound of manure will be available to plants for growth, farmers cannot manage nitrogen from manure as precisely as inorganic nitrogen fertilizers, explains Staver.
Timing is also of the essence, he says. Though nitrate levels on farms are the highest in the summer because of fertilizer application and microbial activity in the soil, it is in fall and winter that nitrate is more likely to leach into groundwater.
This is because in summer, the top layer of soil acts like a big sponge, says Staver. Rainfall that infiltrates the soil is continually re-evaporated, mostly by plants through their leaves (transpiration). As a result, even though summer nitrate levels are often high, water levels are not generally high enough to cause a significant amount of leaching.
But when summer ends, annual crops like corn and soybeans die. Nitrate and water uptake cease, though soil processes that release nitrate continue as long as soil temperatures are warm. Gradually as temperatures cool, soil moisture levels increase, eventually saturating the soil "sponge." Winter rain and snowmelt seep downward through surface soils towards the water table — carrying with them any nitrate that was left in the root zone. Since nitrate is a negatively charged molecule (called an anion), it is not attracted to soil particles, so it moves freely with water through subsurface layers.
Cover crops — small grains such as rye or barley or winter wheat that are planted without fertilizers immediately after harvesting corn or other row crops — can keep nutrients from leaving the farm. If planted early enough, cover crops help take up nitrate in the root zone before it leaches into groundwater.
Like cover crops, forested areas at the edges of streams — so-called riparian buffers — can help keep nutrients from reaching streams and the Bay and remove them from groundwater. Buffers slow surface runoff, allowing the roots of plants to intercept sediment and water heading toward streams. In doing so, they help restore many of the benefits of stream ecosystems, including improved habitat for terrestrial wildlife and native spawning and fish passage, says Russell Mader, the Nonpoint Source Coordinator for the Chesapeake Bay Program.
"I promote buffers, but only as part of a multi-dimensional approach to restoration," Mader says.
According to Mader and others, buffers are often a "hard sell" to farmers because they take land out of production and may require ongoing maintenance. The federal government, realizing that farmers are an essential dimension of the buffer equation, assists them through the Conservation Reserve Enhancement Program (CREP). Authorized by the U.S. Farm Bill, CREP compensates land owners for setting aside sensitive lands — such as buffer strips along streams, rivers and creeks — and planting them with perennial vegetation.
Nitrogen from point and nonpoint sources is transported to streams through runoff, soil water, and groundwater. Once in the groundwater system, nitrogen may take anywhere from less than a year to decades to be transported to a stream. Diagram by Scott Phillips et al., U.S. Geological Survey.
To the Bay
If excess nitrogen from farms is not reclaimed by cover crops or riparian buffers it can reach the Bay via two distinct pathways: direct runoff from the land into surface waters and leaching into the groundwater that feeds streams. The groundwater pathway has only recently been studied on a watershedwide basis.
The United States Geological Survey (USGS) in 2003 published the results of a multi-year study of nitrogen in groundwater in an attempt to untangle its role in maintaining a "lag time" between the implementation of a management action and a positive response of the Bay to these actions.
Groundwater is a very significant component of the water supplied to streams, explains USGS hydrologist Scott Phillips. On average, just over 50 percent of the total volume of water in streams throughout the watershed is from groundwater, although this varies between wet and dry years and between streams (from 16 to 92 percent). In dry years, a much larger percentage of the water in the streams comes from groundwater, while in wet years a larger fraction comes from direct discharge, he says.
How much of the nitrogen that winds up in the Bay actually comes from these groundwater sources? Quite a bit, says Phillips. The USGS study finds that an average of 48 percent of nitrogen loads in streams in the watershed are contributed in the form of nitrate from groundwater (17 to 80 percent in different streams).
But the problem in keeping track of nitrogen that arrives in the Bay from groundwater sources is that there can be a large time delay, explains Phillips. The USGS sampled 46 different springs and found that the age of groundwater in the watershed ranges from modern (less than one year) to more than 50 years, with a median age for all samples of 10 years. This means that nitrogen that enters the groundwater from cropland this year could take a decade or more to arrive in the Bay.
Another variable that affects both the timing and amount of nitrogen that arrives in the Bay is the carbon content of the streams, says Phillips. High carbon levels will enhance the natural processing of nitrogen (called denitrification). From maps of soil characteristics, managers should be able to predict where the positive effects of denitrification will occur.
The time delay is also compounded by what happens on the front end — the rate of nitrogen leaching into the groundwater in the first place, Staver adds. "There is a separation in time and space between what we do on the fields and what we see in the streams," he says.
Much of this separation is caused by physical properties in different parts of the watershed — so-called "hydrogeomorphic regions." The heavily agricultural coastal plain of Maryland's Eastern Shore is one particular case, says Staver. Here the distance from topsoil to bedrock is typically more than 1000 feet and the substrate is made up of silt, clay and sand. "It's a bit like a bucket of golf balls — lots of open space between the particles," he says. Even though most of the groundwater that feeds streams moves through underground aquifers, huge volumes of water can be stored even in the shallow parts of the subsurface flow system. In the Coastal Plain, the average age of groundwater is 6 to 12 years, according to the USGS study. So even though the Coastal Plain lies close to the Bay, its shallow groundwater systems can retain nitrogen for longer than some of the more distant areas of the Bay watershed where the rocky subsurface has less storage capacity.
Lightening the Load
The uncertainty associated with groundwater delivery of nitrogen to the Bay poses a challenge for management. The current version of the Chesapeake Bay Program watershed model, one of the principal tools used to evaluate progress toward goals of reducing nutrients and sediment delivered to the Bay, does not represent groundwater in a thorough manner, says Phillips. The current model does not account for the time delays associated with nitrogen leaching or delivery to the Bay. "This is one of the reasons that we haven't seen large reductions in nitrogen with the implementation of best management practices (BMPs)," he says.
A new version of the Chesapeake Bay Program's model (Phase 5) is currently under development and this iteration will include estimates for the time lags associated with groundwater nitrogen, says Phillips.
Adding the time-delay information to the model is especially important for calibration, says the Bay Program's Russell Mader. "It will enable us to better see the effects of changes in land use and management practices on nutrient reduction over time."
But the jury is still out on whether nitrogen can be effectively removed from groundwater through active management. Strategically placed riparian buffers may help in areas where the root zone comes into direct contact with the water table.
Two important considerations in whether a buffer will remove nitrogen from groundwater are whether the water table is close to the surface and whether it moves quickly or slowly through an area, says chemical ecologist Thomas Jordan from the Smithsonian Environmental Research Center in Edgewater, Maryland. Riparian buffers can remove more nitrate from groundwater that moves slowly. Slower moving water allows the micro-organisms on the vegetation more of an opportunity to do their work, he explains. In a recent study, Jordan found that groundwater nitrate levels declined as water moved through a buffer from a soybean or corn field.
Since these results were difficult to translate broadly across the watershed, Jordan expanded his efforts to study the effect of buffers on nutrient removal from groundwater in 500 different subwatersheds in the Chesapeake basin. His sites are diverse — with varying proportions of agricultural and nonagricultural lands and a range of spatial configurations of land and water, across different regions of uniform geologic structure and climate (physiographic provinces). Through a statistical analysis of flow paths, Jordan is hoping to tease out such factors as differences in nutrient processing in regions where there are gaps in a buffer, compared to uninterrupted forest along a streambed.
Unfortunately once nitrate gets into groundwater cost-effective options for removing it are limited. "Once nitrogen enters the groundwater, there is not a whole lot you can do about it," says Mader. "Groundwater is a nitrogen sink. The best thing that we can do is to make sure that the sink doesn't get any bigger."
A combination of changing agricultural practices, additional riparian buffers, and better model predictions based on a more complete picture of nitrogen's underground passage, should help stem the nutrient tide and give us a better idea of how long the process of reducing nutrient loads to the Bay will take. Like the growth of another Wye Oak, nutrient reduction won't happen overnight, as Staver says. But we know what we have to do if we want the tree back someday.
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This page was last modified April 06, 2005