To answer this question, they took their previous research to new depths, literally. They designed instruments that would accurately and continuously measure nitrate concentration and water flow farther down into the soil than what had been measured before. Their machinery is spread out over six one-acre plots scattered across 70 acres. Several thousand units of data pour wirelessly into a computer in Burger’s lab from the field’s central nervous system.
Burger unlatches the door of a solar-powered metal cabinet perched on a pole at the edge of a field. A spaghetti of multicolored wires is inside. Some of the wires are connected to sensors under the crops. “Too much information,” he quips, shaking his head. He will enter it all into a computer model that simulates water flow and nitrogen transport under various irrigation and weather conditions, so that researchers can get the full picture of what is happening underground.
Soil moisture sensors show, over time, how much water has gone into the ground, and how much has left, either from drainage or in the form of evapotranspiration from plants. Burger points out some small red flags among the triticale in one of his plots. They mark where instruments called lysimeters, ceramic plates about
1 foot in diameter, are buried at various depths down to about 47 inches. They measure the amount of water flowing past.
Another set of sensors measure electrical conductivity, which Burger says can be used as a proxy for measuring nitrate. (Nitrate increases electrical conductivity.) The researchers can trace the nitrate using stable (non-radioactive) isotopes, which are essentially unique atomic signatures for nitrate from different sources.
They grab water samples by applying suction to ceramic cups buried at various levels in the soil. Each sample is brought to the surface, analyzed for its nitrate concentration and labeled with an isotopic signature. Nitrate sitting on the fields at the beginning of the rainy season is also labeled. Much of this has been left over from last year’s fertilizer use. Later in the spring, they can measure how much of the leftover nitrate on the surface ended up in cover crops and how much made its way into each soil layer.
Burger gazes across his plot to the south toward the nearby creek that feeds into the Sacramento River and finally into the Pacific Ocean. “From the label, you can see how far the nitrate is traveling,” he says.
As he strokes a slim triticale shoot at his feet, Burger talks about his experiment’s initial results. “We do know that nitrate is leaching past the root zone during the tomato growing season,” even though applying fertilizer and irrigation water together has helped reduce runoff, he says. What has surprised Burger is that triticale has deeper roots — about two feet deeper — than fava beans. So, despite not being nitrogen-fixing, triticale may be better at capturing leftover nitrate.
Burger hopes his research can apply to farmers and scientists facing similar challenges as far away as Chesapeake Bay and the Mississippi River Basin, where there are enormous dead zones. Based on the results, Burger and Hopmans will ultimately be able to offer recommendations for irrigation methods, fertilizer use and cover-crop plantings that help curb nitrate contamination and increase yields.
Hydrologist Thomas Harter, also from UC Davis, posits that even these best-management practices can only go so far. For many crops that need a lot of nitrogen, such as vegetables, corn and nuts, “we don’t know today whether farming practices will ever be good enough to produce drinking water quality recharge,” he says. (Recharge refers to water that escapes from fields into groundwater.) But farmers can learn to contain nitrate within their systems more carefully, he says. And regulators can step in. He has recommended that California lawmakers consider imposing economic penalties and incentives for farmers, such as excise fees on nitrogen fertilizer applications, with higher rates applied to areas declared to be at risk for nitrate contamination.
In recent years, some growers already have dramatically reduced the amount of water and synthetic fertilizer they use by practicing precision farming techniques, such as drip irrigation, which reduces the overall amount of water used and therefore nitrate runoff. They have managed to achieve these reductions without sacrificing profits. Despite facing what seems like a challenge made for Sisyphus, Harter harbors far more hope than dread for the future. “If we can get the worse half (of growers) to operate like the best half over five years, and then do it again, in 10 years everyone will operate as the best, and we’ll have come a long way,” he says.
Back at Russell Ranch, Burger continually contemplates the connection between farming practices and distant households. “It would be great if we made a contribution toward safer drinking water,” he says.
[This article originally appeared in print as "The Nitrogen Underground."]