Agricultural contribution to nitrates in California groundwater

Nitrogen gas makes up 80% of the air we breathe. Although nitrogen is an inert gas—it doesn’t react easily with other elements—it is essential to all life on earth. Chemical reactions can convert nitrogen into reactive forms, such as ammonium and nitrate ions, and nitrous and nitric oxides. These reactive forms are produced, consumed, and changed into other chemicals by human, animal, and plant activity. Eventually, bacteria break down nitrogen compounds, returning nitrogen to the atmosphere. This process is known as the nitrogen cycle.

aerial view of agricultural land

A recent study in the Journal of Environmental Quality looked at how nitrogen use in agriculture impacted the amount of nitrates in groundwater. The researchers studied the nitrogen cycle in the Tulare Lake Basin and the coastal Salinas Valley in California. “Agriculture, through losses out of crop root systems, contributes over 90% of all nitrate inputs to the large alluvial groundwater systems studied,” says Thomas Harter, professor in the Department of Land, Air, and Water Resources at the University of California–Davis.

Elevated nitrate levels have been associated with respiratory illnesses and cancer. In infants and unborn children, Harter says, “excessive levels of nitrate cause methoglobinemia—a disruption of oxygen transport through the blood, leading to suffocation (blue baby syndrome).”

Several factors can upset the normal balance of the nitrogen cycle by skewing input and output amounts. Inputs include nitrogen applied in fertilizer, manure, or irrigation water, or deposited from the atmosphere. Nitrogen outputs (losses) result from crops harvested and removed from the field, volatile losses to the atmosphere, and surface runoff. “Expansion of farmed land during the middle of the twentieth century, the ongoing intensification of farming, and an exponential growth in milk and manure production,” says Harter, have increased the imbalance.

The study showed that high-intensity crop production and large dairy herds are the biggest contributors to nitrate in groundwater. “High production intensity implies high yields and high nitrogen content in crops. In turn, these crops require relatively large amounts of nitrogen applications to the field,” notes Harter. As a result, significant amounts of nitrate losses can occur from the root zone.

It’s common for manure from large dairy herds to be recycled into fertilizer that’s applied to crop fields. The nitrogen in manure essentially acts as a slow-release fertilizer because it takes time to change into forms that plants can use. “Managing these high-yielding, fast-growing forage crops with manure as the main source of fertilizer further adds to the already high risk of leaching groundwater,” says Harter.

Effective crop management must consider both sides of the issue. Some crops have shallow roots and high nitrogen requirements. Underfertilizing these crops can severely impact yield and quality. Harter says, “Farmers can use highly efficient irrigation methods to prevent nitrate leaching from the root zone, but these must be coupled with the ‘four Rs’ of nutrient management: right fertilizer source, right amount, right time, and right place.”

The study concludes that California agriculture cannot continue along its current nitrogen trajectory and still preserve groundwater quality. Harter recommends several solutions. One approach is to develop technologies and practices for commercial fertilizer production from animal manure. “This would provide an economical solution for safely exporting nitrogen from the dairy regions of the Central Valley,” he says.

In the short and intermediate term, government agencies must address the threats posed by nitrate, particularly in impoverished communities and homes that depend on private wells. Harter observes, “Providing funding, infrastructure, capacity-building programs, and education and outreach to these communities must be high priority.”

Harter recommends ongoing efforts to measure and monitor groundwater nitrate levels. This will enable evidence-based agronomic management practice guidelines. At the same time, Harter says, we need “renewed and ongoing efforts to reach out to growers, and to incentivize improved nutrient management practices.”

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