An Agricultural Research Service experiment finished nearly 30 years ago — and uncovered recently during new study preparations — shows that it's best to be patient when measuring the movement of nitrates through soil and groundwater.
Mark Tomer and Michael Burkart, both soil scientists and hydrologists at the ARS National Soil Tilth Laboratory in Ames, Iowa, found that nitrate applied during the experiment, conducted between 1969 and 1974, apparently took nearly 30 years to move through soils and reach a 70-foot-deep water table.
This shows that water carrying nitrate can take decades to flow through a watershed's soil subsurface to a stream and should be studied for longer periods, according to the scientists, who work in the lab's Agricultural Land and Watershed Management Research Unit.
In the original study, conducted on a 74-acre field in western Iowa, fertilizer was applied to soil at three times the normal rate. The resulting soil nitrate concentration was tracked for the next decade.
In 1996, Tomer and Burkart were preparing to monitor groundwater for a new experiment when they detected the nitrate 60 feet deep in the soil. They confirmed that the nitrate originated from the old experiment by examining groundwater flow rates and ages, and by comparing the concentration's depth with stream flow records.
Leaching of nitrate from agricultural fertilizers has been linked to concerns such as drinking-water quality and hypoxia, a condition in which water bodies contain low oxygen amounts.
Farmers are being encouraged to use nitrogen more efficiently, but resulting environmental improvements have been difficult to document using studies lasting just two to four years, according to Tomer. In summary, he adds, application of a conservation practice within a watershed may take several decades to fully effect improvements in groundwater quality.