N remains in soil, leaks toward groundwater for decades
The novel aspect of their study was that they subsequently determined the long-term fate of this fertilizer N 'pool' retained in the soil. Their measurements of seepage water from locations two meters deep in the soil revealed the amount of fertilizer nitrate leaking toward the groundwater.
The team found that 61 to 65 percent of the N-15 fertilizer applied in 1982 was taken up by the sugar beet and wheat plants over the 30-year study.
However, 32 to 37 percent of the fertilizer N remained in the soil organic matter in 1985 or three years after application, while 12 to 15 percent still lingered in the soils after three decades.
Between eight to 12 percent of the fertilizer N applied in 1982 had leaked in the form of nitrate toward groundwater during the 30 years, and will continue to leak at low rates "for at least another five decades, much longer than previously thought," the study says.
The scientists predict that about 15 percent of the initially applied fertilizer N will be exported from the soils toward the groundwater over a time span of almost one century after the 1982 fertilizer application.
Mayer speculates that if the same research were done in Alberta, the findings would be similar in terms of fertilizer uptake by plants and nitrogen retention in the soils, although Alberta's comparatively dry climate and different geology might slow the rate of nitrate seeping towards the groundwater.
Nitrate contamination of aquatic ecosystems can be reduced by farmers following the 4Rs of nutrient stewardship: applying the right fertilizer source at the right rate, the right time and the right place (see http://www.nutrientstewardship.com/what-are-4rs).
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