Following the Second World War, farmers in the US and across Europe embraced fertilizers in a big way. Nitrate factories that had been used for making bombs were converted to fertilizer production facilities and the use of nitrogen fertilizers began to skyrocket, enabling farmers to significantly increase their yields. But over time scientists have noticed that the amount of nitrogen-based fertilizer being applied to soils doesn't match the amount of nitrate washing into watercourses. Previous studies have shown that 15–20% of nitrogen applied to agricultural soils in Canada, the US and Europe each year is accumulating somewhere in the system: but where?

To answer this question, Nandita Basu from the University of Waterloo in Canada and her colleagues analysed long-term soil data (collected between 1957 and 2010) from 2069 sites throughout the Mississippi River Basin. Using soil cores to estimate change in fertilizer use over time they were able to estimate how much nitrogen accumulation had occurred. They then modelled the movement of nitrogen to discover how long it takes to flush nitrogen through the system.

Their results show that nitrogen has accumulated in the Mississippi River Basin croplands at a rate of between 25 and 70 kg per hectare per year. "While crops are harvested and removed from the soil system each year, both above and below-ground crop residues remain," said Kim Van Meter, also at the University of Waterloo. Those residues are still chock-full of nitrogen, drawn up by the plant. And the deeper soil environment happens to be ideal for retaining nitrogen. "Deeper organic matter is exposed to less oxygen and temperatures are lower at deeper levels, meaning that less of that organic matter will decompose," added Van Meter, whose findings were published in Environmental Research Letters (ERL).

By modelling the movement of this nitrogen, the scientists showed that the nitrogen that has accumulated in Mississippi River Basin soils over the last 30 years would take more than 30 years to flush through the system, even if all fertilizer application was stopped tomorrow. And the Mississippi River Basin is far from alone – the same will be true for many agricultural regions all over the world.

The impact of excess nitrogen in the watercourse can already be seen from the nitrogen that does leach off land. Lakes, rivers and coastal regions near agricultural fields often suffer from algal blooms and "dead zones". As well as harming wildlife and upsetting the balance of ecosystems, this nitrogen imbalance can significantly reduce water quality. But this study shows that there is much more nitrogen waiting in the wings.

"The legacy nitrogen in soil and groundwater will continue as a source of nitrogen leaching for decades to come, even if agriculture were to cease entirely," said Van Meter. The scientists hope that an awareness of the nitrogen legacy and the time-lag involved could encourage more realistic targets to be set. "Many programmes are started and then abandoned because people are disappointed in the results," added Van Meter. "We advocate a long-term perspective to better plan for and anticipate the impacts of nutrient legacies in intensively-farmed regions."

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