Nitrogen desposits from human activities such as farming is causing the diversity of plant species to decline, a new study in the United States has found.
The study looked at 15,000 sites across the United States. In about a quarter of the sites, nitrogen deposits were causing loss of plant species.
Nitrogen levels in soil and water are increasing due to factors such as fossil fuel combustion, agricultural fertiliser application and livestock waste, such as cow urine.
While smaller amounts of nitrogen may act as fertiliser, stimulating growth in plants, large accumulated amounts can decrease soil health and cause a loss in the number of plant species.
A professor of plant ecology at UC Riverside, Edith Allen, said the team of researchers found nitrogen deposits exceeded critical loads for loss of plant species' richness in 24 percent of the 15,000 sites they examined.
Those sites included forest, woodland, shrubland and grassland sites across the continental United States.
They found the effects of nitrogen deposits were more pronounced in dry rather than wet climates.
The researchers said the identification of vulnerable ecosystems and influential environmental factors was critical for managers to set monitoring and conservation priorities.
Global emissions of reactive nitrogen to the atmosphere alone and subsequent deposit on to land have tripled in the last century.
Past studies in Europe and the United States have also shown elevated nitrogen levels cause plant species' richness to decline.