Fertiliser could be too much of a good thing for the world's grasslands, according to study findings, published in the journal Nature on Sunday.

The worldwide study shows that, on average, additional nitrogen will increase the amount of grass that can be grown. But a smaller number of species thrive, crowding out others that are better adapted to survive in harsher times. It results in wilder swings in the amount of available forage.

Johannes M.H. Knops, a University of Nebraska-Lincoln biologist and one of the paper's international co-authors elaborated, "More nitrogen means more production, but it's less stable. [Otherwise put,] There are more good years and more bad years. Not all years are going to be good and the bad years are going to be worse."

The three-year study monitored real-world grasslands at 41 locations on five continents. The sites included alpine grasslands in China, tallgrass prairies in the United States, pasture in Switzerland, savanna in Tanzania and old fields in Germany.

The study found common trends among grasslands around the world:

• Natural – unfertilised – grasslands with a variety of grass species have more stability because of species "asynchrony," which means that different species thrive at different times so that the grassland produces more consistently over time. This finding was consistent with the findings of previous, single-site studies as well as previous biodiversity experiments conducted in Europe.
• Fertilised plots saw declines in the numbers of species compared to unfertilised control plots. The plots averaged from 4.4 species to 32.3 species per square meter and declined by an average of 1.3 species per site.
• Fertilisation reduced species asynchrony and increased the variation in production levels over time compared to control plots. This weakened the benefits of species diversity seen in the un-manipulated plots.

While public attention has grown about elevated levels of carbon dioxide and global warming, Knops said elevated levels of mineral nitrogen in the environment also are concerning. Grasslands are affected by nitrogen deposition that results from burning fossil fuels, as well as from fertiliser runoff and ammonia volatilisation from cropland.

Knops said fertiliser overuse could intensify the detrimental effects of drought on grasslands, a particular concern for farmers and ranchers in the United States, where drought devastated cattle herds in Texas and Oklahoma from 2011-13. Texas, which was hardest hit, lost about 15 percent of its cattle herd (2 million animals) as a result of the drought. In Western Europe, rainfall is set to become more erratic as the climate changes, with more rain falling overall, but in fewer, heavier deluges. This could lead to cycles of drought and flooding, as has been seen in the UK in recent years.

According to the researchers, overuse of fertiliser could also have knock-on effects during 'bad years' by reducing the plant cover, which increases erosion, and decreases water filtration and carbon sequestration benefits provided by grasslands.