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Blind fertiliser use killing our grasslands: Study

Posted on Feb 17 2014 | IANS

Washington, Feb 17 : Surprised at the disappearing grasslands across the world? The blind use of fertiliser is the culprit that may have destabilised grasslands at 41 locations on five continents.

Grasslands are affected by nitrogen deposition that results from burning fossil fuels, as well as from fertiliser runoff and ammonia volatilisation from cropland.

A worldwide study shows that, on average, additional nitrogen would 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.

"More nitrogen means more production, but it's less stable. 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," warned Johannes M.H. Knops, biologist at University of Nebraska-Lincoln in the US.

Knops said fertiliser overuse could intensify the detrimental effects of drought on grasslands - such as the drought that devastated cattle herds in Texas and Oklahoma from 2011-13 when Texas lost about 15 percent of its cattle herd, or about 2 million animals.

Natural, unfertilised grasslands with a variety of grass species have more stability because of species 'asynchrony'.

It means that different species thrive at different times so that the grassland produces more consistently over time.

Fertilised plots saw declines in the numbers of species compared to unfertilised control plots.

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, said the study.

Knops said elevated levels of mineral nitrogen in the environment also are dangerous.

The study sites included alpine grasslands in China, tallgrass prairies in the US, pasture in Switzerland, savannah in Tanzania and old fields in Germany.

The fertiliser misuse could have ripple effects during bad years by reducing the plant cover - which increases erosion - and decreases water filtration and carbon sequestration benefits provided by grasslands.

The study was published in the journal Nature.

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