October 2007 10:55
Nitrogen ö the silent species eliminator

http://www.esf.org/media-centre/press-releases/ext-single-news/article/nitroge
n-the-silent-species-eliminator-341.html

Nitrogen pollution from agriculture and fossil fuels is known to be
seriously damaging grasslands in the UK. A new European study is
starting to show that the effect is Europe-wide, confirming that
current policies to protect ecosystems may need a re-think.

When Carly Stevens finished her PhD in 2004, her findings were so
significant they were published in Science. Not only that, they were
selected as contributing to one of the top ten scientific
breakthroughs of that year ö quite something for a PhD student.
Stevens had found the first evidence that nitrogen deposition from the
atmosphere was depleting numbers of plant species in British
grasslands. ăThere was experimental evidence that this could happen,
but we were the first to show the effect is real and happening now,ä
says David Gowing, one of Stevensâ PhD supervisors at The Open
University in the UK.

Stevens studied acid grasslands ö upland pastures with relatively
infertile soils. She found that in places where more nitrogen is
deposited, there are fewer plant species. The gradient was so
pronounced that one species has been lost for each additional 2.5 kg
of nitrogen per hectare deposited every year. Nitrogen from man-made
sources, like intensive farming and cars, causes significant air
pollution in the UK, and some is deposited from the air on to the
land. Deposition is highest in densely-populated areas, and in Britain
ranges from about 5 to 35 kg of nitrogen per hectare per year.

The approach to protecting wildlife from nitrogen pollution is to
calculate critical load values for different ecosystems ö how much
nitrogen a system can accumulate every year before damage occurs.
Infertile habitats, like heathlands and bogs, are the most vulnerable.
But Stevensâ research showed that species are being lost even where
deposition is Îbeneathâ the critical load for grasslands.

ăThe species arenât going extinct,ä Stevens stresses, ăbut if this is
happening everywhere, we are moving towards much more species-poor
grasslands, and we have no idea what the knock-on effects of that will
be.ä

So last year, Stevens, her UK colleagues David Gowing, Nancy Dise and
Owen Mountford, and a team of experts from Germany, the Netherlands
and France, embarked on a Europe wide project, part of the European
Science Foundation (ESF) EuroDIVERSITY Programme. The projectâs aim is
to see if the effects are the same on a wider range of grasslands,
across the entire Atlantic side of Europe. ăThe low countries and
northern Germany are the epicentre of European nitrogen deposition,ä
says Gowing.

70 new grasslands in at least nine countries have been added to the
picture, including different types of grassland. So far, the first
yearâs field results seem to adhere to the pattern, showing that
species loss is directly related to long term deposition of nitrogen.
ăThe loss in Great Britain is much larger than people had imagined,ä
says Dise. ăItâs almost 25% of species at the average deposition rate.
If this is occurring across Europe, it will be a very important find.ä
Wildflowers and other broad-leaved species, rather than grasses, are
the hardest hit.

The team has started experiments to see if they can establish how
extra nitrogen has these effects. They hope to predict what will
happen in the future. ăNitrogen deposition in Europe probably peaked
in the 1990s, and is coming down now in many places,ä says Gowing. But
it may not be appropriate for policymakers to relax. ăHaving been
accumulating nitrogen for 40 years,ä he continues, ăwe might be near
the edge of the cliff where communities will suddenly change. Perhaps
weâll be able to say: you have another five years of accumulating at
this rate, so now is the time to act.ä

What should be done? ăWe are hoping for a clear signal that you can
maintain species richness [under nitrogen deposition] by biomass
stripping,ä says Gowing. That means extra mowing and grazing. ăIf we
find one, we can offer a management strategy for nature conservation.ä