Organic nitrogen in UK rainfall (495Kb)

Organic nitrogen in UK rainfall
Researchers usually measure the nitrogen content of
rain in terms of the concentrations of ammonium or
nitrate ions. Plants can use this ‘inorganic’ nitrogen
as a fertiliser, and soil microbes convert it into gases
such as nitrogen, nitrous oxide and nitric oxide. Too
much nitrogen in rain can lead to some plant
species, such as grasses, outgrowing others, such
as wild flowers. It can lead to higher nitrate levels in
streams and lakes, and more nitrous oxide, a
greenhouse gas, being released into the atmosphere.
Until this GANE project started, we did not know
how much organic nitrogen there was in rain,
although reports from other parts of the world, and
occasional measurements in the UK, suggested that
concentrations might be important.
Over two years we took careful measurements,
making sure that microbes did not degrade the rain
samples and birds did not contaminate them. The
measurements showed that around one third of the
nitrogen in UK rainfall is organic (see map). There is
more organic nitrogen in rain in the west. This
correlates with higher concentrations of ammonium
in the atmosphere, suggesting a biological source,
such as agriculture.
We do not yet know whether plants or soil microbes
can use organic nitrogen in the same way as
inorganic nitrogen. However, researchers cannot
ignore it when estimating the total deposition of
nitrogen from the atmosphere.
Ace Stock Ltd/Alamy

Organic nitrogen in UK rainfall
Neil Cape (Principal Investigator) and Margaret Anderson, Centre for Ecology & Hydrology Edinburgh.
Philip Rowland and David Wilson, Centre for Ecology & Hydrology Lancaster.
What we found out
Organic nitrogen makes up 24-40% of the
total amount of dissolved nitrogen in rain
and snow across the UK. The fraction of
water-soluble organic nitrogen was greatest
at western sites, where ammonium
concentrations in rain were also higher.
However, the seasonal pattern across all
sites showed maximum concentrations of
ammonium and nitrate in late spring, but
maximum concentrations of water-soluble
organic nitrogen in late summer (see graph
on right). The processes leading to watersoluble
organic nitrogen in rain are
therefore different from those that emit
ammonia into the atmosphere.
Why these results are important
Policy-makers setting and assessing critical
loads for nitrogen deposition need to take
into account the high levels of watersoluble
organic nitrogen in rain and snow.
How we did it
We measured the concentration of watersoluble
organic nitrogen in rain at seven
sites across the UK, over a period of 1-2
years. We collected rainwater in
continuously open stainless steel funnels
draining to a glass bottle over one to two
weeks. We added thymol to preserve the
rainwater during and after collection. After
chemical analysis, we excluded samples
showing signs of contamination (significant
concentrations of potassium or phosphate
ions). We used two methods to measure
the total dissolved nitrogen: heating the
sample with a strong oxidising agent
(persulphate) to convert organic nitrogen
into nitrate ions, which we then measured
by conventional methods; and heating the
sample to over 1000°C in oxygen, to
convert all nitrogen compounds to nitric
oxide (gas) which we then detected by the
light it generates when it reacts with ozone
– high temperature chemiluminescence.
The latter generally gave the larger values,
and we used it to assess the organic
component of dissolved nitrogen.
Weighted mean concentrations (_M) of inorganic and total N in
precipitation (2000-2002)
Data for total N are presented for both analytical methods:
high temperature chemiluminescence and persulphate digestion/colorimetry
Site Nitrate Ammonium total N organic N %organic persulph. organic N %
total N (persulp.) organic
Cairngorm
North-east Scotland 30 14 58 14 24% 47 2 5%
Bush
East Scotland 15 16 46 15 33% 46 15 33%
Merlewood
North-west England 17 27 87 35 40% 64 12 18%
Moor House
North-west England 20 32 84 31 37% 69 16 23%
Climoor
North Wales 22 33 73 29 39% 61 16 27%
Norwich University
South-east England 38 58 129 33 26% 126 30 24%
Winfrith
South-west England 23 26 75 23 31% 58 7 11%
For more information contact:
Neil Cape
Centre for Ecology & Hydrology, Bush Estate
Penicuik EH26 0QB.
Tel: 0131 445 8533, email: jnc@ceh.ac.uk
Shaded areas show organic fraction
of N in rain.Total area is proportional
to total N concentration.
Publications
Water, Air and Soil Pollution: Focus (2004).
See also: Cape et al., The Scientific World, 1,
230-237 (2001)