Kwt. Goulding et al., NITROGEN DEPOSITION AND ITS CONTRIBUTION TO NITROGEN CYCLING AND ASSOCIATED SOIL PROCESSES, New phytologist, 139(1), 1998, pp. 49-58
Human activity has greatly perturbed the nitrogen cycle through increa
sed fixation by legumes, by energy and fertilizer production, and by t
he mobilization of N from long-term storage pools. This extra reactive
N is readily transported through the environment, and there is increa
sing evidence that it is changing ecosystems through eutrophication an
d acidification. Rothamsted Experimental Station, UK has been involved
in research on N cycling in ecosystems since its inception in 1843. M
easurements of precipitation composition at Rothamsted, made since 185
3, show an increase of nitrate and ammonium N in precipitation from 1
and 3 kg N ha(-1) yr(-1) respectively, in 1855 to a maximum of 8 and 1
0 kg N ha(-1) yr(-1) in 1980, decreasing to 4 and 5 kg N ha(-1) y(-1)
today. Nitrogen inputs via dry deposition do, however, remain high. Re
cent measurements with diffusion tubes and filter packs show large con
centrations of nitrogen dioxide of c. 20 mu g m(-3) in winter and c. 1
0 mu g m(-3) in summer; the difference is linked to the use of central
heating, and with variations in wind direction and pollutant source.
Concentrations of nitric acid and particulate N exhibit maxima of 1.5
and 2 mu g m(-3) in summer and winter, respectively. Concentrations of
ammonia are small, barely rising above 1 mu g m(-3). Taking depositio
n velocities from the literature gives a total deposition of all measu
red N species to winter cereals of 43.3 kg N ha(-1) yr(-1), 84 % as ox
idized species, 79 % dry deposited. The fate of this N deposited to th
e very long-term Broadbalk Continuous Wheat Experiment at Rothamsted h
as been simulated using the SUNDIAL N-cycling model: at equilibrium, a
fter 154 yr of the experiment and with N deposition increasing from c.
10 kg ha(-1) yr(-1) in 1843 to 45 kg ha(-1) yr(-1) today, c. 5 % is l
eached, 12% is denitrified, 30% immobilized in the soil organic matter
and 53 % taken off in the crop. The 'efficiency of use' of the deposi
ted N decreases, and losses and immobilization increase as the amount
of fertilizer N increases. The deposited N itself, and the acidificati
on that is associated with it (from the nitric acid, ammonia and ammon
ium), has reduced the number of plant species on the 140-yr-old Park G
rass hay meadow. It has also reduced methane oxidation rates in soil b
y c. 15 % under arable land and 30 % under woodland, and has caused N
saturation of local woodland ecosystems: nitrous oxide emission rates
of up to 1.4 kg ha(-1) yr(-1) are equivalent to those from arable land
receiving > 200 kg N ha(-1) yr(-1), and in proportion to the excess N
deposited; measurements of N cycling processes and pools using N-15 p
ool dilution techniques show a large nitrate pool and enhanced rates o
f nitrification relative to immobilization. Ratios of gross nitrificat
ion:gross immobilization might prove to be good indices of N saturatio
n.