Human activity has greatly perturbed the nitrogen cycle through increased fixation by legumes, by energy and fertilizer production, and by the mobilization of N from long-term storage pools. This extra reactive N is readily transported through the environment, and there is increasing evidence that it is changing ecosystems through eutrophication and acidification. Rothamsted Experimental Station, UK has been involved in research on N cycling in ecosystems since its inception in 1843. Measurements of precipitation composition at Rothamsted, made since 1853, 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 10 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. Recent measurements with diffusion tubes and filter packs show large concentrations of nitrogen dioxide of c. 20 mu g m(-3) in winter and c. 10 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 deposition velocities from the literature gives a total deposition of all measured N species to winter cereals of 43.3 kg N ha(-1) yr(-1), 84 % as oxidized species, 79 % dry deposited. The fate of this N deposited to the very long-term Broadbalk Continuous Wheat Experiment at Rothamsted has been simulated using the SUNDIAL N-cycling model: at equilibrium, after 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 leached, 12% is denitrified, 30% immobilized in the soil organic matter and 53 % taken off in the crop. The 'efficiency of use' of the deposited N decreases, and losses and immobilization increase as the amount of fertilizer N increases. The deposited N itself, and the acidification that is associated with it (from the nitric acid, ammonia and ammonium), has reduced the number of plant species on the 140-yr-old Park Grass hay meadow. It has also reduced methane oxidation rates in soil by 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 pool dilution techniques show a large nitrate pool and enhanced rates of nitrification relative to immobilization. Ratios of gross nitrification:gross immobilization might prove to be good indices of N saturation.
