1. The extent of nodulation and nitrogen fixation by a population ofHippophaë rhamnoides bushes occurring on a sand dune system on the east coast of England has been studied.
2. Nodules were found on all plants (the latter ranged from 3 to 16 years in age) and using the isotope N15 it was confirmed that these nodules fixed nitrogen. The nodules on the youngest plants were most active in fixation but because of the greater total nodule mass older plants fixed the largest quantities of nitrogen.
3. Large increases in total nitrogen (i.e. soil plus plant nitrogen) occurred with increase in age of the plants, such nitrogen increases ranging from 27 kg per hectare per annum in the presence of bushes 0 to 3 years old to 179 kg per hectare per annum in the presence of bushes 13 to 16 years old.
4. The N15-technique indicated that a proportion of the nitrogen accumulated was due to fixation byHippophaë but nitrogen contributions from other sources could not be ruled out.
5. The levels of nitrate-nitrogen and ammonium-nitrogen in the soil underHippophaë showed a marked seasonal variation, increasing to a maximum in the winter and to a minimum in the summer. These levels were in general inversely related to the level of average monthly rainfall. Because of the low levels of combined nitrogen in the soil during the summer months it seems unlikely that combined nitrogen should markedly inhibit fixation in this situation.