Development of the nitrogen cycle in the soils of a coastal dune succession

The development of the soil nitrogen cycle in a coastal dune succession (xerosere) on the East Frisian Island Spiekeroog was investigated. In 13 sites of different successional age the nitrogen pools in the topsoil (0-30 cm) and the organic soil layers were determined. The nitrogen mineralization was studied at the sites during the vegetation period in 1987 (38 weeks) and winter 1990-91 (14 weeks). Turnover rates and nitrification rates were calculated. The nitrogen pool of the soil was very low in the first stage of the succession (<100 kgNha'1 30 cm'1) and increased rapidly to more than 1500 kgNha'1 within approx. 200 years. This accumulation rate was strongly exposition-dependent. Mineralization of nitrogen was low in the young sites (15 kg Nmin ha'1 year'1 30 cm “ ’) and increased to more than 100 kg Nmin ha “1 year “1 in sites covered by buckthorn (Hippophaë rhamnoides) and in dune forest. Mineralization was also exposition-dependent. The annual nitrogen turnover rate was very high in the young dune sands (up to 18%) and was only 2% in the brown dune sand podsols under Empetrum heathland. Nitrification dropped as well in the course of the dune development from 80-90% to 20% nitrate of total mineral nitrogen production over a year. Some hypotheses on nitrogen accumulation and retention during ecosystem and plant cover development are discussed.