DEVELOPMENT OF THE NITROGEN-CYCLE IN THE SOILS OF A COASTAL DUNE SUCCESSION

The development of the soil nitrogen cycle in a coastal dune successio n (xerosere) on the East Frisian Island Spiekeroog was investigated. I n 13 sites of different successional age the nitrogen pools in the. to psoil (0-30 cm and the organic soil layers were determined. The nitrog en mineralization was studied at the sites during the vegetation perio d in 1987 (38 weeks) and winter 1990-91 (14 weeks). Turnover rates and nitrification rates were calculated. The nitrogen pool of the soil wa s very low in the first stage of the succession (< 100 kg N ha-1 30 cm -1) and increased rapidly to more than 1500 kg N ha-1 within approx. 2 00 years. This accumulation rate was strongly exposition-dependent. Mi neralization of nitrogen was low in the young sites (15 kg N(min) ha-1 year-1 30 cm-1) and increased to more than 100 kg N(min) ha-1 year-1 in sites covered by buckthorn (Hippophae rhamnoides) and in dune fores t. Mineralization was also exposition-dependent. The annual nitrogen t urnover 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. Nitr ification dropped as well in the course of the dune development from 8 0-90% to 20% nitrate of total mineral nitrogen production over a year. Some hypotheses on nitrogen accumulation and retention during ecosyst em and plant cover development are discussed.