REPRESSION OF NITRIFICATION IN SOILS UNDER A CLIMAX GRASSLAND VEGETATION

Two hypotheses on repression of nitrification in climax vegetations (i .e. nitrogen immobilization and allelopathy) were investigated. In thi s study the potential nitrification activities and numbers of ammonium -oxidizing bacteria were established in a nature reserve with a series of natural grasslands with vegetational different stages of successio n of plants species. The pastures had not been fertilized for 3, 7, 20 and 46 years, respectively, and the gradual decrease in availability of nutrients had led to pastures dominated by different grass species. In each field soil parameters, potential nitrification activities (PN A) and numbers of ammonium-oxidizing bacteria were determined in the r oot zone of Holcus lanatus as well as in that of a grass species chara cteristic of the stage of succession. In the rhizosphere of H. lanatus decreasing PNA and numbers of ammonium-oxidizing bacteria were observ ed as the period of non fertilization increased. Within each field no significant differences in PNA were observed between the root zones of H. fanatics and those of the dominant grass species. From these resul ts it is concluded that, in these fields, decreasing nitrification was related only to decreasing ammonium availability and not to species c omposition. No indications were obtained that allelochemicals were inv olved in the low nitrification potentials of late stages of succession . The optimum pH of the ammonium-oxidizing community, measured as PNA, decreased as the period of non fertilization increased. It is suggest ed that impoverishment of the grassland soil with respect to nitrogen availability selects against ammonium-oxidizing bacteria with a relati vely high pH optimum.