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 this 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 succession 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 (PNA) and numbers of ammonium‐oxidizing bacteria were determined in the root zone of Holcus lanatus as well as in that of a grass species characteristic of the stage of succession. In the rhizosphere of H. lanatus decreasing PNA and numbers of ammonium‐oxidizing bacteria were observed as the period of non fertilization increased. Within each field no significant differences in PNA were observed between the root zones of H. lanatus and those of the dominant grass species. From these results it is concluded that, in these fields, decreasing nitrification was related only to decreasing ammonium availability and not to species composition. No indications were obtained that allelochemicals were involved in the flow 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 suggested that impoverishment of the grassland soil with respect to nitrogen availability selects against ammonium‐oxidizing bacteria with a relatively high pH optimum.