RELATIONSHIP BETWEEN SOIL CHEMICAL FACTORS AND GRASSLAND DIVERSITY

Many studies carried out during these last few years have focused on t he factors influencing plant diversity in species-rich grasslands. Thi s is due to the fact that these ecosystems, among the most diversified in temperate climates, are extremely threatened; in some areas, they have almost disappeared. The re-establishment of these habitats implie s to know the living conditions of the associations to be recreated. V ery often, the typical species of these communities have become so rar efied that the seed bank or the seed rain are not sufficient to recrea te the plant community. Most of the time, to achieve the restoration o f these communities, they have to be totally recreated by sowing. For the restoration or the maintenance of the community, the soil chemical characteristics have also to be appropriate or if not modified. This research tends to establish a relation between some soil chemical fact ors and the plant diversity of a great number of stations. This resear ch has illuminated the relationship between soil extractable phosphoru s and potassium and plant diversity. Over 5 mg of phosphorus per 100 g of dry soil (acetate + EDTA extraction), no station containing more t han 20 species per 100 m(2) has been found. The highest number of spec ies is found below the optimum content of the soil for plant nutrition (5-8 mg P/100 g). Concerning the potassium, the highest number of spe cies is found at 20 mg/100, a value corresponcing to an optimum conten t of the soil for plant nutrition. High potassium contents, in opposit ion to phosphorus contents, are thus compatible with high values of di versity. Other factors (i.e. pH, organic matter, total nitrogen and ca lcium) do not show so clearly a relation with plant diversity. Excess of N-NO3 is known for its negative effect on the diversity of plant co mmunities. In these environments, apart from the atmospheric deposits which can be important in some areas, N-NO3 is derived mainly from the symbiotic fixation of atmospheric nitrogen by legumes as well as from the mineralization of the organic matter of the soil. It is possible that, when in small quantities, the available soil phosphorus could be a limiting factor of the N-NO3 supply by these two sources. In this h ypothesis, nitrogen would remain the main element limitating plant div ersity but its availability would be controlled by phosphorus.