Plant and Soil 202: 69–78, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 69 Relationship between soil chemical factors and grassland diversity F. Janssens 1 , A. Peeters 1 , J.R.B. Tallowin 2 , J.P. Bakker 3 , R.M. Bekker 3 , F. Fillat 4 and M.J.M. Oomes 5 1 Laboratoire d’Ecologie des Prairies, UCL, Louvain-la-Neuve, BeIgium * 2 Institute of Grassland and Environ- mental Research, North Wyke, Great Britain, 3 Laboratory of Plant Ecology, University of Groningen, Haren, The Netherlands, 4 CSIC-lnstituto Pirenaico de Ecologia, Jaca, Spain and 5 DLO-Research Instituto and Soil Fertility (AB-DLO), Wageningen, The Netherlands Received 19 July 1997. Accepted in revised form 7 April 1998 Key words: grassland, nitrogen, nutrient competition, phosphorus, plant diversity, potassium Abstract Many studies carried out during these last few years have focused on the factors influencing plant diversity in species-rich grasslands. This 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 implies to know the living conditions of the associations to be recreated. Very often, the typical species of these communities have become so rarefied that the seed bank or the seed rain are not sufficient to recreate the plant community. Most of the time, to achieve the restoration of 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 factors and the plant diversity of a great number of stations. This research has illuminated the relationship between soil extractable phosphorus and potassium and plant diversity. Over 5 mg of phosphorusper 100 g of dry soil (acetate + EDTA extraction), no station containing more than 20 species per 100 m 2 has been found. The highest number of species 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 species is found at 20 mg/100, a value corresponcing to an optimum content of the soil for plant nutrition. High potassium contents, in opposition to phosphorus contents, are thus compatible with high values of diversity. Other factors (i.e. pH, organic matter, total nitrogen and calcium) do not show so clearly a relation with plant diversity. Excess of N–NO 3 is known for its negative effect on the diversity of plant communities. In these environments, apart from the atmospheric deposits which can be important in some areas, N–NO 3 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–NO 3 supply by these two sources. In this hypothesis, nitrogen would remain the main element limitating plant diversity but its availability would be controlled by phosphorus. Introduction The grassland management intensification of these last decades has considerably reduced the importance of diversified swards in West-Central Europe (Baldock, 1990). The main factors affecting grassland diversity are the cut regimes (especially silage involving early * FAX No: +32 10 472428. E-mail: Janssens@ecop.ucl.ac.be cuts) and the increase of fertilizer use (especially ni- trogen and phosphorus). The grassland flora, which is the most species-rich ecosystem in the concerned area, has become more and more simplified to eventually be reduced, in some cases, to its lowest value: the monospecific sward. The maintenance of some plant communities in nature reserves has nowadays become a common practice to slow down this evolution. This solution is certainly very useful but it is insufficient.