Frédéric Janssens

Relationship between soil chemical factors and grassland diversity

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 phosphorus per 100 g of dry soil (acetate + EDTA extraction), no station containing more than 20 species per 100 m2 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–NO3 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–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 hypothesis, nitrogen would remain the main element limitating plant diversity but its availability would be controlled by phosphorus.

Phosphorus removal by a synthetic iron oxide–gypsum compound

Phosphorus pollution is a major concern for soil and water management. This study assesses the phosphate and organic phosphorus removal capacity of an iron oxide-gypsum compound (named OX) in batch trials. Phosphate solutions ranging from 0.001 to 10 mg P l(-1) were tested and OX proved to be an effective fixing agent. Solutions with different ionic strengths did not affect this reactivity. Phosphate removal was not altered by pH values between 4 and 8, but increased significantly with higher values. Near-completion of this reaction was observed after some minutes. The combined effects of precipitation with calcium (gypsum) and sorption onto the oxide explain these interesting properties. The phosphate removal capacity was demonstrated on field samples. The compound also promoted the hydrolysis of a model organic phosphorus molecule. Contact of OX with solutions with pH values between 4 and 10 did not alter its stability but caused pH levelling to neutral values. Other secondary effects involve sulfate and calcium release. OX reactivity with different phosphorus species under various conditions is interesting for application to water and soil remediation processes aiming to control phosphorus pollution