Ecological restoration on former agricultural soils: Feasibility of in situ phosphate fixation as an alternative to top soil removal

In Europe, high phosphorus (P) concentrations form the most important constraint on the ecological restoration of biodiverse vegetation on former agricultural soils, because they lead to dominance of highly competitive species like Juncus effusus or to algal blooms in flooded situations. Top soil removal is often not sufficient or not possible, so alternative methods have to be found. We therefore investigated whether modified bentonite clay to which 5% lanthanum had been added (LMC) and lime could effectively decrease bioavailable P and phosphate mobilization to the water layer in different soil types.A container experiment was performed using peaty and sandy soils with different Olsen-P concentrations, mixed with different doses of LMC and lime. The soils were exposed to two different common water regimes (moist and flooded). J. effusus seedlings were used as phytometers.Addition of LMC and lime lowered extractable P concentrations in some of the P-rich sandy soils. Only the highest LMC dose was able to decrease phosphate mobilization to the water layer in the sandy soils. However, neither LMC nor lime was sufficiently effective in reducing Olsen-P concentrations and J. effusus growth. Lime addition eventually even led to additional nutrient mobilization by alkalinization and increased mineralization of the soil.Our experiments therefore show that LMC and lime are not feasible alternatives to top soil removal, because they are inefficient in preventing dominance of highly competitive species under moist or shallowly flooded conditions. LMC may only be used to prevent phosphate mobilization to the water layer in deeply flooded situations, which may allow for a more biodiverse vegetation development.