Phosphorus availability in different types of open-cast mine spoil and the potential impact of organic matter application

Greenhouse experiments were conducted in order to determine for carboniferous and non-carboniferous mine spoil substrates from the Lusatian lignite mining area (i) the suitable extraction method for plant available P, (ii) the soil capacity for immobilisation of P and (iii) the impact of sewage sludge and compost on P availability. Ca-lactate extraction (DL) and NH4F-extraction (Bray) were both suited equally well for the determination of plant available P as they extracted similar amounts of P on both spoils, they showed a close correlation with each other (R=0.97 2) and they showed a close relation with plant P uptake (R2=0.63 and R2=0.66, respectively). Phosphorus recovery from limed carboniferous mine spoil five days after mineral fertiliser application was only 50%, and decreased to 30% after 54 days. As pH was increased from 3.0 to 5.0 the amount of P immobilised decreased only by about 5%. Several pH dependent processes of P immobilisation and release could occur concurrently counteracting each other. One process could be P sorption to newly formed hydroxy-Al-surfaces but P desorption could also take place as pH increases by decreasing surface positive charge. Finally, due to high Ca concentrations in spoil solution formation of Ca-phosphates, even at lower pH values, cannot be excluded as a possible mechanism of P immobilisation. As part of the P is bound in organic matter, application of P with organic matter resulted in a lower P recovery compared to mineral P-fertiliser. However, the amount of P recovered did not differ between carboniferous and non-carboniferous mine spoil, if P was applied in the form of organic matter, indicating that the application of P with organic matter might be a measure to overcome P immobilisation in carboniferous mine spoils.