Phosphorus bioavailability of sewage sludge-based recycled fertilizers

Six phosphorus (P) fertilizers recycled from sewage sludge [Struvite SSL, Struvite AirPrex,P-RoC, Mephrec, Pyrolysis coal and Ash (Mg-SSA)] were tested for their plant availability in potted soil of pH 7.2 under greenhouse conditions. The crop sequence simulated a rotation of red clover (Trifolium pratense L.), maize (Zea maize L.), and ryegrass (Lolium perenne L.). Other P fertilizer treatments included: Phosphate Rock (PR), Calcium dihydrogen phosphate [Ca(H2PO4)2], and an unfertilized control. Additionally, soil was regularly inoculated with two strains of plant growth-promoting rhizobacteria (PGPR; Pseudomonas sp. Proradix, and Bacillus amyloliquefaciens) to test their ability to increase P availability to plants. Sequential P fractionation was conducted to link the amount of readily available P in fertilizers to plant P acquisition. Shoot P content and dry matter of maize decreased in the following order: Struvite SSL >= Ca(H2PO4)2 > P-RoC >= Struvite AirPrex >= Mephrec > Pyrolysis coal >= Mg-SSA >= PR >= unfertilized. Rhizobacteria did not affect shoot biomass or P content. The results show that red clover might have mobilized substantial amounts of P. Sequential P fractionation was not suitable to predict the efficacy of the fertilizers. Generally, the sewage sludge-based fertilizers tested proved to be suitable alternative P sources relevant to organic farming systems. However, the efficacy of recycled fertilizers is strongly dependent on their specific production conditions.