Biotic strategies to increase plant availability of sewage sludge ash phosphorus

Sewage sludge incineration-ash (FB-I) represents a potential alternative phosphorus (P) fertiliser with a high concentration of P, although with relatively low crop availability. In this study, we investigated two P-solubilisation approaches (acidification and P mobilisation by citrate) to enhance plant P uptake from the FB-I ash in a pot study by using various biotic strategies: (1) a pre-treatment of ash with a Penicillium bilaiae inoculum, (2) an isogenic line of wheat that excretes citrate from the root tip, (3) nitrogen (N) provided as NH4 + combined with nitrification inhibitor dicyandiamide (DCD). All strategies were tested combined with each other and with different methods for ash application: (1) completely mixed within the top one third of soil in a pot, or (2) applied as distinct band at 10 cm depth. Triple super phosphate (TSP) at a rate of 15mg P kg(-1) soil per pot was sufficient to support maximum shoot growth. Ash mixed into the first top third part of soil in the pot at a rate of 180 mg P kg(-1) soil (equivalent to 60 mg P kg(-1) soil throughout the pot) significantly increased the soil water-extractable P and the subsequent shoot P uptake and shoot biomass for both wheat lines and microbial pre-treatment to support maximum plant performance. Shoot P concentration in these treatments was further enhanced when the plants received NH4 + and DCD, although not leading to a significant increase in shoot biomass. The citrate secretion by the root tips and pre-inoculation with P. bilaiae of the ash did not influence plant growth. In conclusion, root-zone soil acidification by NH4 + nutrition is regarded as a promising strategy to improve the fertilising effect of such alternative P fertilisers originating from urban waste streams.