Effects of different innovative bone char based P fertilizers on bacteria catalyzing P turnover in agricultural soils

Phosphorous (P) is one of the most critical macronutrient elements for plant growth, yield and quality. However, natural P sources are finite and an improved P recycling is necessary. Therefore, we investigated the effect of bone char (BC) and bone char plus (BCplus) as recycling products and alternative P fertilizers, on the abundance of microorganisms, which catalyze major steps in P turnover in a field experiment in Central Germany. The effects were compared to conventional triple super phosphate (TSP) and no P fertilization. Samples were analyzed from soils with three different initial soil P concentrations (very low, low, optimal) and three times during winter wheat cultivation (stem elongation, heading, ripening) to reveal interactions of fertilizers and soil properties. Abundances of microorganisms involved in P uptake, solubilization and mineralization were assessed by quantitative real time PCR (qPCR). Additionally, potential acidic- and alkaline phosphatase activity, water extractable P and plant available P were measured. Bacterial strategies to maintain P pools differed among the treatments. While the addition of BCplus increased the solubilization potential, the low P concentration in control plots and slow release of P from BC favor P recycling from biomass and P inducible uptake systems, which is displayed by either high abundance bacteria harboring the phoD or pstS gene, respectively. All effects were most pronounced at the time of heading and in soils with optimal initial P concentration. It can be assumed that sulfurization of bone char (BCplus) influences bacterial P turnover by promoting solubilization of the fertilizer thereby increasing P availability for plants. Additionally, plant development stage and initial soil P concentrations hamper the effect of BC and BCplus on bacterial P turnover.