Plant phosphorus availability of pyrolysed pig slurry related to ammonium and nitrate nutrition

Excessive slurry applications in regions with intensive livestock production are overloading soils with phosphates, which can lead to water pollution. Pyrolysis of pig slurry solids creates a fertiliser that is potentially efficient to store and transport, hence creating the opportunity to export it from affected regions. This study aims to quantify the plant availability of phosphorus (P) from the pyrolysed pig slurry in different soils and in combination with the nitrogen application in the form of nitrate (NO₃–) and ammonium (NH₄⁺), respectively. A pot experiment with maize seedlings (Zea mays L., cv. Amadeo) was conducted under glasshouse conditions to assess changes in plant-available phosphate from pyrolysed and freeze-dried solids in three contrasting topsoils with pH values of 5.2, 6.7 and 7.4 (in 0.01 mol/L CaCl₂). In two separate positive control treatments, P was applied in the form of rock phosphate and Ca(H₂PO₄)₂, respectively, instead of processed pig slurry. To eliminate nitrification in the treatment fertilised with NH₄⁺, the synthetic nitrification inhibitor 3,4-dimethylpyrazol phosphate (DMPP) was utilised. The plant P availability of the pyrolysed and freeze-dried product exceeded the plant P availability of rock phosphate on all tested soils, but pyrolysis lowered it compared to the freeze-dried treatment. Furthermore, the NH₄⁺ nutrition improved plant P availability compared to the NO₃– nutrition. This indicates that pyrolysis potentially leads to the formation of tri- or octa-calcium phosphates rather than crystalline apatite and that the acidification of the rhizosphere by NH₄⁺ nutrition led to the solubilisation of P. Pyrolysis is a promising treatment for making a plant available P fertiliser, however freeze-drying led to an even better result. For the future, both procedures need to be compared economically to achieve optimal utilisation of the scarce resource P.