Phosphordüngewirkung karbonisierter Biogasgärreste

P-recycling from biogas residue may play a future key role for sustainable P supply in organic farming. However, in particular transportation costs of residue need to be reduced e.g. by pretreatment. One approach is carbonization by pyrolysis or hydrothermal carbonization (HTC). In a pot trial with maize on a P-deficient, acidic sandy loam P uptake after application of two chars from pyrolysis (400 and 700 °C) and HTC (6 and 8 h dwell time) all made of the same residue was compared to P uptake after application of the raw residue and water soluble KH2PO4 respectively. P uptake was the same for KH2PO4, raw residue and both HTC treatments, but was significant lower in treatments with pyrolytic chars. Neither dwell time nor temperature had an effect. However, whereas pyrolysis raised P concentration, in HTC chars it was the same as in the raw residue. Also P was extracted from soil of pots without plants at beginning, half and end of the trial using H2O, CAT, CAL and Na-formate respectively. For CAT, CAL and Na-formate significant correlations between P uptake by maize and extractable P at all dates exist, whereas for H2O this was only true for extractable P at the end of the trial.