Long-term phosphorus supply with undigested and digested slurries and their agronomic effects under field conditions

Biogas digestates contain substantial amounts of nutrients and are widely used as fertilizer. However, long-term results of their agronomic effects are limited. A ten-year field experiment was evaluated regarding the phos-phorus (P) fertilizer value of digestates in dependence of different crop species, paying special attention to turnover and translocation of P in the soil profile. The input substrate (inputS) and digested substrate (digestS) from a biogas plant were applied annually to at a rate of 30 m(3) ha(-1). For control, mineral nitrogen and po-tassium, but no P, were given in equal amounts as applied with the biogas substrates. The average annual P uptake of crops was increased by the inputS (31.5 kg ha(-1)) and digestS (28.5 kg ha(-1)) compared to the control (23.5 kg ha(-1)). Regarding the plant available P content (soluble in double-lactate) no differences between the inputS and digestS treatments were found in the topsoil. In the subsoil all P pools measured as well as the degree of P saturation were highest when P was added with the digestS. The activities of soil enzymes were only increased after the application of inputS in comparison to the control and were also affected by the crop species cultivated. Our results indicate that digested slurries have a comparable P fertilizer effect to that of undigested slurries, but they involve a higher risk of P losses due to higher P concentrations in the subsoil.