Environmental application and phytotoxicity of anaerobic digestate from pig farming by in vitro and in vivo trials

Anaerobic digestate is a by-product from the biogas production which can be applied as replacement for mineral fertilizers. But digestate has both phytostimulating and phytotoxic effects on plants. Fertilizers toxicity and phytotoxicity should be measured to preserve the environment. The phytotoxicity of digestate from anaerobic digestion of swine manure was evaluated using several official protocols. Germination toxicity was tested on Pisum sativum L. and Lepidium sativum L., roots elongation on Hordeum vulgare L. and shoots elongation on Lactuca sativa L. To determine digestate growth toxicity, pot experiments were performed on two Solanum lycopersicum L. cvs. Digestate dilutions ranging from 5 to 30 % were tested in the different assays. Standard protocols were applied to non-standard species: Cichorium endivia L. (shoots elongation) and two Capsicum annuum L. cvs (pot trials). Digestate concentrations stimulating germination and early life stages were between 2 and 3 %. Pot trials suggested that during more advanced growth stages, digestate can be applied at 20–30 %. The main finding was that to minimize digestate phytotoxicity, direct contact with germinating seeds or young plants should be avoided. During plants late growth, digestate administration might be increased carefully to minimize salinity and ammonia excess. Soil samples from the pot trials were analysed at the starting, mid- and final experimental stages. Conclusion was that digestate utilization may have also a soil liming effect, increasing electric conductivity, pH and contrasting soil acidification.