Elimination of phytotoxicity during co-composting of spent pig-manure sawdust litter and pig sludge

A plant seed germination technique was used to assess the phytotoxicity of spent pig litter-sludge at different stages of composting in forced-aerated piles on the relative seed germination, relative root elongation, and germination index (GI, a factor of relative seed germination and relative root elongation) of Chinese cabbage (Brassica parachinensis) and Chinese spinach (Amaranthus espinosus). The relative seed germination and root elongation of the two plants were significantly retarded by the spent litter-sludge extracts at day 0, but their values incresased as composting progressed. From day 49 onwards, their values were similar to that of the control (deionized water) (between 80 and 100%). Increases in both relative seed germination and root elongation of the two plants corresponded with decreases in the concentrations of NH₄⁺-N, water-extractable Cu and Zn of the compost demonstrating that these chemical compounds were gradually eliminated during composting. The multiple regression analyses showed that the NH₄⁺-N and water-extractable Cu concentrations in the spent litter-sludge extracts were the most important chemical factors causing the phytotoxicity. Composting at the top of the forced-aerated pile was slower than the middle, bottom and surface of the pile during the first 49 days of composting. However, from day 49 onwards, there was no difference among the four locations of the spent litter-sludge pile in terms of both plant responses and chemical parameters. These suggest that the spatial variations in the forced-aerated piles, in terms of phytotoxicity, gradually disappeared as the spent litter-sludge became mature.