ELIMINATION OF PHYTOTOXICITY DURING CO-COMPOSTING OF SPENT PIG-MANURESAWDUST LITTER AND PIG SLUDGE

A plant seed germination technique was used to assess the phytotoxicit y of spent pig litter-sludge at different stages of composting in forc ed-aerated piles on the relative seed germination relative root elonga tion and germination index (GI, a factor of relative seed germination and relative roof elongation) of Chinese cabbage (Brassica parachinens is) and Chinese spinach (Amaranthus espinosus). The relative seed germ ination and root elongation of the two plants were significantly retar ded by the spent litter-sludge extracts at day 0, bur their values inc reased as composting progressed. From day 49 onwards, their values wer e 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 N H4+-N, water-extractable Cu and Zn of the compost demonstrating that t hese chemical compounds were gradually eliminated during composting. T he multiple regression analyses showed that the NH4+-N and water-extra ctable 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, bott om and surface of the pile during the first 49 days of composting. How ever, from day 49 onwards, there was no difference among the four loca tions 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 disapp eared as the sl,ent litter-sludge became mature. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.