Origin of selective inhibitors of legume-seed germination in sludge composts

Abstract

Water extracts (20 g/l) of the compost produced from activated sludge and coffee residue were found to selectively inhibit seed germination of some legumes, especially white clover (Trifolium repens L.). In order to identify the inhibitors and their origin, the water or alkaline extracts (mixture of 2 N NaOH and MeOH, 1: 1 in volume) of the compost and its raw materials were analyzed by GC-MS after chromatographic separations. The inhibitors in the compost extracts were identified as 3,4-dichlorobenzoic acid (3,4-DCB), 3,4 -dichlorophenylacetic acid (3,4-DCP), 3,4,5-trichlorobenzoic acid (3,4,5-TCB), 3,4,5-trichlorophenylacetic acid (3,4,5-TCP) and mono-2-ethylhexyl phthalate. The contents of 3,4-DCB, 3,4-DCP, 3,4,5-TCB and 3,4,5-TCP in the water extracts were about 60, 75, 35 and 60% of those in the alkaline extracts, respectively. These chlorinated compounds were not found in the water extracts of the raw materials and in the alkaline extracts of the coffee residue, but a high concentrations of 3,4-DCP and 3,4,5-TCP were found in the alkaline extracts of the sludges. A composting experiment of mixture of sludge and coffee residue revealed that a part of these compounds became water soluble in a week. These results indicate that the precursors in the sludge might be converted to water soluble forms during the composting process.