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To clarify the effects of molecular weight of dissolved organic matter(DOM)on the toxicity and bioavailability of copper(Cu)to plants,DOM extracted from chicken manure was ultra-filtered into four fractions according to their molecular weights by means of sequential-stage ultrafiltration technique.Lettuce seeds were germinated by being exposed to the solutions containing Cu2+with or without different fractions of DOM.The concentration of copper in roots,leaves,sprouts and the length of roots were investigated.The results showed that not all fractions of DOM could improve copper availability or toxicity.The fraction of DOM with larger molecular weight more than 1 kDa had higher complexation stability with Cu2+and caused lower concentration of free Cu2+ion in the solution of copper plus the fraction,resulting in lower availability and toxicity of copper to lettuce,hut the fraction with molecular weight less than1 kDa had the opposite function.Therefore,the molecular weight of 1 kDa may be the division point to determine DOM to increase or decrease copper availability and toxicity.