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 Cu^2＋ 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 Cu^2＋ and caused lower concentration of free Cu^2＋ ion in the solution of copper plus the fraction, resulting in lower availability and toxicity of copper to lettuce, but the fraction with molecular weight less than 1 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.
Xudong Wang,Xianni Chen,Shuai Liu,Xizu Ge College of Resource and Environment,Northwest Agriculture and Forestry University,Shaanxi 712100,China
Chicken manure was composted for 110 d to evaluate changes in organic carbon (C) and humic substance-complexed copper (HS-Cu) extracted with water,sodium hydroxide (NaOH),and sodium pyrophosphate and sodium hydroxide mixture (Na 4 P 2 O 7-NaOH).The average HS-Cu content from the Na 4 P 2 O 7-NaOH extraction accounted for 46.8% of the total Cu in the compost,followed by those from the NaOH and water extractions,which accounted for 34.5% and 6% of the total Cu in the compost,respectively.NaOH-Na 4 P 2 O 7-extractable HS-Cu increased throughout the composting process.NaOH-extractable HS-Cu increased during the first 50 d of composting and then leveled off.Water-extractable HS-Cu enhanced during the first 21 d and then decreased.Water-extractable HS-Cu was not correlated to NaOH-or NaOH-Na 4 P 2 O 7-extractable organic C.The average amount of Cu leached from the compost was about 20% of the total Cu in the chicken manure and Cu leaching increased during the first 35 d of composting and then decreased.Leaching of Cu was significantly correlated to the concentrations of HS-Cu extracted with NaOH and water,but was not correlated to the concentration of HS-Cu extracted with NaOH-Na 4 P 2 O 7.Thus,NaOH-and water-extractable HS-Cu were relatively labile and NaOH-Na 4 P 2 O 7-extractable HS-Cu was relatively immobile.Composting process after rapid decomposition could result in low potential of Cu leaching because of formation of more NaOH-Na 4 P 2 O 7-extractable HS-Cu.
