Characteristic of heavy metal contents in agricultural wastes and agricultural risk assessment

In order to understand the pollution status of heavy metals of main agricultural wastes in Jiangxi province and evaluate its long-time use safety of the agricultural wastes reusable products (for example, organic fertilizer), rice straw samples, paddy soil samples, vegetable waste samples, vegetable soil samples, pig manure samples and cow manure samples were collected in 9 countries of Jiangxi provinces. Eight kinds of heavy metals (Cr, Ni, Cu, Zn, As, Cd, Hg and Pb) contents in the samples were determined by ICP-MS and AFS, and then risk assessments were carried out. The results showed that the concentrations and standard exceeding rates of heavy metals in plant wastes were significantly lower than those in animal wastes, such as the levels of Cu and Zn in animal wastes were 10 to 100 times of those in plant wastes. Besides, the standard exceeding rates of Zn, Cu and As in pig manures were 86.1%, 83.3% and 13.9%, respectively, and the standard exceeding rates of Zn and Cu in pig manures were both 16.7%. Only the concentrations of Cd in rice straw samples exceed the limit standard (4.2%), and the concentrations of Cd, Zn and Ni in vegetable waste samples exceed the limit standards, the standard exceeding rates ranged from 3.6% to 7.1%. The order of heavy metal contents in plant wastes were Zn>>Cr>Ni>Cu>As, Pb, Cd>Hg, while that in animal wastes were Zn>Cu>>Ni>As, Pb, Cr>Cd>Hg. According to the Nemero composite pollution indexes, the comprehensive pollution levels in agricultural wastes of Jiangxi province were: pig manures>cow manures>vegetable wastes>rice straw, their composite pollution indexes were 3.32, 1.20, 0.34 and 0.29, respectively. Pig manures were seriously polluted, and cow manures were slightly polluted, and vegetable wastes and rice straw were at safety levels. If these agricultural wastes were served as organic fertilizer raw materials respectively, which were applied to greenhouse soil for a long-term, accumulation rates of heavy metals and their safe service lives in greenhouse soil were calculated. Accumulation rates of most heavy metals in greenhouse soil with animal manures were higher than that of plant wastes, especially of Zn and Cu. By using pig manure organic fertilizer, the levels of Cu, Cd and Zn in the greenhouse soil will be out of the limit standards after 8.4 years, 15.3 years and 23.9 years, respectively. By using cow manure organic fertilizer, the level of Cu in the greenhouse soil will exceed the limit standard after 23.3 years, the level of Cd in the greenhouse soil will exceed the limit standard after about 29 years by using vegetable wastes organic fertilizer or rice straw organic fertilizer. Obviously, the main risk factors of agricultural utilization of pig manure organic fertilizer are Cu, Cd and Zn, the risk factors of agricultural utilization of cow manure organic fertilizer is Cu, the risk factors of agricultural utilization of plant waste organic fertilizer is Cd. It should be strictly restricted the contents of heavy metals in agricultural wastes when used as raw materials of organic fertilizer. The application safety of organic fertilizers made by agricultural wastes remains long-term monitoring.