Assessment of heavy metal content and pollution in organic and conventional farming soils in North China

To study the effects of conventional and organic farming systems on soil heavy metals contents and heavy metals pollution degree, an experiment was conducted in typical organic and conventional vegetable and wheat fields in five regions of North China (Suning, Quzhou of Hebei Province; Feicheng, Gaotang, and Cao County of Shandong Province). The contents of Cu, Zn, Pb, Cr, Cd and As in 020 cm soils under conventional and organic cropping systems of greenhouse vegetable, open-air vegetable and wheat were measured in the five regions. The pollution degrees of heavy metals under different cropping systems were assessed by using single and comprehensive pollution indexes, geo-accumulation index, and potential ecological hazard coefficient and potential ecological risk index. The results showed that compared with conventional farming, organic farming balanced soil pH, alleviated soil salinization, significantly increased soil organic matter content and cation exchange capacity (CEC) and reduced biological effectiveness of heavy metals in soils in different regions. Also compared with background values, soil contents of heavy metals increased and accumulated to different degrees in five regions. Compared with conventional farming, organic farming effectively reduced contents Cr and Cd in soil. There was obvious enrichment and accumulation of Cu, Zn and As in organic wheat fields because of long-term application of large amounts of organic manure. The use of large quantities of organic manure, chemical fertilizers and pesticides in conventional greenhouse vegetable fields increased the accumulation of Cu, Zn and As in soil than in organic greenhouse vegetation fields. The contents of Cu, As and Zn in open-air organic vegetable fields were not significantly different from those in conventional vegetable fields. The comprehensive pollution index evaluation showed that soils in different investigated regions were lightly polluted and the main contributors were Cd, Cu and Zn. The geo-accumulation index evaluation showed no-pollution to moderate pollution levels in the investigated fields of different regions. Cd is the most obvious pollution heavy metal. Potential ecological hazard index assessment showed a slight potential of ecological risk in the investigated regions and that Cd had a higher ecological risk and showed a slight ecological damage. The research results showed that compared with conventional farming, organic farming comprehensively reduced pollution levels and pollution risks of heavy metals and alleviated ecological hazards of heavy metal in soils in the investigated regions.