Improving pig manure composting efficiency employing Ca-bentonite

The aims of this study were to investigate the effect of added Ca-bentonite (CB) on nutrient transformation during pig manure (PM) composting and the related effects on Cu and Zn accumulation in crops. Five treatments representing different CB concentrations (0%, 2.5%, 5%, 7.5% and 10% of dry PM weight basis) were mixed with initial feed stock composting system. The end product with application of 5% (wet weight) was also studied in plant growth experiment using Chinese cabbage. The results showed 10% CB amendment could promote dissolved organic carbon (DOC) and organic matter (OM) degradation, improve the total nitrogen, total phosphorus, NO3−−N, NH4+−N transformation and significantly enhance immobilization of Zn and Cu heavy metals. CB addition could reduce metal availability through adsorption and complexation of the Zn and Cu metals ions on inorganic components. The increase in pH and OM degradation was likely responsible for the reduction in exchangeable and bioavailable Cu and Zn in CB amended compost. Further pot experiments demonstrated that 5% CB-added compost application improved the yield of Chinese cabbage (Brassica rapa chinensis) with highest biomass (7.43 ± 1.2 g/pot) and significantly reduced the Cu and Zn contents in Chinese cabbage when compared with control. Finally results indicate 10% CB amendment prior to PM composting should be recommended, because the rate of addition was more effective in the immobilization of Cu and Zn concentration through the composting process.