Effect of Ca-bentonite on Cu and Zn Forms in Compost and Soil, and Their Absorption by Chinese Cabbage

Abstract

Aerobic composting was conducted to evaluate the effects of the different ratios (0%, 2.5%, 5%, 7.5%, and 10%) of Ca-bentonite (CB) on the speciation of Cu and Zn during pig manure composting, while pot experiment was performed to investigate the role of CB-added compost on the bio-availability of Cu and Zn to Chinese cabbage and its biomass. The results showed that the exchangeable fractions of Cu and Zn decreased, while their oxidized and residual fractions gradually increased during composting; CB addition reduced the distribution ratios of bio-available Cu and Zn in mature compost by approximately 19.84%-48.90% and 4.94%-16.39%, compared to those in the 0% CB-added treatment, and the best result was found in the 10% CB-added treatment.. Meanwhile, the pot experiment confirmed that the addition of compost considerably increased soil organic matter (OM) and electrical conductivity (EC) but decreased soil pH value. On the contrary, the application of CB effectively decreased EC and increased soil pH but inhibited OM mineralization in soil as compared to non-amended treatment. While addition of compost significantly increased plant dry biomass as compared to that with soil alone, maximum biomass was obtained[(6.51±0.34) g·pot-1] in 10% CB-added compost. After the application of CB-added compost, the contents of the bio-available factions of Cu in the harvested soil increased, while the contents of the exchangeable fractions of Zn decreased by 38.91%, 43.69%, 46.02%, 45.12%, and 54.65%, respectively. The absorption of Cu and Zn by Chinese cabbage was considerably reduced after the application of compost, while the uptake of Zn in the plant declined as the CB rates increased, and the absorption of Cu increased. The study indicated that 10% CB addition could significantly reduce the bioavailability of Cu and Zn in compost, and also showed a continuous effect on restricting the bioavailability of Zn after land utilization. Meanwhile CB amendment enhanced Chinese cabbage biomass and reduced the uptake of Zn.