Effects Of Simulated Acid Rain On Availability Of Heavy Metals From The Swine Manure-amended Vegetable Soils And Accumulation In Vegetables

Ecological hazards of acid deposition and environmental pollution of heavy metals as global environmental problems are widely concerned both at home and abroad. The acid rain region in southern China is the centre of the strongest and largest acid rain area in Asia. Furthermore, the intensity and frequency of acid rain has been progressively enhanced in recent years. Acid rain not only directly affects plant growth, but also have great impacts on adsorption/desorption, and solution/precipitation processes of heavy metals in soil, which influences their availability to plants and the potential risks of environmental pollution. At the same time, livestock and poultry manure rich in nitrogen (N), phosphorus (P) and organic matter (OM) has been widely used in agricultural production as an important organic fertilizer for a long time. However, in the intensive confined livestock and poultry production systems, more and more trace elements were added to the feeds of animals, which lead to the high concentrations of heavy metals in manures, especially copper (Cu), zinc (Zn) and arsenic (As). Thereby, heavy application of heavy metal-enriched swine manures to vegetable soils would increase potential risks of heavy metal contamination to ecological environment. Up to now, much research has been carried out on heavy metal contamination caused by acid rain or livestock and poultry manures, litter is known about the effects of acid rain on the release and bioavailability of heavy metals from the vegetable soils amended with heavy metal-enriched swine manure, as well as the vegetable biomass and heavy metal accumulation in vegetables. Therefore, as to the wide application of livestock and poultry manure to soils of vegetable plantations in Hangzhou suburb, the effects of simulated acid rain on the release and bioavailability of heavy metals in soil and their accumulation in vegetables were systematically investigated in this study combined with the methods of survey, indoor analysis, equilibrium incubation, cultivation and pot experiments, on the basis of the survey of the contamination of heavy metals to the soils and vegetables and their spatial distribution, in order to provide scientific foundation for the protection of farmland ecoenvironment, the improvement of vegetable quality, the comprehensive utilization of poultry and livestock manure and the treatment of environmental pollution. The main results are summarized as follows:(1) The results showed that the contents of chromium (Cr), nickel (Ni) and mercury (Hg) in the vegetable soils of Hangzhou suburb were less than the local background values, while the contents of Cu, lead(Pb) and Zn were far more than the local background values. Furthermore, more than 40% soils with Cu contents and about 10% soils with with Zn contents exceeded National Standard of Soil Environmental Quality (GB 15618-1995). Soil available concentrations of heavy metals are as follows: Zn＞Cu＞Pb＞Ni＞Cr＞Hg, in which available coefficient of Zn and Cu was more than 0.08, meaning that they were of a high activity.(2) Spatial distribution of heavy metals in the tested area was asymmetric, and greatly spatial variation was found among different heavy metals. Contaminated region with heavy metals such as Cu and Zn was relatively concentrated. Furthermore, similar spatial distribution was observed between total contents and available contents of heavy metals.(3) It was found that the available Cu,Zn,Pb concentrations were positively correlated with their total concentrations, and soil organic matter contents, while they were negatively correlated with the soil pH. And the organic matter content made the greatest contribution to heavy metal bioavailability in these three factors.(4) Equilibrium incubation with acid rain accelerated the soil acidification and increased the concentrations and cumulative release amounts of heavy metals such as Cu,Zn,Cd and Pb, whereas the opposite result was observed for As. Vegetable soil amended with metal-enriched swine manure would result in higher concentrations and cumulative amounts of Zn and Cd incubated by acid rain of pH3.6 relative to pH5.6, although manure could buffer the acidification of soils. Therefore, heavy application of metal-rich swine manures to vegetable soils in the areas subjected to acid rain deposition with high-frequency would increase a potential risk of heavy metal contamination to vegetables and ecological environment.(5) Incubation experiment indicated that acid rain affected the bioavailability of heavy metals by decreasing soil pH. The available concentrations of Zn,Cd,Pb increased with the increasing of H~+ concentration, whereas the opposite result was observed for As. What's more, vegetable soil amended with metal-enriched swine manure as well as the inorganic heavy metal treatment would result in significantly higher available concentrations of Cu, Zn, and As compared to the control.(6) Pot experiment showed that the biomass of pakchoi and radish decreased, whereas the available contents of Cu,Zn,Cd and Pb in soil increased exposed to acid rain, which would result in the accumulation of these metals in vegetables, especially Zn, Cd and Pb. Although the biomass of vegetables amended with swine manure increased, both available contents of heavy metal in soils and total contents in vegetables significantly increased. Meanwhile, Zn, Cd and Pb contents in edible parts of the pakchoi and radish exceeded the state standard of food hygiene of vegetables.