Vertical distribution characteristics of heavy metals in agricultural soil profiles under rice-wheat system in Heigangkou Yellow River Irrigation Region
开封黑岗口引黄灌区稻麦轮作下农田土壤剖面重金属分布特征

Yellow River Irrigation Region(YRIR) is one of China’s important grain production bases.The contents and chemical forms of heavy metals in the agricultural soils of the region are related to sustainable agricultural development.The distribution of main soil properties total organic carbon(TOC),total nitrogen(TN),total phosphorus(TP) and CaCO3] and heavy metals(Pb,Cd,Cu and Zn) in the agricultural soils of YRIR were investigated using 45 soil samples.Five soil profiles(in the 0~100 cm soil layer) and arable soils(in the 0~15 cm and 15~30 cm soil layers) were collected from agricultural lands under rice-wheat rotation system in the northern suburb of Kaifeng.The region belongs to the Heigangkou YRIR,which is sufficiently representative of the YRIR.The chemical forms of heavy metals were analyzed by sequential extraction and the concentrations determined with ICP-AES and ICP-MS.Results showed that under long-term irrigation and rice-wheat rotation system,there were variations in the basic soil prop-erties in the study area at different soil depths.For example,the contents of TOC and TN primarily accumulated in the 0~30 cm depth of arable soils.These were clearly higher than those in the 30~100 cm soil layer.TP content was not remarkably different at different soil depths.Obvious CaCO3 alluvial horizons were also noted in some soil profiles.In the 0~30 cm depth of arable soils,Pb,Cu and Zn contents were similar to that of background soil value.However,Cd content was evidently higher than that of background soil value.Also the average content of the heavy metals was still within WHO threshold,and far below that of China for agricultural soil quality.Factor analysis showed that the sources and transfer characteristics of Pb,Cu and Zn were similar.Cd sources and transfer characteristics were distinct from those of other heavy metals,which was attributed to fertilizer broadcasting and other agricultural factors.While surface accumulation of total Cu and Zn in the soil profiles was obvious,Pb accumulation was not so variable among different soil layers.However,Cd was highly variable at different soil depths.The distribution trends of the heavy metals across the different soil layers were significantly related to heavy metal property,soil profile characteristics and agricultural activity.Cu and Zn were mainly organic and residual fractions in soil profiles.The fraction of residual and organic and Fe/Mn oxide forms were also in relatively high percentages.Total Pb and the associated organic fractions exhibited similar vertical distributions.However,Cd and the associated carbonate and exchangeable forms accounted for considerable percentages.This suggested that Cd was highly mobile in soils and therefore posed potential risks for human health.