| In recent decades, due to the rapid development of urbanization and industrialization, soil heavy metal pollution in China has become an important environmental issue. Heavy metal pollution in agricultural soils can cause disturbance of soil function and accumulation of heavy metals in crops. Rice is the dominant food crop in China, it's quality is closely related to human health. Therefore, the study on heavy metal contamination in soil-rice systems is important for soil environmental quality monitoring and safety of agricultural products. Soil heavy metal pollution will lead to changes in microbial community structure, thereby affecting the functional diversity of the soil ecosystem and biochemical processes. Previous studies on the impact of heavy metals on soil microbial community structure, were mostly conducted in laboratory or in field with plot experiments at a small-scale. Therefore, it is necessary to investigate the microbial responses to heavy metals and soil properties at a regional scale, explore the key factors influencing microbial community structure in agriculture soils, and provide guidelines beneficial for sustainable agricultural development and soil environmental protection.The present study was conducted in Wenling City, Zhejiang Province. Based on geostatistics and multivariate statistical analysis, we studied status, trends, potential ecological risks and human health risks of heavy metal pollution in soil-rice system; analyzed the response relationship between the spatial distribution characteristics of the microbial community structure and heavy metals and soil properties. The main results are as follows:(1) Based on geostatistical and spatial analysis methods, the temporal and spatial variation of heavy metals in soil-rice system showed that, heavy metals in soil and rice have a certain spatial distribution in the study area. Compared with 2006, the average concentrations of Cd, Cu and Pb increased with time. The regions with increased Cd and Cu concentrations in soil were most obvious in the study area. The average concentrations of Cd in rice have nearly doubled, and there was a widespread distribution of regions with increased Cd concentrations. This trend needs our attention.(2) According to the second grade of soil environmental quality standard of China, Cd and Cu had the highest over-standard ratio. Compared with the national food hygiene standards, Cd in rice had the highest excess ratio, indicating slight pollution characteristics. Cadmium had the highest potential ecological risk, reaching moderate hazard level. Non-carcinogenic risks faced by adults and children were mainly from eating rice, and they were higher for adult than for children. Residents in the study area have been facing the risk of cancer, and the cancer risk from Cd was greater than from Ni. Potential risk index and health risk index were higher in the north and southwest of the study area.(3) Correlation analysis and RDA analysis showed that, soil Zn and Cu concentrations were closely correlated with the social and environmental factors. Cadmium and Pb were related to agricultural activities. Ni was mainly affected by soil physical and chemical properties. Soil environmental factors explained 43.3% of the total variation of heavy metals.(4) The absolute contents of individual fatty acids and microbial groups were higher in the southwest of the study area.Factor analysis and regression analysis showed that, among the individual fatty acids,18:1?9c, i15:0, i16:0, al7:0, i17:0, cy19:?8c,10Me16:0 and 10Me18:0 were mainly affected by soil physical and chemical properties; 16:00,16:lco5c, 16:1?7c,18:l?7c, a15:0, cyl7:0 and 18:2co6,9c closely related to soil properties and heavy metal concentrations. For the microbial groups, bacteria, Gram-negative bacteria, Gram-positive bacteria and fungi were affected by heavy metals and soil physical and chemical properties. Actinomycetes were mainly affected by soil physical and chemical propertiesSoil organic carbon was the main microbial promoting factor. Among heavy metals, the microbial PLFAs were mainly affected by Cd and Cu. Cd showed inhibition, while Cu showed promotion.(5) Analysis of the relative contents of microbial PLFAs showed that,16:00, 16:1?7c,18:1?7c,18:1?9c, i15:0, cy19:0?8c and10Me16:0 were the dominant groups of soil microorganisms in the study area. Overall, bacterial had the highest abundance, followed by actinomycetes, fungi was the lowest.According to the results of geostatistics, the spatial distributions of individual fatty acids were quite different from each other. Relatively higher contents of bacteria and Gram-negative bacteria existed in the east and north-west of the study area; high values of Gram-positive bacteria and actinomycetes located in the northwest of the study area; and areas with high abundance of fungi were mainly located in the southern parts.Gram-positive bacteria were negatively correlated with heavy metals, and the positive correlation between metals and the ratio of Gram-negative and positive bacteria suggested that in heavy metal contaminated soils, Gram-negative bacteria had an advantage over Gram-positive bacteria; the relative abundance of fungi was negatively correlated with heavy metals; the ratio of fungi to bacteria was significantly negatively correlated with Cd, suggesting that metal pollution may reduce the sustainability of soil ecosystem; The positive correlation between cy/pre ratio and heavy metals suggested that the microbial community has been suffering the surppress by metal pollution.RDA analysis showed that environmental factors explained 39.8% of the variation of microbial community structure.Soil physical and chemical properties explained 33.4% and heavy metals explained 14.7%. The main factors affecting soil microbial community structure in the study area were pH, EC, OC, heavy metals, soil mechanical composition and aluminum oxide.(6) Spatial distribution patterns of Shannon index, Pielou evenness index and Simpson dominance index were similar with higher values existing in the southwest of the study area. Correlation analysis showed that the three diversity indices were significantly positively correlated with heavy metals, suggesting that heavy metal contamination caused moderate stress on microorganisms in the study area, resulting in increased microbial diversity and evenness, decreased dominance. |
PDF Full Text Request