Impacts of land use pattern on heavy metal concentration in agricultural soils: A comparison of three different approaches in Wuxi, China
Land use practices, while shaping landscape patterns and modifying ecological processes, may accelerate soil heavy metal contamination. However, understandings on the relationships between land use pattern and soil heavy metal contamination were currently insufficient to guide soil pollution assessment and diffusion control. This study tried to uncover the above relationships as well as the effects of scale on these relationships in Wuxi, China, through a comparison of three different approaches. First, we conducted a statistic and geostatistic analysis of heavy metal contents (As, Cd, Cu, Hg, Pb, Zn) of 1938 soil samples of Wuxi in 2009. Secondly, land use structure was analyzed based on a high resolution land use map of Wuxi in 2009. Then, correlation coefficients between landscape metrics (nine landscape level and five class level) and heavy metal contents were analyzed with SPSS based on three different sampling plots: (1) township boundary; (2) grid of 1km, 2km and 5km; (3) buffers of 0.5km, 1km, 2.5km for 100 randomly selected sample points. If there were more than one soil samples points within the plot of township boundary or grid, heavy metal contents of all sample points were averaged to represent the plot.
The results showed that Hg and Cd were the major pollutants in the study area, and coefficient variance for Cd, Pb and Hg were much larger than As, Cu and Zn. Significant correlations were observed among Cu, Hg, Pb and Zn. All the six elements, especially Hg, As and Zn, were spatially auto correlated. Agricultural land, urban and rural construction land were the two major land use types in 2009 with the percentage coverage of 30.1% and 25.8%, followed by industrial and mining zone, water body, green space and transportation facility respectively. The results showed that correlation coefficients between heavy metal contents and land use pattern would change with varied elements, landscape metrics, scales and approaches. However, correlation coefficients between As and CONTAG, SHDI, area of industrial land, and between Hg and area of industrial land and water body were consistently significant in different approaches. The results suggested that non-point pollution might be the major sources of Hg and As in Wuxi, and buffer of sample points might be the best approach among the three different plot sampling methods. Correlations varied with increasing scale, indicating that there wasn’t a single optimal scale for all the correlations.