Geochemical baseline establishment and ecological risk evaluation of heavy metals in greenhouse soils from Dongtai, China
Introduction
Increasing demands for food have resulted in more intensive use of existing croplands (de Vries et al., 2013, Tilman et al., 2011), which has promoted the continuous accumulation of heavy metals (HMs) in soils due to the intensive application of agrochemicals, fertilizers and manures (Bai et al., 2015, Rodríguez Martín et al., 2013). Soil pollution in intensive agricultural areas may pose an even greater risk to human health because it can result in greater human exposure to soil and agricultural products than heavily polluted areas (e.g., mining sites and industrial area) (Sultan and Shazili, 2009). Hence, HM contamination of agricultural soils is an environmental issue of major concern due to its potentially negative impacts on the agro-ecosystem.
Greenhouse agriculture, which is a type of intensive agriculture, has expanded rapidly in recent years in China (Bai et al., 2010, Kong et al., 2014, Yang et al., 2013), due to the surging demand for vegetables with an increasing population (van Grinsven et al., 2015). By the end of 2014, greenhouse agriculture occupied an area of more than 4.1 million hectares in China. To achieve high yields and profits, greenhouse agriculture involves a high multi-cropping index (Bai et al., 2015, Chen et al., 2014), high chemical inputs (e.g., fertilizers and pesticides) (Chen et al., 2014, Sungur et al., 2016), and huge amounts of irrigation (Kong et al., 2014). All of these practices could contribute to HM enrichment in greenhouse soils (Bai et al., 2015, Gil et al., 2004, Rodríguez Martín et al., 2013). Thus, there are urgent needs for accurate evaluation of HM contamination to greenhouse soil.
Geochemical baseline concentrations (GBCs) are defined as the natural levels of HMs in soils that have not been influenced by human activities (Bech et al., 2005, Galán et al., 2008). Due to diverse geological properties and dominant soil forming factors (Galán et al., 2008), GBCs can vary widely across regions (Jiang et al., 2013, Micó et al., 2007, Song et al., 2014, Tack et al., 1997). However, because background values (BVs) are lacking for most studied areas, series of data (e.g., the average concentrations in the continental crust or in shale, preindustrial concentrations, etc.) have commonly been used as reference data when determining the HM contamination statuses of soils (Horckmans et al., 2005, Li et al., 2015b, Reimann et al., 2005 Reimann et al., 2005). As a result, some widely used quantitative indices (Jiang et al., 2013), such as the geo-accumulation index (Igeo), pollution index (PI), pollution load index (PLI) and ecological risk index (RI), could contain bias when used to assess HM pollution, and an evaluation error might occur when applying GBCs at a large scale in a specific area (Horckmans et al., 2005, Wang et al., 2011). Hence, the establishment of GBCs at local scales is a priority in agricultural soils to better evaluate HM pollution (Albanese et al., 2007, Baize and Sterckeman, 2001, Galán et al., 2008, Ramos-Miras et al., 2014). The local HM baselines in greenhouse soils in Spain were established by Sierra et al. (2007) and Ramos-Miras et al. (2011). In China, although considerable research of greenhouse soils has been conducted, mainly focused on the assessment of HM contamination (Bai et al., 2015, Kong et al., 2014, Yang et al., 2015), the establishment of local GBC is seriously lacking. Consequently, the objectives of the present work are to i) determine the GBCs of HMs in a specific greenhouse system; ii) evaluate the pollution statuses and potential ecological risks of HMs using the Igeo, PI, PLI and RI; and iii) identify possible sources of HMs.
Section snippets
Study area and sample collection
The study area is located in Dongtai, Jiangsu Province, China, and is adjacent to the coastline of the China Yellow Sea (Fig. 1). This is an area of the transition from a subtropical to warm temperate zone with a monsoon climate and has a mean annual precipitation of 1061 mm and a mean annual temperature of 15 °C. The soils in the study area are mainly Halosols (Gong et al., 2003), which developed from the marine deposits. Although the soils present low fertility, agriculture in this area is
Soil properties and HM concentrations
The edaphic characteristics of surface soils are presented in Table 1. The greenhouse soils in this region had low clay and SOM contents, indicating a low retention of HMs by these components in the soil matrix. The soil pH ranged from 7.70 to 8.67, with an average value of 8.17, indicating that the soils in this area are slight alkaline to alkaline, which could decrease the HM mobility (Chai et al., 2015, Sharma et al., 2007). The soil EC ranged from 89.7 to 1360 μS cm−1, with a mean of 251.6 μS cm
Conclusion
In this study, the GBCs of HMs (in mg kg−1) in Dongtai, China, were 0.059–0.092 for Cd, 39.20–54.50 for Cr, 12.52–15.57 for Cu, 20.63–23.26 for Ni, 13.43–16.62 for Pb and 43.02–52.65 for Zn. Generally, not very high HM concentrations were presented, except for Cd, in the investigated greenhouse surface soils with low clay contents, poor fertility and high salinity. However, this region showed moderate HM contamination. HM pollution spots were observed adjacent to the town, where the hotspots of
Acknowledgement
The research was financially supported by the National Natural Science Foundation of China (41473073), the National Science-technology Support Plan Projects (2015BAD05B04), the Special Research Foundation of the Public Natural Resource Management Department from Ministry of Environmental Protection of China (201409044) and the Key Frontier Project of Institute of Soil Science, Chinese Academy of Sciences (ISSASIP1629).
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