Geochemical baseline establishment and ecological risk evaluation of heavy metals in greenhouse soils from Dongtai, China

Highlights

• Geochemical baselines of heavy metals (HMs) were established in greenhouse soils.

• The established baselines facilitated a more accurate HM evaluation.

• Cd was considered to be the main contribution of soil contamination in this study.

• Accumulation of Cd, Pb and Zn in surface soils ascribed to anthropogenic sources.

Abstract

Currently, heavy metal (HM) contamination in greenhouse soils is a significant concern due to the rapid expansion of greenhouse agriculture. However, it is difficult to accurately assess HM pollution in greenhouse soils in China due to the lack of local geochemical baseline concentrations (GBCs) or corresponding background values. In the present study, the GBCs of HMs in Dongtai, a representative greenhouse area of China, were established from subsoils using cumulative frequency distribution (CFD) curves. The pollution levels of HMs and potential ecological risks were investigated using different quantitative indices, such as geo-accumulation index (I_geo), pollution index (PI), pollution load index (PLI) and ecological risk index (RI), based on these regional GBCs. The total concentrations of six metals (Cd, Cr, Cu, Ni, Pb and Zn) in surface soils were determined and shown to be lower than the concentrations reported in other greenhouse regions of China. The GBCs of Cd, Cr, Cu, Ni, Pb and Zn were 0.059–0.092, 39.20–54.50, 12.52–15.57, 20.63–23.26, 13.43–16.62 and 43.02–52.65 mg kg⁻¹, respectively. Based on this baseline criterion, Cd, Pb and Zn accumulated in the surface soils because they were present at concentrations higher than their baseline values. The soils were moderately polluted by Cd according to the I_geo values, and the PI results indicated that moderate Cd contamination was present in this area. The large variation of I_geo value of Cd revealed that Cd in this area was likely influenced by agricultural activities. The PLI showed that most of the study area was moderately polluted. However, an analysis of the RI showed that the investigated HMs had low ecological risks. Correlation analysis and principle component analysis suggested that the Cd, Pb and Zn in the greenhouse soils mainly originated from anthropogenic sources (agricultural activities, atmospheric deposition etc.), while Cr, Cu, and Ni originated from natural sources. The findings of this study illustrated the necessity of GBC establishment at the local scale to facilitate more accurate HM evaluation of greenhouse soils. It is advisable to pay more attention to Cd, which could cause environmental problems in the greenhouse system.

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 (I_geo), 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 I_geo, PI, PLI and RI; and iii) identify possible sources of HMs.