Application of cadmium prediction models for rice and maize in the safe utilization of farmland associated with tin mining in Hezhou, Guangxi, China

Highlights

• The contamination of Cd in rice, maize and soil caused by tin mining was investigated.

• Transfer ability of Cd in soil-rice system is stronger than soil-maize system.

• Soil CaO, TOC, pH and Mn are the controlling factors of Cd bioavailability.

• A suitable safe usage plan for local farmland was proposed by predictive models.

Abstract

Cadmium (Cd) contamination in soil and crops caused by mining activities has become a prevalent concern in the world. Given that different crops have varying Cd bioaccumulation factors, crops with low Cd bioaccumulation abilities can be selected for the safe usage of Cd -contaminated lands. This study aimed to investigate Cd contamination in soil and crops and the influencing factors of soil Cd activity in a tin mining area (TMA) and control area (CA) and to put forward suggestions for the safe usage of farmlands by developing prediction models of Cd content in different crop grains. We collected 72 and 40 pairs of rice and maize grain samples, respectively, along with their rhizosphere soil samples and 6176 topsoil samples. The results showed that compared with the CA, the Cd pollution was more severe in the cultivated soil and crop grains around TMA. Furthermore, rice has a strong ability to transport Cd from soil to grains, whereas maize has a poor Cd uptake ability. The total organic carbon, CaO, pH, and Mn in soil play key roles in the transfer of Cd from soil to crop grains. Using these parameters and Cd concentration in soil, two sets of accurate Cd prediction models were developed for maize and rice. Based on the Cd concentration in the topsoil and predicted Cd concentration in crop grains, the safe utilization scheme of farmland was proposed. The proportions of priority protection, safe exploitation, planting adjustment, and strict control were 72.59%, 22.77%, 3.16%, and 1.48% in the TMA, respectively. The values reached 80.51% (priority protection), 19.12% (safe exploitation), 0.37% (planting adjustment), and 0% (strict control) in the CA. Thus, given the difference between Cd accumulation in rice and maize, adjustment of planting crops in contaminated farmlands can be applied to maximize the use of farmland resources.

Introduction

Increasing attention has been paid to the high content of heavy metals in agricultural products due to soil heavy metal contamination (Nriagu, 1996; Zhang et al., 2012a; Christou et al., 2014). Heavy metal pollutants, such as cadmium (Cd), not only pose a threat to the safety of agricultural products but also cause harm to the immune, reproductive, and nervous systems of the body when they enter the body through food; these pollutants also lead to itai-itai disease and renal failure (Zhang et al., 2015; Lei et al., 2016; Herath et al., 2018). Similar to other countries in the world, China is facing serious soil metal pollution due to its rapid and extensive development over the past several decades (Cheng, 2003; Zhao et al., 2015). Hence, the State Council issued an action plan regarding the prevention and control of soil pollution (“Soil Ten Chapter”) in 2016 to improve the land quality and ensure food production safety in the country (CSC, 2016).

From 1981 to 2016, soil Cd concentration gradually accumulated in China, especially in agricultural soils from Southern China (Shi et al., 2019). Anthropogenic activities, such as industry, sewage irrigation, and mining, are the primary sources of Cd pollution. Metal mining activities lead to heavy metal accumulation in the surrounding water, atmosphere, soil, and other environmental media (Zhuang et al., 2009; Li et al., 2015a; Obiora et al., 2016). Tin mining significantly contributes to Cd accumulation in agricultural soils (Song et al., 2015; Puttiwongrak et al., 2019). Given the process of mining and rock weathering, Cd in sulfide minerals associated with tin or polymetallic mines is released into the soil through atmospheric subsidence and mine drainage, posing a health risk to residents living near mines. (López et al., 2008; Navarro et al., 2008; Kwon et al., 2016). In Guangxi, compared with other trace elements, Cd can be more easily absorbed by crops from soil. Given the high transfer rate of Cd from soil to crops, the over-standard rate of Cd in rice and peanut grains in Guangxi is higher than that of other heavy metals (Gu et al., 2019a; Gu et al., 2019b). Rice is a staple food in Asia, and Cd can eventually enter the human body through rice intake, posing a threat to human health. According to a report by Kwon et al. (2016), the long-term consumption of rice grown in mining soil will cause health problems among local residents. Similarly, the excessive accumulation of Cd in agricultural soil around a mine resulted in the increased Cd uptake by crops in Dabaoshan, Guangdong (Zhuang et al., 2009).

The uptake, accumulation, and redistribution of Cd by crops depend not only on the total concentration and bioavailability of Cd in soil but also on different soil–crop systems. The pH, Eh, total organic carbon (TOC), and Fe/Mn oxides in soil have significant effects on the bioavailability of toxic elements in soil (Usman et al., 2008; Zeng et al., 2011; Wang et al., 2019a). Furthermore, researchers have identified differences in the transport and accumulation of Cd in various crops (Chen et al., 2016; Xiao et al., 2017b; Wang et al., 2017). Therefore, the linear-regression soil–crop transfer model can be used to describe the relationship between the total amount of Cd in soil and the uptake of Cd in various crops depending on soil properties (McBride, 2002; Warne et al., 2008; Ye et al., 2014; Wen et al., 2020a; Gu et al., 2019b). Maize and rice are the main agricultural products in Guangxi. Numerous studies reported the transfer and accumulation of heavy metals in single rice or maize plants (Guo et al., 2010; Nwite and Alu, 2015; Ihedioha et al., 2016; Mao et al., 2019). However, the differences in the transfer and accumulation of heavy metals between rice and maize are rarely reported. In addition, farmlands are classified and managed in accordance with “Soil Ten Chapter” and GB15618-2018 (MEE, 2018). Thus, based on the risk screening and control values of pollutants in GB15618, farmlands are classified as priority protection (Cd_soil < risk screening value), safety exploitation (risk screening value ≤ Cd_soil ≤ risk intervention value), and strict control (Cd_soil > risk intervention value) farmlands. The current scientific problems are as follows: First, the accumulation of Cd in crops is related to the bioavailability and potential mobility of this element in soil. In priority protection areas, soil Cd concentrations are low and Cd bioavailability is high, resulting in certain samples containing excess Cd concentration in rice grains. By contrast, in strict control areas, a considerable proportion of the Cd concentration in rice grains remains below the threshold due to the low Cd bioavailability in soil (Li et al., 2021). To sum up, land management standards need to be supplemented and improved. Second, in areas where Cd content in rice grains exceeds the standard, the Cd content in maize may be at the safe level. Therefore, rice and maize prediction models and topsoil data of regional surveys can be used for land security zoning and crop planting adjustment to maximize and ensure the safe use of land resources and protect human health.

In view of this perspective, an investigation was launched in Hezhou city, which was previously known for its tin mining activities. Meanwhile, a region distant from anthropogenic pollution in Hezhou was defined as a CA. This study aimed 1) to investigate the level of Cd and its ecological risk in soil, rice, and corn grains in mining and comparison areas; 2) to study the influencing factors of soil Cd bioaccumulation and establish prediction models; 3) to estimate the concentration of Cd in grains of rice and maize in the study area based on the Cd prediction model and various topsoil parameters obtained from the regional survey; 4) to propose a plan for the safe usage of farmland in the study area on the basis of ensuring safe food production to realize the fine management of sustainable utilization of land resources in mining areas.