1. IntroductionSoil pollution and agricultural product safety have become major concerns in China because of increased population growth, industrialization, and limited arable land [1,2]. Humans take up cadmium (Cd) mainly through consuming food, which ultimately affects human health [3,4,5]. Cd has no known biological function in plants or humans but has the highest accumulation rate compared with other heavy metal pollutants [6] and is also a non-essential, highly soluble, non-degradable, and persistent element, mainly occurring in soil [7]. Heavy metal pollution in soils in China is region-dependent, with high levels of pollution reported in the Southern and Eastern regions and low pollution levels observed in the Northern and Western regions [8]. The results from a National Soil Pollution Survey Report [9] conducted from 2005 to 2013 showed that 7.0% of the soil point samples had pollutants of Cd that exceeded China’s recommended limits. Previous findings indicate that 15% of the agricultural soil in the Yangtze River Delta has high heavy metal pollution, especially at the boundary of Zhejiang, Shanghai, and Jiangsu [9]. Xiao et al. [10] (2010) evaluated the characteristics of heavy metal pollution in the soil in the middle and lower reaches of the Yangtze River and reported that the soil was mainly contaminated with Cd, lead (Pb), chromium (Cr), copper (Cu), arsenic (As), and zinc (Zn).Soil pollution control and the restoration of cultivated land are currently underway in the middle and lower reaches of the Yangtze River to improve soil quality and increase agricultural production output. Cd pollution in rice fields has been widely explored in several parts of the world, including India, Thailand, China, and Japan, and Cd levels in rice pose significant health risks after the consumption of contaminated rice [11,12,13,14,15]. Increased consumption of rice and other grains in the United States has increased Cd intake from dietary sources [16]. Rice and wheat are the most consumed grains by Chinese residents. Therefore, evaluating the content of heavy metals in rice and wheat grains will improve the health of local residents and the national population. Previous studies reported higher Cd contents in wheat and rice than other toxic metals, even when the crops were planted on moderately Cd-contaminated soil [17,18,19,20]. Some soil properties, such as soil pH, soil organic matter (SOM), cation exchange capacity (CEC), and clay content, can significantly influence the mobility and bioavailability of soil Cd and further influence Cd accumulation in crops [21,22]. In addition, some cations in soil solutions (e.g., Mn4+, Cu2+, Zn2+, Si4+, and Fe3+) may compete with Cd2+ for adsorption sites in soil or access to cell walls, cytoplasm, and cell fluids, thereby directly or indirectly affecting Cd accumulation in crops [23,24]. Therefore, in addition to the selection of crop varieties with low Cd accumulation capacity, the regulation of relevant soil properties is an important measure to reduce Cd accumulation in crops.Notably, the Cd concentrations in grains can exceed recommended Cd thresholds without showing any toxic symptoms in crops [17,19,20]. Therefore, it is necessary to explore the mechanism of heavy metal absorption from soil to crops and the key regulatory factors of heavy metal absorption by different food crops. Rice plants tend to accumulate more Cd than other grains [25,26,27,28]. Identifying key transporters and their role in Cd accumulation or detoxification will provide useful information for the development of biological breeding to reduce Cd levels in rice grains [29]. The key transporters of Cd uptake by rice roots have been reported to be similar to the transporters of essential elements such as zinc, manganese (Mn), and iron (Fe) [30]. Metal chelators and several organic acids also play an important role in reducing the toxicity of Cd to essential organelles and macromolecules. Some key detoxification genes and related mechanisms have been explored [31,32]. However, it is not clear whether plant hormones are involved in Cd detoxification or tolerance. Due to physiological differences, crop varieties are important factors affecting Cd accumulation ability [33]. Selection of rice varieties with low Cd accumulation and irrigation management may also be effective strategies for reducing Cd in rice, but these options still need further development [34]. Wheat-derived products are the main source of human ingestion of Cd. Cd is more toxic to wheat than other toxic metals, such as chromium [35]. Cd toxicity reduces the uptake and transport of essential elements by plants, including wheat [36]. Under Cd stress, wheat root growth and morphology were seriously affected. In shoots, Cd toxicity presents a number of physiological impairments, such as decreased photosynthesis, soluble protein and sugar, and antioxidant enzyme activities [37,38,39]. Cd that accumulates in the shoots can be transferred to grains and then through the food chain to people and animals. Therefore, reducing Cd content in wheat is an important requirement, especially in Cd-contaminated soils. Different mitigation strategies have been used to reduce Cd toxicity in wheat. These strategies may include plant growth regulators (PGRs), appropriate application of mineral nutrients, silicon, inorganic modifiers, biochar, manure, and compost [40,41,42,43,44]. Agronomic management practices, such as wheat varieties with low Cd accumulation, crop rotations, planting patterns, and the application of microorganisms, can also be used to reduce Cd uptake and toxicity in wheat [45,46,47].

Rice and wheat are major crops and staple foods in China. In order to better understand the Cd pollution in rice and wheat planting systems in China, the crops’ main production areas in eleven provinces in the Yangtze River Basin and the southeastern Yellow River Basin were taken as examples. Accordingly, the specific objectives of this study were to (i) summarize the current situation of Cd pollution in rice and wheat growing areas in China from 2000 to 2022 by collecting the data from the previous literature in order to understand the distribution of Cd pollution, (ii) assess the human health risks of Cd through contamination status and intake, aiming to highlight the areas with high human health risk to which more attention needs to be paid in the future, (iii) review the mechanisms and influencing factors of Cd uptake and transport in rice and wheat, aiming to find out the methods for reducing the Cd content in grain, and (iv) summarize the research hotspots and dominant research institution through bibliometric analysis in order to look forward to the development trend in this research area.

2. Materials and Methods 2.1. Literature Search

Articles published in English and Chinese from January 2000 to June 2022 were reviewed in this study. Articles that reported Cd pollution status, health risks, Cd pollution mechanism, and research hotspots in the rice and wheat cropping system in China were included in this review. The search was conducted using the Web of Science (WOS), the Science Direct, and the China National Knowledge Infrastructure Engineering database (CNKI). The search method used “topic.” Keywords such as “Rice and wheat cropping system”, “Wheat/rice cadmium”, “Cadmium pollution in wheat or rice”, “Wheat enriched with cadmium”, “Rice enriched with cadmium”, “Wheat cadmium”, “Rice cadmium”, “Wheat/rice rotation cadmium”, “Remediation of cadmium pollution in rice”, “Remediation of cadmium pollution in wheat”, “Wheat cadmium water and fertilizer management”, “Rice cadmium water and fertilizer management”, “Low accumulation varieties of rice cadmium”, “Low accumulation varieties of wheat cadmium”, “Rice cadmium remediation materials”, and “Wheat cadmium remediation material” were used to search for relevant papers in this review.

2.2. Data Retrieval and Analysis

Data retrieved from each article included: (i) the name of the first author, title, publication source, country of issue, and publishing year; and (ii) the location of the study area. The location of the study area was inferred from the scope of the administrative division if it was not clearly stated in the article. Included articles were reviewed to ensure that they met the following criteria: (i) the cropping method of farmland reported in the study was the rice and wheat cropping system (not only the rice-wheat rotation but also growing rice and wheat in the same place); (ii) sources of pollution in the study area were mainly from human activities, including industrial waste gas, waste water and waste residue, mining development, and agricultural waste; and (iii) accurate analysis and presentation of data, including mean value, deviation, and coefficient of variation was reported in the articles. This review was conducted in May 2022 by reviewing literature and retrieving relevant data from previous studies. A total of 10,465 articles that met the study criteria were obtained from the search process. The VOS viewer (1.6.18, Netherlands) software was used to perform a bibliometric analysis of the search results from the aspects of Cd accumulation in agricultural land, research hotspots, and future prospects. A spatial distribution map of Cd pollution in rice and wheat cropping system was generated using Arc Gis (10.3) software.

2.3. Risk AssessmentThe Hazard Quotient (HQ) proposed by the US Environmental Protection Agency (EPA) was used to assess the non-carcinogenic human health risks of exposure to hazardous substances. The non-carcinogenic risk of Cd was expressed as the HQ, as shown below: where ADI represents the average daily intake of Cd (μg kg−1BW day−1), and RfD denotes the chronic reference dose of Cd. The oral reference dose of Cd was 1 μg kg−1BW day−1 [48]. An HQ value ≤ 1 meant that significant adverse reactions were unlikely to occur in the exposed population. An HQ value > 1 showed a high non-carcinogenic risk to the exposed population. The ADI of Cd was calculated using the following equation: where ADI denotes the estimated daily intake of Cd (μg kg−1BW day−1); Ci represents the concentration of Cd in rice, leafy, rootstalk, and legume vegetables (mg kg−1); IRi represents the daily consumed amount of rice, leafy, rootstalk, and legume vegetables (g day−1); BW indicates the average body weight of the corresponding population (kg body weight, kg BW). The Ci of rice was converted using a factor of 0.86 × 0.70 because rice stored at home usually contains 14% water content [49], and during brown rice processing (milling), the content of Cd in polished white rice is reduced by 20~40% (mean: 30%) [50,51]. The average monthly intake (AMI) of Cd was calculated using the following equation: The possible exposure to Cd from rice and vegetables in Xiangtan County was investigated. Potential health risks associated with dietary Cd intake were estimated through Monte Carlo simulations [52,53], and 30,000 iterations were performed using the Crystal Ball 11.1 tool to obtain reliable results. Data on consumption rate (IRi) and body weight (BW) were retrieved from the Chinese National Nutrition and Health Survey (CNNHS) conducted in 2002 [54]. As a major nutritional reference database for the Chinese population, the CNNHS contains the dietary patterns of 67,608 people in 31 provinces, autonomous regions, and municipalities across China.