1 Introduction
Protected cultivation (PC), a symbol of agricultural modernization, is the future direction for agricultural development (Balliu et al., 2016; Guo et al., 2012). To ensure healthy and sustainable development, soil quality is an important requirement for PC. In the past few decades, several developed countries, including the United States, Netherlands, Israel, and Japan, have taken a large lead in PC (Sun et al., 2019). This involved constructing large-scale multi-span greenhouses and advanced soilless cultivation techniques such as hydroponic cultivation, rock wool cultivation, and substrate cultivation (Katsoulas et al., 2007). Since the 1980s, PC is also rapidly growing in China with an increasing cultivated area of 4.0×106hm2 (Jensen et al., 2002; Lu et al., 2020). Moreover, China has emerged as the world’s largest producer, consumer, and exporter of vegetables with an export trade of 1.55×1010 dollars in 2017 (Xu et al., 2019). However, PC in China mainly relies on arable land soil transformed from the extensive field planting pattern lacking precise experience. To pursue high yields, over-applying pesticides, chemical, and organic (especially livestock and poultry manures) fertilizers has become common agricultural intensification activity (Verma, 2015). Moreover, long-term continuous cropping of single crops and high multiple cropping index are reducing soil quality in PC, such as acidification (Guo et al., 2010), salinization, nutrient imbalance, pollutants accumulation, and soil-borne diseases (Tang et al., 2021). These may also cause agricultural non-point source pollution (Huang et al., 2021), which may severely restrict the sustainable development of PC in China, especially in HMs (heavy metals) accumulation regions (Li et al., 2021; Gil et al., 2018).
Anthropogenic activities can severely impede PC. Studies showed that the risk of soil HMs enrichment is higher in PC than that in the open-air cultivation fields (Ramos-Miras et al., 2011; Tian et al., 2016). Apart from high water, fertilizer, and pesticide inputs, PC has the characteristics of high rotation frequency and surface evaporation. In PC, the soil and certain crops are more likely to accumulate HMs than the traditional fields, which may pose a higher risk to the ecological environment and human health (Hu et al., 2017). Studies showed that PC soils are relatively more enriched in copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), nickel (Ni), and chromium (Cr); and, the trend aggravates with the extension of planting years (Jia et al., 2020). For instance, PC soil samples of Wuhan showed a higher enrichment of As, Cd, Ni, and Pb with the increasing planting years; after 10-15 planting years, Cd, Cr, Cu, Pb, and Zn reached the highest amounts (Wang et al., 2020). Organic fertilizer, especially the livestock and poultry manure, is one of the main inputs in PC; the required amount can reach up to 75000 kg·ha-1(air-dried weight). , the total annual production of livestock manure in China exceeded 2.235×109 t in 2010 (Geng et al., 2013). In poultry/livestock breeding, feed additives containing Cu, Zn, and other elements are widely used for disease resistance and/or growth stimulation (Sager, 2007). In Huanghuaihai, the middle and lower reaches of the Yangtze River and the south region mainly use pig and livestock manures for PC with a field return rate of ~50% (Liu et al., 2018). A survey of commercial organic fertilizers and livestock manures (Huang et al., 2017) showed that commercial chicken manures exceeded the standard (NY 525-2012) for Cd, Pb, and Cr by 10.3, 17.2, and 17.2%; the commercial pig manure exceeded the standard for Cd and As by 20.0 and 6.7%; the chicken and pig manures exceeded the standard for As by 7.1 and 15.4%, respectively. In the 1990s, most chicken and pig manures exceeded Cu and Zn by 1.5-16.2 and 1.3-4.7 times respectively. Cu and Zn concentrations reached up to 208.6 and 776.8 mg·kg-1 in the chicken manure, and 140.8 and 592.8 mg·kg-1 in the pig manure, respectively.
In China, extensive management has advanced PC exponentially, which has certainly raised alarms for soil quality degradation. Therefore, for risk assessment, it is critical to examine the HMs accumulation in the PC soils. Regrettably, most previous studies only focused on field cultivation rather than on PC or the local area rather than the overall situation. This study systematically surveyed the soils in 16 cities of 8 provinces in China for the accumulation of Cu, Zn, Pb, Cd, As, Hg, Cr, and Ni in the largest PC area. Apart from risk characteristics and sources, we also evaluated the relationship between HMs accumulation and the planting years of PC. This study can provide scientific insights for harmless PC.