Soils were heavily fertilized and rich in minerals and toxic trace elements
Vegetable fields in both Nuwara Eliya and Welimada regions of Sri Lanka have frequently and heavily been fertilized using both inorganic and organic sources of nutrients for decades (Dissanayake and Chandrajith 2009; Sirisena and Suriyagoda 2018; Dandeniya and Caucci 2020). Therefore, soil fertility levels have been built up to very high levels (i.e. to the levels higher than that required for optimal plant growth), e.g., organic matter, N, P and K concentrations and EC levels were very high. As far as authors are aware this much higher available P and exchangeable K concentrations have not been previously reported from any agricultural fields. This situation has caused soil solution to retain very high concentrations of N, P and K as observed in the leaching column experiment.
Apart from the application of straight fertilizers such as urea (N), triple superphosphate (P) and muriate of potash (K), the use of organic matter and inorganic fertilizer mixtures available in the market has resulted in the development of micronutrient levels, and inevitably, the toxic trace element concentrations due to their poor quality (Tables 3,4,5) (Premarathna et al. 2005; Dissanayake and Chandrajith 2009; Bandara et al. 2010; Jayasumana et al. 2015). The concentrations of micro nutrients and toxic trace elements made available to crops through the application of organic matter varied depending on the type of organic matter applied and region, e.g., the organic matter added to soil in this region contained a lower concentration of As than that reported in rice cultivating regions of Sri Lanka (Jayasumana et al. 2015). Moreover, the same authors reported that the inorganic fertilizers contained a higher concentration of toxic trace elements than those found in organic matter. Despite these heterogeneities, due to the continuous application of fertilizers and manures concentrations of micronutrients and toxic trace elements have been built up to very high levels. Similar to the findings of this experiment, the presence of high micronutrient and toxic trace element concentrations in the soils from the same region and intensively cultivated cropping fields in North China have previously been reported (Premarathna et al. 2005; Ju et al. 2007).
Farmers in these vegetable cultivated fields have not adopted adequate soil conservation practices despite located in mountainous areas exposed to heavy rainfall (Samarakoon and Abeygunawardena 1996; Diyabalanage et al. 2017; Weerakkody and Mawalagedara 2020). It is also reported that the disturbed and damaged soil structures with high organic matter content can no longer be protected by microbial decomposition and aggregation (Kobierski et al. 2020). These conditions have aggravated the risk in erosion and leaching, polluting aquatic ecosystems downstream which are important tributaries and reservoirs in Sri Lanka (Samarakoon and Abeygunawardena 1996; Bandara et al. 2010; Diyabalanage et al. 2017; Sirisena and Suriyagoda 2018). This highlights the importance of revising (e.g., cutting down) fertilizer application plans and adopting soil conservation methods urgently in this region.
Soils in the region were rich in organic matter, particularly at Nuwara Eliya. The addition of organic matter is the cause of the development of high organic matter content in agricultural soils while the senesced biomass contributed to the accumulation of organic matter in natural forests. Application of poultry manure and cattle manure has become popular among vegetable farmers in this region as it increases productivity than applying inorganic fertilizers alone (Table 2) (Wijewardena and Amarasiri 1997; Wijewardena 2000). Greater accumulation of organic matter in Nuwara Eliya than that in Welimada would also be due to the cooler temperatures in Nuwara Eliya slowing down the decomposition of organic matter and mineralization (Table 1). Moreover, as farmers practice crop rotation in their fields, a significant difference in mineral element concentrations among fields within a region was not observed. As a result, irrespective of the crop grown, Nuwara Eliya soils retained more organic matter than that in Welimada.
Vegetables as a source of essential mineral elements
Among the edible parts of the crops studied, element concentrations were the highest in cabbage and lowest in potato. Similar pattern was observed in the amounts of mineral and toxic trace elements removed with the harvest from one ha land, except for K and Cu. However, the amounts of mineral element intake through these crops in a daily ration (i.e. 240 g FW) did not follow the same pattern due to the difference in their dry matter content (i.e. cabbage < carrot < potato) (Fig. 6).
Even though these soils have been heavily fertilized, only some of the essential mineral elements showed high concentrations in plant tissues such as P and K. Moreover, none of the crops displayed deficiency or toxicity symptoms of mineral elements. Therefore, vegetables produced in this region can be considered as a major source of mineral elements to the local community and used to minimize mineral malnutrition (i.e. hidden hunger). However, people suffering from chronic disorders such as kidney disease may reduce the consumption of vegetables such as cabbage and carrot produced in this region as those vegetables contain high concentrations of P and K (Parpia et al. 2018).
When compare mineral accumulation, N concentration in green leaves of a range of vegetables from different parts of the country was lower than that reported in the present study (Liyanage et al. 2000). Similarly, potatoes produced in Canada and the Czech Republic, and cabbage produced in Turkey also contained lower concentrations of essential mineral elements in their edible parts and in the soil solution than reported in this experiment (Kiziloglu et al. 2008; Bártová et al. 2013; Liang et al. 2019). However, Takahashi et al. (2018) and Wen et al. (2019) reported similar concentrations of N, P and K in cabbage and lettuce tissues when grown in soils well fertilized with N, P and K. Therefore, higher tissue N, P and K concentrations reported in the crops grown in this region would also be due to heavy N, P and K fertilization.
Vegetables act as a key source of micronutrients in the human diet. Mineral elements such as Zn, Mn and Cu are either components in the large number of enzymes or essential for enzymatic reactions in both plants and animals (Abeywickrama et al. 2018). According to the current rate of vegetable consumption by a Sri Lankan adult (i.e. 240 g per day), per capita consumption of 0.05–0.2 mg Cu, 0.45–0.65 mg Zn and 0.5-2 mg Mn day− 1through these vegetables was observed. These amounts represent only 5–23% Cu, 7.5–11% Zn and 22–87% Mn of the recommended daily intake (IMNA 2001), and thus, these vegetables contributed as a key source of mineral elements requirement to the local community. It is reported that the residents of almost all South Asian countries consume extremely low quantities of fruit and vegetables, lower than the World Health Organization recommendation (Dizon et al. 2019; Jayawardena et al. 2020). Therefore, in terms of quantity, vegetable consumption has to be increased, particularly for vegetable-based diets to increase the provision of a mineral element through vegetables. Moreover, the identification of vegetables rich in minerals and methods that can be used to enrich/fortify vegetables with minerals need to be investigated (Dobosy et al. 2020).
Carrot and potato shoots contained high concentrations of mineral elements and toxic trace elements than those present in their roots/tubers, and higher than those present in cabbage shoots. Despite nutritional excellence, carrot and potato shoots are not generally consumed as vegetables but used in making compost. Even though the chance of contaminating with toxic trace elements is lower in shoots than that in roots or tubers (Setiyo et al. 2020), an opposite response was observed in the present study. As samples were carefully washed using running water before drying for element analysis, the observed concentration of mineral elements would have not been due to surface accumulation, but represent the amount accumulated in leaf tissues. This could be due to the heavy use of foliar fertilizers and pesticides for these crops in this region (Shahid et al. 2017). However, this needs further testing and confirmation.
Although mineral elements are essential, excessive and long-term intake, higher than the required amount may cause complications e.g., long-term intake of Zn higher than the recommended level would cause problems in the metabolism of Cu (FAO/WHO 2001).
However, according to the findings of this study the available concentrations of essential mineral elements would not cause a serious threat to its consumers as the tissue concentrations have not been built up to toxic levels, but would cause detrimental impacts to the environment. Therefore, it is important to reduce the rate of fertilizer application, implement soil conservation methods, educate farmers on good agricultural practices (GAP), avoid the flow of agricultural residues and wastewater directly to natural waterways to minimize environmental pollution and increase the health, safety and wellbeing of these vegetable growers and consumers (Weerakkody and Mawalagedara 2020). In this process, evaluation and monitoring of nutrient levels in soils and vegetables and implementation of policy and regular measures by the responsible organizations is required.
Accumulation of toxic trace elements in vegetables
According to the FAO/WHO (2019) guidelines established for human consumption, the maximum permissible limit of Pb in Brassica vegetables, and root and tuber crops is 0.1 mg kg− 1. Similarly, the maximum permissible limit of Cd in Brassica vegetables is 0.05 mg kg− 1, and in root and tuber crops is 0.1 mg kg− 1. European Commission (EU 2006) has set the permissible levels of Cd and Pb in potato tubers as 0.05 and 0.1 mg kg− 1, respectively. Moreover, the maximum permissible levels of Cd, Pb and As set for leafy vegetables and root crops in China are 0.05, 0.1 and 0.5 mg kg− 1, respectively (GAIN 2014). All the above limits have been given on a fresh weight basis. When comparing those limits with the values observed in the present experiment (i.e. concentration of toxic trace elements in dry weight basis × dry matter content/100), the concentrations of As, Cd and Pb observed in edible parts of these crops on fresh weight basis did not exceed the maximum permissible limits established. However, Premerathna et al. (2005) reported the presence of high Cd in cabbage shoots from the same region exceeding maximum permissible limits. Similarly, the leafy vegetable Mukunuwenna (Alternanthera sessilis L.) samples collected from different urban regions in Sri Lanka also contained similar or higher concentrations of Cd and Pb than observed in the present study (Kananke et al. 2014, 2016). It is also reported that irrigation of agricultural lands with wastewater leads to the accumulation of toxic trace elements in soils and crops grown (Kiziloglu et al. 2008; Sharma et al. 2016). Therefore, the concentration of toxic trace elements in vegetable crops depend on the variation in soil characteristics such as soil pH, clay and organic matter contents, crop management practices such as quality of irrigation water and fertilizer applied, and crop adaptations (Tack, 2014; Khan et al. 2015; Setiyo et al. 2020). It is also suggested that P fertilizers induced immobilization of heavy metals such as Pb, Cd, and Zn in soil (Wang et al. 2008; Yang et al. 2019) and decrease translocation from roots to shoots in cabbage (Qiu et al. 2011). However, this response depends on the concentration of P and other competing elements in soil (Suriyagoda et al. 2018). As there is a risk of accumulating high concentrations of toxic trace elements in vegetables grown in the region, agronomic mitigation strategies need to be implemented in order to ensure the sustainability of these cropping systems and the health of consumers.