Assessment of Geochemical Variabilities in Tea Garden Soils and their Effects on the Quality of Tea in Bangladesh

Abstract

Tea cultivation in Bangladesh is done mainly in the hilly regions of Moulvibazar, Sylhet and Chattogram. Recently, tea cultivation has also been started in the plain lands of Panchagarh. A large amount of chemical fertilizers and agrochemicals are used in the soils of tea gardens to increase the production of tea. These fertilizers and agrochemicals are sources of heavy metals accumulation in tea garden soils. To date there is very limited information on the concentration of heavy metals in tea garden soils, tea leaves and made tea of Bangladesh. In the present study, 160 tea garden soils (surface soil, n = 112 and sub-surface soil, n = 48) and 32 non-tea garden soils from the geographically different tea gardens of Bangladesh were collected and analyzed for nitrogen, boron, chromium, copper, iron, manganese, nickel and zinc to assess their variabilities among Old Himalayan Piedmont Plain, Northern and Eastern Hills and Northern and Eastern Piedmont Plains based on landforms, soil texture, drainage class, soil series as well as the age of tea plants. Tea leaves (n = 112) and made tea (n = 39) samples were also collected and analyzed to assess the concentrations of heavy metals. A range of biochemical parameters (total polyphenol, theaflavin, thearubigin and protein content) of made tea were also evaluated. The concentrations of chromium, copper, iron, manganese, nickel, and zinc in the surface soils of the different tea gardens were ranged between 2 - 73, 0.86 - 23, 3084 – 47120, 21 – 1274, 3 – 23 and 2 – 465 mg/kg, respectively. In the sub-surface soils of different tea gardens, the concentration of chromium, copper, iron, manganese, nickel and zinc were ranged between 3 - 89, 1 – 22, 2640 – 52540, 49 – 466, 4 – 24 and 2 - 71 mg/kg, respectively. Significant variations (0.001 > p < 0.05) in the concentrations of the heavy metals were observed between the surface and sub-surface soils. The mean concentrations of chromium, copper, iron, manganese, nickel, and zinc in the tea leaves were 12.86, 14.67, 185, 641, 4, and 43.79 mg/kg, respectively. The mean concentrations of chromium, copper, iron, manganese, nickel and zinc in the made tea were 21, 17.51, 373, 613, 6.29 and 134 mg/kg, respectively. The concentrations of iron and nickel in the tea leaves and made tea and zinc in the tea leaves were within the permissible limits of heavy metals in plants. Significant variations (p < 0.001) in the concentrations of copper, iron, manganese, nickel, and zinc in the tea leaves and made tea were observed. The concentrations of heavy metals in the soils were found to be varied significantly within the different physiographic regions, tea gardens, landforms, soil texture, and soil series. The tea leaves and made tea of different tea gardens also had variabilities in their heavy metal contents. Zinc had significant positive relationship with caffeine (p < 0.01) in the made tea while had the significant negative relationships with theaflavin and thearubigin. Significant positive relationships between caffeine and zinc indicated that with increase zinc concentration caffeine content also increased in the made tea. Excess zinc might decrease the quality of tea through decreasing theaflavin and thearubigin contents. Nickel had significant negative (p < 0.01) relationships with polyphenol and manganese had significant negative relationships with thearubigin in the made tea. Excess nickel might decrease the quality of tea through decreasing the total polyphenol content. The made tea of different tea gardens had significant differences (p < 0.01) in the concentrations of biochemical parameters such as caffeine, protein, total polyphenol, theaflavin and thearubigin. The transfer factors for chromium, copper, iron, manganese, nickel, and zinc were 0.72, 1.72, 0.009, 3.44, 0.44 and 1.15, respectively. The findings of present research can be regarded as the baseline information on heavy metals in soils, tea leaves and made tea of the tea gardens located in different physiographic regions of Bangladesh, which can be useful for further understanding of the origin, extent, and bioavailability of toxic heavy metals in the tea garden soils of Bangladesh. As tea is a popular drink, this study will be supportive for the further investigation to assess the release of toxic heavy metals from made tea into tea brew.