Characteristics of soil aggregates of tea gardens in Anxi County, Fujian Province, and their influence on element distribution

Top (0-20 cm)and sub-layer (20-40 cm)soils were collected at twenty-seven tea gardens in Anxi, Fujian Province, which is the source area for Tieguanyin tea production. Air-dried soils after sieving through 10 mesh (2 mm nylon screen) were partitioned into 5 aggregate-size fractions by vibration sieving with supersonic wave generators. It is found that the macro aggregates of 2000-840 μm and 840-250 μm fractions are composed of 73% of the total particle weight, and micro aggregates of 250-125 μm, 125-63 μm and <63 μm fractions occupy less than 30%. Soils with higher Al₂O₃, Fe₂O₃ and lower SiO₂ are in favor of macro aggregate formation. There is a concentration difference between macro and micro aggregates for most elements such as Cd, Cr, Hg, Ni, P and Al. The concentrations of most geochemical indicators such as P, Zn, Zr and organic carbon become increasingly higher with the decrease of the aggregate size. Extractable concentrations of As, Cu, Pb and Zn are enhanced in order of water, ammonium nitrate, dilute hydrochloric acid, and DTPA extraction. However, the extractable Cd, Hg, Mn, Ni and Se present higher concentrations in the particular extraction method. Generally the extractable content of top soils is higher than that of sublayer soils due to soil physical-chemical characteristics including organic carbon content and soil structure.