Chemical and physical properties of rhizosphere and bulk soils of three tea plants cultivated in Ultisols

This study aimed to investigate the physical and chemical properties of rhizosphere and bulk soils of three tea plants cultivated at two different elevations for direct comparison each cultivar planted in different soil environments. The three tea plants are Camellia sinensis CV Chin-Shin Dahpan (CSD), Camellia sinensis CV Chin-Shin Oolong (CSO), and Camellia sinensis CV TTES, No. 12 (TTE). The two study sites were located at Yangmei (YM), Taiwan Tea Experimental Station, Taoyuan County of northern Taiwan, and the Luku (LK), Fonghuang Tea Experimental Station, National Taiwan University, Nantou County of central Taiwan. Rhizosphere and bulk soils from A horizon (0–15 cm) below tea plants were sampled. The fresh tissues of tea plants were also collected for comparison. The physical and chemical properties, bioavailability of metals (i.e., Al, Fe, Mn, Cu and Zn), and fractionations of metal speciation of the rhizosphere and bulk soils were studied. The low molecular weight organic acids (LMWOAs) of the soil and fresh tissue samples were determined by gas chromatograph (GC), while the organic functional groups were determined by cross polarization magic angle (CPMAS) nuclear magnetic resonance ( 13C NMR) analysis. In general, the mean value of cation-exchange capacity (CEC), organic carbon, and C/N ratio of the rhizosphere soils were lower than those of the bulk soils. Bioavailability of Al shows that Al concentrations in YM soils were higher than that of LK soils. However, bioavailability of Fe in YM and LK soils showed no significant differences. Bioavailability of both Al and Fe in rhizosphere soils was higher than those in bulk soils. According to sequential multi-step extractions, the amounts of Al and Fe fractions showed the following order: residual (step 8) > metal-organic complexes-bound (steps 3 and 5) > crystalline Fe-oxide-bound (step 7). The LMWOAs concentrations in rhizosphere soils were higher than those in bulk soils. The LMWOAs exudates in soils showed the following order: oxalic > succinic > fumaric > malonic acids. Regression analysis of bioavailability of Al and Fe in YM and LK sites showed linear relationships with total LMWOAs concentrations ( R 2 > 0.974 to Al; R 2 > 0.791 to Fe). The microbial resynthesis of alkyl-C, hemicellulose of acetal-C and aromatic-C rhizosphere soils was higher than that of bulk soils. On the other hand, the lignin of aromatic-C, phenolic-C and carboxyl-C contents of bulk soils were higher than those of rhizosp