Effects of tannic acid on active aluminum forms distribution in tea garden soils with different pH

The effects of tannic acid on the distribution of active aluminum forms in typical tea garden soils with different pH in Pu'er of Yunnan Province and Nanchang of Jiangxi Province were studied by field observations and laboratory analyses. The tested 4 active aluminum forms in tea garden soils are exchangeable aluminum Al³⁺, unimer hydroxyl aluminum Al(OH)²⁺ and Al(OH)₂⁺ , acid-soluble aluminum Al(OH)₃⁰ and humic-acid aluminum Al-HA. Experimental results showed that at tannic acid addition level of 0 0.4 mmol·kg^-1 in Nanchang tea garden and 0 2.0 mmol·kg^-1 in Pu'er tea garden, soil exchangeable aluminum content decreased whereas hydroxyl aluminum, acid-soluble aluminum and humic-acid aluminum increased significantly with increasing soil pH. When the concentration of tannic acid exceeded 2.0 mmol·kg^-1, the inhibition effect of high concentration of tannic acid on active aluminum content strengthened with increasing soil pH. The variation trend in 20 40 cm soil layer was similar to that in 0 20 cm soil layer. Generally, active aluminum content in upper soil layer was less than that in subsoil layers and the total content of active aluminum in Pu'er tea garden soil was higher than that in Nanchang tea garden soils. Correlation analysis showed that soil pH was positively correlated with soil pH buffle capacity (pHBC), hydroxyl aluminum, humic-acid aluminum (r = 0.852, 0.796, 0.960; P < 0.01, 0.01, 0.01) in 0 20 cm soil layer. pHBC showed a significant negative correlation with exchangeable aluminum (r = 0.904, P < 0.01) and hydroxyl aluminum (r = 0.645, P < 0.05) and a positive correlation with humic-acid aluminum (r = 0.795, P < 0.01). At tannic acid concentration of 0 0.4 mmol·kg^-1, soil pH significantly increased to a peak before steadily decreasing. The relationship between pH (Y_pH) and tannic acid concentration (CDN) was best described by the following equation - Y_pH = 0.04CDN + 3.82 (R² = 0.95, P < 0.01). When the concentration of tannic acid increased to 8.0 12.0 mmol·kg^-1, soil pH remained unchanged. It was thus clearly noted that the effects of tannic acid on active aluminum content were not the same in different regions of tea garden soils. Low concentration of tannic acid improved soil content of various forms of activie aluminum. With increasing amounts of tannic acid, however, contents of various forms of active aluminum were inhibited. With increasing soil pH, high concentrations of tannic acid inhibited the release of active aluminum. Both pH and tannic acid in garden soils had a mutual weakening effect on active aluminum.