Abstract: Exogenous acid input, such as acid deposition and chemical fertilizer, have contributed to the serious soil acidification in the Three Gorges Reservoir. In this study, the titration curve method was adopted to investigate the pH buffering capacity (pH BC) and influencing factors of typical soil (purple soil and yellow soil) in the Three Gorges Reservoir. Linear regressions were used to fit the linear portion of each titration curve and the slopes of these lines were used as the determination of pH BC. The results indicated that in a certain pH range (purple soil:pH 6.5-2.5, 6.2-11.5, yellow soil:pH 5.6-2.8, 5.5-10.7), the pH changes of the two soils showed linear correlations to the amount of acid or alkali addition. Furthermore, the slope-derived pH BC of acid and alkali of purple soil were 101.3 mmol/kg and 34.6 mmol/kg, respectively, lower than that (105.3 mmol/kg and 38.0 mmol/kg) of yellow soil. The pH BC was mainly affected by the content of calcium carbonate and cation exchange capacity (CEC). The difference between pH BC of the two soils was due to their different soil buffer systems and basic properties (initial pH, mechanical composition). Above all, the pH BC of the yellow soil was slightly better than that of purple soil. Meanwhile, acid deposition and nitrogen fertilizer application can threaten soil acidification in the Three Gorges Reservoir. According to the input speed of exogenous acid, a rough estimation of the time of acidification to different pH value could be made. For purple soil, the time acidified to pH of 5.5 (which is defined as acidification) was 194 a, to pH of 4.5 was 252 a, while the time acidified to pH=4.5 of yellow soil only needed 97 a. Excessive and unreasonable nitrogen fertilizer can change the soil CEC, SOC content and soil environmental factors, decline the soil pH and soil buffer capacity, even may speed up the rate of soil acidification. This study obtained preliminary results on pH BC and the influencing factors, however, water level fluctuation in the Three Gorges Reservoir has great impacts on the physical and chemical properties of soil. In the forthcoming studies, the evolution process of the acid and alkali buffer could be of great interests in further investigations in the Three Gorges Area. The knowledge on pH BC and influencing factors of the typical soils in the Three Gorges Reservoir, could not only be conducive to improve our understandings on soil environmental capacity and the typical soil acidification potential, but to provide theoretical basis for assessing regional exogenous acid critical value.

Key words: Three Gorges Reservoir, pH buffer capacity (pH BC), titration curve method, purple soil, yellow soil