The Responses Of Red Soils Under Different Vegetation Stands To Atmospheric Sulfur Deposition

In China, atmospheric S deposition has been increasing since early 1980s. It is expected to reach at peak value in 2020-2030. China has become the third largest acid region in the world. The most highly polluted areas are currently found in central and southwest China, such as Guizhou, Sichuan, Hunan, Jiangxi, Guangdong Province etc. The economic losses caused by acid rain are huge. The control of acid rain and its detrimental effects has become an urgent national task. It was reported soil acidification due to atmospheric deposition is one of the causes resulting in forest soil degeneration. Forest red soil is major type of soil in south China. Soil quality decrement is its obvious characters due to acidification. The clutter grass, shrubbery forest, coniferous forest (mason pine), broadleaf forests are main vegetation types in South China. It needs urgently to understand the differences of the retention of atmospheric sulfur deposited in red soil in typical vegetation in south China and their consequence to soil qualities. Based on the forest and grass ecological experiment of the Red Soil Ecology Station, CAS, the variation of soil sulfur content under different vegetation stands, and the relationship between soil sulfur content and soil physicochemical property were researched through sampling soils under different vegetation stands and chemical analysis methods in the lab, meanwhile an eight-month leaching experiment using soil columns (10cm as diameter, 60cm tall) was carried out to investigate the responses of soils under different vegetation stands to the sulfur inputs. It was expected to explore the mechanism of the effect of atmospheric sulfur deposition to red soil qualities. The results were as below. 1. The soil sulfur content in different vegetation stands varies distinctly. The total sulfur of soils in 0-30cm decreases as the sequence –coniferous forest, broadleaf forest, grassland. The total sulfur in the sub top layer is the highest for all three different vegetation stands, and total sulfur content is related to the type of stands in the top layer (0-10 cm) and the third one (30-50 cm). The total sulfur is higher in topsoil than in the third layer in coniferous forest, but reverse in broadleaf forest and no significant difference in grassland. Compared with the total sulfur, there are the same distribution orders in profile at inorganic sulfur and adsorbed sulfur. 2. The soil BSP under different vegetation stands are sequenced from high to low: grassland > broadleaf forest > coniferous forest. Related to vegetation stands, the distribution of the soil BSP in the profiles is that it is increasing from the top layer to the third layer in coniferous forest, while it is the lowest in broadleaf forest and grassland and there is no significant difference between the top layer and the third layer. 3. The content of sulfur in topsoil is significantly negatively correlated with BSP value (r = -0.661**, n = 36) and pH (r = -0.455**, n = 36). The correlation is mostly due to the contribution of vegetation, because it is not significant under the same vegetation stands. The soil pH in topsoil put into order as grassland > broadleaf forest > coniferous forest. Compared with the soil background pH in 1989 when the forest and grassland experimental zone started, the pH in topsoil all decreased at different degrees in three vegetation stands. Related to the type of vegetation stands, the range of pH decrease is the largest in coniferous forest (0.28 pH unit), then in the broadleaf forest (0.19 pH unit), and the smallest in grassland (0.12 pH unit). So it can be concluded that the acidification rate were different in different vegetation stands on the same sulfur input. 4. In all three soils under different vegetation stands, the leaching amount of total base ions is much higher without sulfur input than with sulfur input significantly, three are no significant differences among different sulfur inputs. 5. The holding capacity of external sulfur in soils related to the type of vegetation stands. The holding capacity of external sulfur is higher in forest than in grassland at low inputs, and there are no significant differences between coniferous forest and broadleaf forest. While it is the highest in coniferous forest at high input, and there are no significant difference between grassland and broadleaf forest. 6. With the increasing of sulfur input, soil pH in the top and sub top layers decreases gradually in three different vegetation stands, but there are little influence on soil pH in the third layer. So atmospheric sulfur deposition can lead to the soil acidification in the top and sub top layers. In sum, there are significant differences in the holding capacity of external sulfur and the responses to the external sulfur in red soil under different vegetation stands. The results have reference values in exploring the mechanism of red soil quality evolvement, and have practical meaning for the forest layout in red soil area in south China.