Characteristics Of Acid Deposition And Its Effects On Forest Soil In The Three Gorges Reservoir Area

In recent years, acid deposition has become one of the important environmental problems in the world, which is a great threat to the global ecological system. Studies have shown that acid deposition makes the soil acidification, the destruction of soil structure, water acidification, water quality variable difference, forest decline, crop yield reduction and corroding buildings and so on, not only to human life caused great influence, but also threaten human survival and development. At present, our country has become the third largest acid rain area in the world after Europe and the United States, which is mainly concentrated in the central and southwestern regions of China. Therefore, this study selected the Three Gorges Reservoir Area as main study area. It was studied, which was atmospheric pollution situation and sources, atmospheric acid deposition characteristics, acid critical loads, the exceedance of acid critical loads, the impact of acid deposition on soil and the future situation of acid rain. The specific research results are as follows:(1) After 30 years of rapid urbanization and industrialization process in the Three Gorges Reservoir area, the average annual concentration of SO2 and NO2 were 0.034-0.039mg·m-3 and 0.026-0.031mg·m-3, respectively. Based on the national standard, annual average emission concentration of SO2 was only Wuxi which reached emissions standards in recent years in the area of a class; annual average emission concentration of NO2 has reached the standard in most districts and counties. Application Hysplit4 air trajectory model was applied, which simulated air trajectory in 2013 and got two track lines.1 track line originated in the territory of the reservoir area, appears frequency reached 77%; 2 track line originated in the south, appears frequency reached 23%. It illustrated that the impact of external source pollutants on the pollutant was small in the reservoir area.(2) The average annual rainfall in the Three Gorges Reservoir area was between 968.53-1254.12mm in 2007-2013. The annual average pH value of precipitation was 4.26-4.79. It was always lower than 5.6, which belongs to acidic precipitation. According to the degree of the pollution of the precipitation, the precipitation in the reservoir area was all slightly polluted. Orthogonal rotation was used for precipitation ion composition with PCA method in reservoir area and got two principal components; and source apportionment was conducted based on PMF analysis model and got four factors, factor 1:Na+ and Cl", mainly was marine source pollutant in high altitude long-range transport; factor 2:NH4+ and SO42-, mainly was coal burning and agricultural sources; factor 3:Ca2+ and Mg2+ on the, mainly was the crust or wind blown dust source; factor 4:NO3-, mainly was the fossil fuel combustion sources. The two methods of PCA and PMF are different from the analytic principle or hypothesis, which lead to the existence of some differences in the analytical results, and show the advantages and limitations of each method in the application.(3) Based on SMB model, acid critical loads were calculated in the Three Gorges Reservoir area, the value was between the 0-10.3 keq·ha-1·yr-1. The acid critical load above 5.0 keq·ha-1·yr-1 was mainly distributed in the upstream and downstream parts of Three Gorges Reservoir area. Nutrient nitrogen critical load values were between 0-3.5 keq·ha-1·yr-1 below the critical loads of sulphur. The exceedance of acid critical loads were between 0-2.1 keq·ha-1·yr-1 and has reached the area of the whole area of nearly 4.8%; the exceedance of nutrient nitrogen critical loads were between 0-0.3keq·ha-1·yr-1 accounted for 29.0% of the whole area of area. Therefore, it should focus on the protection region which exceeded the critical load to prevent the acid deposition on forest ecosystems caused greater harm. When evaluating critical loads in the Three Gorges Reservoir area, some uncertainties were generated and were mainly includes:some uncertainties of indicator organism and critical chemical value, lack of measured data, some uncertainties of the input parameters caused by the accuracy of data estimation, some uncertainties of research methods to evaluate acid critical load.(4) Water chemistry occurred obvious change when atmospheric precipitation passed through the mixed coniferous-broad leaved forests and broad-leaved forests, and pH decreased 0.88 and 0.71 pH equivalent, respectively. The leaching amount of Ca2+and leaching coefficient were 4.606 mg·L-1 and 2.360 in the trunk stream of the broad leaved forest and were significantly higher than the mixed coniferous-broad leaved forest. Precipitation was through soil layer, and pH value of interflow rosed to 7.55 in the mixed coniferous-broad leaved forest, pH value of interflow decreased to 6.59 in the broad-leaved forests. The pH value of mixed coniferous-broad leaved forest and broad-leaved forest decreased from 2010 to 2012. The soil pH of forests decreased in the "L" type with the increase of the amount of H+. Soil acid buffer curve turned when the amount of H+reached 0.03mol/kg. Mixed coniferous-broad leaved forest and broad-leaved forest soil acid buffer capacity were smaller when soil pH was bigger than 3.0; two forests soil acid buffer capacity rised sharply when soil pH was less than or equal to 3.0, it was 2.8-4.2 times of pH>3.0. The soil acid damage capacity of mixed coniferous-broad leaved forest and broad leaved forest showed the trend of decreasing first and then increasing from the soil A layer to the C layer. According to the classification of soil acid damage capacity, two kinds of forest stands were acid damage soil, and the soil was relatively sensitive to acid deposition.(5) According to the R2014b Matlab, prediction model of acid deposition which was NAR neural network was constructed and was carried on many times training. It completed the modeling task. And it could simulate and predict the time series data by time delay. In the Three Gorges Reservoir area, the prediction of precipitation reached 1599.4mm,1422.6mm and 1766.1mm in 2014,2015 and 2016, generally higher than the measured precipitation in 2007-2013. Precipitation pH value was 4.77,5.23 and 5.21, respectively, which belonged to the weak acid rain. Compared with the amount of ions in 2013, the prediction of the ion contents increased in 2014-2016 predicted. The main anion of precipitation was SO42- and NO3-, the major cations was NH4+ and Ca2+. According to the [SO42-]/[NO3-], acid rain in reservoir area still belongs to coal burning type of acid rain in 2014-2016.