ANALYSIS ON THE FOREST SOIL ACIDIFICATION AND MECHANISMS IN CHONGQING JINYUN MOUNTAIN

In order to study the effects of acid deposition on the soil acidification,this paper analyzed the conditions of acid deposition of Chongqing area and the soil acidification trend of four kinds of typical forest(shrubwood,evergreen broad-leaf forest, mixed coniferous broad leaved forest and Phyllostachys pubescens forest)by the method of soil sampling and laboratory analysis from 2010 to 2012 in August of each year.This study divided the soil of the four kinds of typical forests into four layers according to the soil-forming process and analyzed the physical and chemical properties,respectively.The results showed as follows.(1)Although the sulfur dioxide levels in the atmosphere was going down with each passing year and the nitrogen dioxide levels was in a very steady state,the acid rain condition was very serious in Chongqing area and indicated that the type of acid rain was turning to mixed acid rain from typical sulfate acid rain.(2)Because of the effects of acid deposition for decades,the soil acidification phenomenon was very serious in the four kinds of typical forest.The surface soil pH was various from 3.87 to 4.47 and with the increase of soil depth,soil pH value decreased.All stands surface soil pH values reduced year by year,but the subsurface soil layers pH values of evergreen broad-leaf forest and mixed coniferous broad leaved forest increased.(3)The organic matter content existed significant difference between the four forests,indicating that the stand type had a very important effect on soil organic matter.Because the litter is the main source of organic matter,the surface soil had the highest content and decreased with the soil depth.The content of surface soil of four typical forests were ranked as follows:shrubwood(88.45g/kg) mixed coniferous broad leaved forest(55.65g/kg) evergreen broad-leaf forest(40.87g/kg) Phyllostachys pubescens forest(24.69g/kg).(4)Under the influence of the leaching of acid rain for a long time,cation exchange capacity(CEC)was very low.For the same kind of stand,the change trend of cation exchange capacity was the surface soil had the highest content and decreased with the depth of soil.The content of CEC of surface soil of four typical forests were ranked as follows:evergreen broad-leaf forest(27.65g/kg)shrubwood(23.19g/kg) mixed coniferous broad leaved forest(19.71g/kg) Phyllostachys pubescens forest(15.19g/kg).(5)The content of exchangeable Al3+ of the four typical forests was very high under the effect of acid rain for a long time,and ranked as follows:evergreen broad-leaf forest(40.97 mmol/kg) shrubwood(31.32 mmol/kg)mixed coniferous broad leaved forest(30.06mmol/kg) phyllostachys pubescen(26.15mmol/kg).In this study,there was a negative correlation between exchangeable Al3+ and soil pH.