Effects Of Continuous Planting Of Eucalyptus Grandis Organic Carbon Qingshen County

Eucalyptus grandis plantation has been the most important commercial forest for industry in Southern China and has already got good social and economic benefit.Some problems such as the declines of land capability and growth caused by the Eucalyptus plantation for long time and large-scale has happen expecially on the continuous cropping of Eucalyptus grandis plantation.To sustainably developt the Eucalyptus grandis plantation,many researches about site division type and site quality evaluation,ecosystem biodiversity maintenance,nutrient management and business technology have been made and got notable achievement.But the absence of research of how Eucalyptus affect vegetation and the content of soil organic carbon can not satisfy the development of Eucalyptus grandis plantation.We chosed three generations Eucalyptus grandis plantation under same condition in Qinshen County of Meishan City in Sichuan Province as the object and used the chronosequence approach to research how the continuous cropping affect the carbon content of vegetation,the organic carbon content of soil,the content of soil activated carbon(readily oxidizable carbon,particlllate orgaIlic carbon,microbial biomass carbon and dissolved organic carbon)and stable carbon,providing the basement bate for study the effect of the continuous cropping of Eucalyptus on soil fertility and the reference for sustainable development of Eucalyptus.Main research productions as follow:The carbon content of one,two and three generation of Eucalyptus grandis plantation tree layer is 0.457~0.504 g2g-1,0.447~0.490 g2g-1 and 0.453~0.481 g2g-1 respectivly,and there is no significant difference of this in each organ;The carbon content of the same organ of different generation Eucalyptus brushwood layer and herbaceous layer has notable difference as one generation>two generation>three generation.Those results show that continuous cropping has smaller effect on the carbon content of Eucalyptus grandis plantation tree layer but contrast in vegetation.The organic carbon of all three generation is 7.362 ~15.466g2kg-1,7.018~12.352g2kg-1 and 6.918~11.041g2kg-1 respectivly and it is decreasing aginst with the inreasing of soillayer depth of each generation.The content of soil organic carbon has downtrend in both 0~10cm and 10~20cm soillayer;While there was no significant difference under 20 cm soillayer.Those results show that soil organic carbon decrese with the increasing generation of continuous cropping especially in topsoil(0~20cm).The content of soil activated carbon and stable carbon deeply decline with the increase soillayer depth in different generation of Eucalyptus grandis plantation.The content of soil activated carbon and stable carbon deeply decline with the increasing generation in both 0~10cm and 10~20cm soillayer,and thesoillayer below 20 cm also has the similar trend but not significant.The stable carbon content has no change.Those results point out that continuous cropping affects the activated carbon of soil organic carbon only especially in topsoil(0~20cm).The content of Humin,humic acids and fulvic acid carbon are all decreasing with the increasing of soil depty and generation in topsoil especially.The ratio of humic acids and fulvic acid is one generation(0.242)> two generation(0.215)> three generation(0.202).Those results show that soil humus of Eucalyptus grandis plantation become worse.The correlation analysis showed that there was line correlation between the content of soil organic carbon and each active carbon(0.622<r<0.699,p<0.01),and aslo each two active carbon(0.888<r<0.952,p<0.01).In summary,continuous cropping of Eucalyptus grandis plantation result in carbon content of understory vegetation,soil organic carbon,soil active carbon and soil humus decrease.The main resons are the fast growth rate consumes lots of soil nutrient and water and fertilization not on time;and also the soil acidification of single cropping for long time and the organic carbon decrease of utilizing full tree can make this happen.Thus,we should protect the understory vegetation and litter and leave residua after felling and strengthen the fertilizing step at its growthe exuberant period to maintain the balance of soil nutrient of forest land and realize sustainable development of Eucalyptus grandis plantation.