Dynamics Of Forest Fine Roots Decomposition In Mid-subtropics

Studies on fine root decomposition are essential accurate estimation of fine root production and turnover,and is useful to improve our understanding of biogeochemical cycling in forest ecosystem.As global C cycle research is in the ascendent,which promotes the development of forest ecology,fine root decomposition study had gained a widespread attention from ecological circles and become one of the hotspots in forest C cycle study.Howerver,fine root decomposition studies are mainly focused on tropical and temperate forests,but much less in subtropical forests,particularly mid-subtropical evergreen broadleaf forests.The present study was carried out in Wanmulin Nature Reserve of Fujian province,where fine roots of 7 native species(Castanopsis carlesii, Schima superba,Cinnamomum chekiangense,Castanopsis fabri,Tsoongiodendron odorum,Altingia gracilipes,Pinus massoniana)and 1 plantation(Cunninghamia lanceolata)were selected to be the research object.The fine roots of Castanopsis carlesii and Cunninghamia lanceolata were divided into 3 size classes(0-1mm,1-2mm and 2-4mm),and those of the rest species were divided into 2 size classes(0-1mm,1-2mm).The dry mass loss pattern,nutrient elements dynamics and chemical composition change were measured in the decomposition process by using both litterbag and carborundum tube methods.The present study can contribute to advance forest fine root research in our country,better understand the position of fine root in forest C cycles,and provide foundamental data to model forest C cycles in subtropical area of China.After 2-year(2002-2004)studies,the following results have been obtained:1.Fine roots decomposed faster in the early stage and slower in the later stage both in litterbag and in carborundum tube,which was related to the change of chemical composition during the decomposition process.The results of chemical composition analysis indicated that rapid leaching of the extractives contributed to faster decomposition of early stage and decomposition of the acid-insolubles did to slower decomposition of later stage.2.Annual dry mass loss of coniferous(Pinus massoniana and Cunninghamia lanceolata)fine roots(range from 38.2%～65.5%)fell in the range of the broadleaf ones(32.8%～80.7%)by using litterbag method;the lowest annual dry mass loss of coniferous fine roots(21.5%for 0～1 mm fine roots of Cunninghamia lanceolata)was comparable to that of broadleaf fine roots (27.0%for 0～1 mm fine roots of Schima superba)by carborundum tube methods,while the highest annual dry mass loss in coniferous fine roots was lower than that of Castanopsis carlesii fine roots.This results showed that plant life form can not be served to predict the decomposition rate of their fine roots.3.Decomposition rates of fine roots of different size class in the same species were not correlated with their size classes consistently.By using litterbag method,1～2 mm fine root of Castanopsis carlesii decomposed the fastest, 0～1mm ranked the second,while 2～4 mm decomposed the slowest.The finer root decomposed faster for Cunninghamia lanceolata and Schima superba, while being on the contrary for Cinnamomum chekiangense and Pinus massoniana.As for Castanopsis fabri,Tsoongiodendron odorum andAltingia gracilipes,there were not significantly difference between the fine roots of two diameter classes,as was the same for the fine roots of different diameter classes by using carborundum tube methods.These results indicated that it is not applicable to predict the decomposition rate according to the diameter of fine roots.4.The annual dry mass loss in litterbag method was higher than that in carborundum tube method.This probably was due to that carborundum tube sheilded the attack from soil fauna and formed micro-environment differed from that outside of the tube which hindered the decomposition of fine roots.5.There were significantly positive correlations between dry mass losses of fine roots and the initial concentrations of P,K,while negative correlations between dry mass losses of fine roots and the initial concentrations of acid-insoluble and ratios of acid-insoluble/P and C/P with litterbag methods.Besides,there were also significantly positive correlations between dry mass losses of fine roots and the initial concentrations of P,K,and negative correlations between dry mass losses of fine roots and the initial ratios of acid-insoluble/P and C/P,while no significant correlations between dry mass losses of fine roots and the initial concentrations of N and ratio of C/N with carborundum tube methods. Therefore,initial P concentration instead of N at present study in combination with K became the major controls of fine roots decomposition.6.Nutrients release were significantly correlated with dry mass losses of fine roots,initial concentration of P,K and C quality(concentration of acid-insoluble).K released from the onset of fine roots decomposition,while release pattern of N,P showed varieties.Accumulation-release pattern dominated the N,P release dynamics,net accumulation and release patterns also occured in the 2-year decomposition process.7.When fine roots of Castanopsis carlesii and Cunninghamia lanceolata decomposed in their original stand,their dry mass loss rates and nutrients release rates were higher than that of their mixed fine roots decomposing in Cunninghamia lanceolata stand with litterbag method,which indicated an antagonistic effect upon decomposition of the mixed roots.The result showed that mixed forests of Castanopsis carlesii and Cunninghamia lanceolata would not bring a synergistical effect on decomposition of their roots.