Effects Of Exotic Plant Eucalyptus On Soil Carbon And Nitrogen Cycling Processes

Carbon and nitrogen cycling are important functional processes of forest ecosystem, regulating and maintainning forest ecosystem productivity and stability. Eucalyptus is an exotic tree species large-scale cultivated in subtropical regions in China. Reseaches on the impacts of Eucalyptus afforestation on soil carbon and nitrogen cycling could be helpful to make comprehensive assessment of the potential ecological consequences of Eucalyptus and provide a scientific basis for eucalyptus forest management. Eucalyptus plantations at East Gate Farm, Guangxi Province, and the surrounding areas are targeted. Our reseach focus on two aspects: to reveal the impacts of Eucalyptus on soil carbon and nitrogen cycling through comparative study with paired experimental design of the Pinus massoniana and Eucalyptus; to explore the long-term impact of Eucalyptus on soil carbon and nitrogen cycling through the method of space representing time to study soil carbon and nitrogen cycling of generation 1,2,3,4 Eucalyptus plantations. The main results are as follows:(1) Eucalyptus afforestation significantly reduced the sizes of soil carbon and nitrogen pools, changed the composition of soil carbon and nitrogen pools. Total carbon, organic carbon, total nitrogen, available nitrogen content of Eucalyptus soil was significantly lower than Pinus massoniana soil; compared with Pinus massoniana soil, the proportion of labile carbon pool I, soil ammonia and nitrate content was increased and Recalcitrant pool was reduced in Eucalyptus soil.(2) Eucalyptus afforestation changed the rates of soil carbon and nitrogen cycling processes. compared with Pinus massoniana soil, activities ofβ-glucosidase, phenol oxidase, peroxidase, protease, urease of Eucalyptus soil was significantly reduced; denitrification rate of Eucalyptus soil was reduced too, but N mineralization, ammonification and nitrification rates of Eucalyptus soil were higher than Pinus massoniana.(3) Eucalyptus afforestation decreased soil carbon and nitrogen pools, changed the rates of soil carbon and nitrogen cycling processes at early stages, but these changes trend to recover latter. Eucalyptus soil carbon and nitrogen pools declined in generation 1,2, and recovered slightly in generation 3,4; activities of phenol oxidase, peroxidase, protease, urease declined in generation 1,2, recovered in generation 3,4; net nitrification rates increased in generation 1,2 and then become stable in generation 3,4; denitrification rate decreased significantly at generation 1, and recovered slightly at generation 2,3,4.(4) Substrate availability and soil moisture are the main enviromental factors for eucalyptus to drive changes of soil carbon and nitrogen cycling processes. Soil enzyme activity was significantly correlated with substrate availability; peroxidase, protease and urease activity were signightly correlated with soil water content.Exotic species Eucalyptus significantly decreased soil carbon and nitrogen pools, changed the rates of soil C and N cycling processes, but these changes showed a recover trend in continuous planting. Substrate availability and soil moisture are main environmental factors for Eucalyptus to alter soil carbon and nitrogen cycling processes.