Effects Of Continuous Cropping Poplar On Plant Growth And Soil Fertilities

In the north of Jiangsu province, the characteristics of tree growth of poplar (I-69 and I-72) and soil with same site index but different rounds of continuous cropping and different age have been studied through plot observations and intensive investigations. According to the experimental datum, correlation analysis and principle component analysis had been done to find out factors of site productivity decline and ways to keep long-term maintenance of the productivity in poplar plantations.The results showed that:1, With the increasing continuous cropping ,the height, D.B.H and volume of 4-year-old poplar begain to significantly decreased from the third planting, but the second planting grew well than the firest and the third generations. With the increasing continuous cropping , the height, D.B.H and volume of 7-year-old, 10-year-old poplar inclined and the defference between the defferent generations were significant.Compared with the total contents of N, P and K of poplar leaves of different generations the law was not significant,with the age grew, the total contents of N,P and K of poplar leaves inclined.2, Along with the increasing cropping generations, soil bulk density of 4-year-old and 7-year-old of poplar plantations all increased. Soil bulk density of I-69 and I-72 of 7-year-old increased. With generations increasing, the soil total porosity and capillary porosity of I-69 and I-72 of 4-year-old, 7-year-old and 10-year-old all declined, non-capillary porosity didn't obviously vary. Field capacities of 0~20cm of I-69 and of 0~20cm and 20~40cm of I-72 all increased with the increasing cropping generations, but proliferation was not large. Field capacities of 20~40cm and 40~60cm of I-69 and 40~60cm of I-72 didn't have regularity.3, Total N and OM of the surface layer of different ages plantations increased with the increasing continuous cropping. Siol hydrolyzable N, available P, available K and pH of I-69 and I-72 plantations declined with the generation increasing. The contents of soil nutrient in 20~40cm and 40~60cm of I-69 and I-72 plantations were not obviously vary and the regularity were not good.4, Enzyme activity of soil were obviously influenced by the continuous cropping of I-69 and I-72 poplar plantations. With the generation increasing, the activity of soil urease and alkaline phosphatase were getting weaker.In the same generation. In the same generation, with the tree growing, the content of soil urease and alkaline phosphatase were getting weaker. Proteinase were getting stronger,with generations increased. In the same generation, with the tree growing, proteinase was also getting stronger.5, Soil total N, hydrolyzable N, available P, available K and proteinaseshowed postitive correlation, r=0.530-0.685. Soil total N> hydrolyzable Nn available P, available K> capillary porosity ^ proteinase and urease showed postitive correlation, r=0.603~ 0.749. Available P> available Kn proteinase^ urease and alkaline phosphatase showed postitive correlation , r=0.659-0.880.OM and proteinase showed postitive correlation, r=0.616. OM and urease showed negative correlation, r=-0.588. OM and total N showed postitive correlation, r=0.526.OM% available K, capillary porosity and soil water showed negative correlation, r=-0.524—0.649. Principle component analysis indicated that soil hydrolyzable N> pH> OM^ available P^ available K> urease, alkaline phosphatase and proteinase were more important than other soil factors. The proportion of first principle component was 0.555, the second was 0.149, the third was 0.133.