The results of positioning test on the nutrient cycling of 1-69 poplar plantation within the first rotation show that; (1) The suitable range of average annual leaf nutrient values in the fast-growing period are; N(2. 98% -3.37%) >Ca(2.26% -2.95%) >K(0.65% -0.73%) >Mg(0.35% -0.46%) >P(0. 11% -0. 13)%) , indicating the I -69 is a N- and Ca-favored species. (2) Both the annual and monthly variations of leaf nutrient values in the fast-growing period are extremely significant; the tree growth is regular and the dynamic changes of leaves in nutrient absorption, accumulation, distribution and transfer reflect the characteristics of nutrient cycling in vivo. The sequence of forest biomass allocation is the same for each of the year and follows the order of stem > branch > lateral root > taproot > stem bark > taproot bark, and the biomass of leaves decrease with the increase of forest age; the nutrient allocation in fast-growing period follow the order of lateral roots > branches > stem > stem bark > taproot > taproot bark; in the late stage, the nutrients in branch and stem are higher than in the lateral roots, and that in leaves decrease with the increase of stand age, but the nutrient distribution ratio for leaves is higher than that for stem; The accumulation and distribution of biological nutrient are consistent with the tree growth.(3) The nutrient cycling rates in the forest ecosystems are 0. 47 in the early growth stage and 0. 28 in the late growth stage, and the nutrient utilization rates are also different, i. e. in early stage, N 7. 6 kg, Ca 4. 97 kg, K 1. 89 kg, Mg 0. 84 kg, and P 0. 26 kg are needed to produce 1 ton of dry matter compared with N 4. 3 kg, Ca 6. 34 kg, K 1. 39 kg, Mg L 14 kg, and P 0. 18 kg in late stage. The mean percentages of annual move away of soil nutrients of post-harvested plantation in soil mean available nutrients are N 36. 80% , P 52. 8% , and K 19. 73% , with a rather higher expenditure. However, there is a high level of soil nutrient supply, so the nutrient cycling between trees and soil in the forest ecosystem is still in virtuous circle with dynamic equilibrium.%对首个轮伐期内I-69杨人工林养分循环的定位试验结果表明:(1)速生期叶片养分年平均适宜范围值:N(2.98％～3.37％)＞Ca(2.26％～2.95％)＞K(0.65％～0.73％)＞Mg(0.35％～0.46％)＞P(0.11％～0.13)％),属喜N、Ga树种.(2)速生期叶片养分的年、月变异极显著,林木生长规律一致,叶片对养分吸收、积累、分配与转移的动态变化反映了体内养分循环的特性.林木生物量分配顺序各年都相同:干＞枝＞侧根＞主根＞干皮＞主根皮,叶随林龄增加递减；养分分配速生期为:侧根＞枝＞干＞干皮＞主根＞主根皮,后期枝、干大于侧根,叶随林龄增加递减,但养分分配比叶高于于.生物养分量的积累与分配与林木生长一致.(3)该森林生态系统内养分循环速率:生长前期为0.47,后期为0.28；养分利用率也不同,前期生产1t干物质需要的主要养分量分别为:N:7.6 kg,Ca:4.97 kg,K:1.89 kg,Mg:0.84 kg,P;0.26 kg,后期则为N:4.3 kg,Ca:6.34 kg,K:1.39 kg,Mg:1.14 kg,P:0.18 kg.人工林采伐后年平均移出养分量占土壤相应年平均速效养分量为:N:36.80％,P:52.8％,K:19.73％.支出较大.但是土壤养分供给水准较高,在该森林生态系统中,以林木和土壤为主导的养分循环仍处于动态平衡的良性循环之中.
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