Abstract:

Distribution and biological cycling of five microelements(Fe，Mn，Zn，Cu and B) as well as their changes with growth of stands in Acacia mangium plantation of three different stand ages(4-year-old，7-year-old and 11-year-old) were studied in the city of Nanning，Guangxi．The results showed that concentrations of these five microelements in different components of A．mangium plantation were in the order of leaves > dead branch > bark >alive branch > root > stem．In plant，concentration of Mn was the highest among the five elements，followed by Fe,Zn and B，while Cu was the lowest in various components of A．mangium plantation．In the soil, concentration of Fe was the highest，then Mn，Zn，B，and Cu concentration was the lowest among six elements. The total microelements storage at three stand ages plantations were 13.953，22.357，and 28.835 kg·hm^-2，respectively． Of the total storage of microelements, 47.37%～58.68% was distributed in arbor layer，14.76%～23.54% at forest floor layer，26.63%～29.10% at standing litter layer．The storage of microelements in A. mangium varied with different stands growth stages，most microelements storage in leaves and branches at 4-year-old，while they gradually moved gradually in stem and root at 7-year-old and 11-year-old. Annual net accumulations of microelements in three stand ages plantation were 1.652，1.750 and 1.536 kg·hm^-2a^-1，respectively．Comparison of microelements annual net accumulation in different components was in the order of stem > branches or roots >leaves > bark．Changes in annual net accumulation of various microelements in the same component was similar to that in various components，Mn＞Fe＞Zn or B＞Cu．The annual absorption of microelements in three stand ages plantations were 5.423，5.862 and 5.273 kg·hm^-2a^-1，respectively．The annual returns equaled to 3.771 , 4.112 and 3.737 kg·hm^-2a^-1， respectively. The microelement utilization coefficients were 0.821，0.479 and 0.312,respectively. The recycling periods were 1.753，2.979 and 4.522 years，respectively.

Key words: Acacia mangium plantation, stand age, microelement, biological cycling, Nanning