| Alnus cremastogyne Burk, an endemic species of China, is an ideal tree species used for conversion of the farmland to forestry and construction of ecological protective forest and mixed forest. It is one of the tree species used for cultivating short rotation industry timber and has been considered as one of the major deciduous species utilized in Forest Resource Development and Protection Project (FRDPP) and base construction of pulpwood. Soil nutrient, root morphology and spatial distribution characteristics of biomass and nutrient element in Alnus cremastogyne Burk plantations are useful for well understanding root response to the spatial heterogeneity of soil nutrient and also valuable to selection of silviculture measures and studies on forest ecological system.Huayuan Alnus cremastogyne Burk plantations of four age classes (4-year-old, 6-year-old,9-year-old 15-year-old) and Miluo Alnus cremastogyne Burk plantations of 6-year-old were taken as cases in this study. On the basis of measurement of all individuals in a plot, three sample stems were selected. The root system was investigated by the hierarchical digging method, and nutrient element contents were measured. Subsequently, the data on physical and chemical properties of soil, root biomass, morphological distribution of root and nutrient element were processed and analyzed using softwares Excel and SPSS. The main results are as follows:The pH increased gradually with increase of forest-age and soil depth. The organic content decreased gradually with the increase of soil depth but increased with the increase of forest-age. And the pH and the organic content in rhizosphere soil were higher than those in Non-rhizosphere soil. In Miluo plantations, for primary nutrient content in soil K was the highest followed by Ca, Mg, N and P; for micronutrient content Fe was the highest followed by Mn, Zn, Cu, Ni, Pb and Cd. The contents of the primary nutrients apart from P and Ca and all of the micronutrients in rhizosphere soil were lower than those in Non-rhizosphere soil. P and Ca were exceptions due to their higher contents in rhizosphere soil than those in Non-rhizosphere soil. In Huayuan plantations, for primary nutrient content in soil Mg was the highest followed by K, Ca, N and P; for micronutrient content, Fe was the highest followed by Mn, Zn, Ni, Cu, Pb and Cd. The contents of N,K,Fe,Zn,Cd and Pb in soil increased gradually with the increase of forest-age, while those of P and Mn decreased with the increase of forest-age. Among three phases of soil, the root grew well only when soil solid rate accounted for below 54.5%, solid liquid rate ranged from 21% to 32.5%, and soil solid rate was between 12.5% and 35.5%.The biomass in Miluo Alnus cremastogyne Burk plantations of 9-year-old was the highest followed by 15-year-old,6-year-old and 4-year-old. For the root system of 4-year-old,6-year-old and 15-year-old, the biomass of large-sized root was the highest followed by middle-sized root, small-sized root, and fine-sized root, while the order of biomass proportion in 9-year-old root was:large-sized root> middle-sized root>fine-sized root>small-sized root. In addition,the biomass of large-sized root and middle-sized root ascent gradually, while those of small-sized root and fine-sized root did not show apparent change pattern. The change order of total root length (TRL) and specific (SRL) was:fine-sized root>small-sized root> large-sized root>middle-sized root, and that of root length density(RLD) was: fine-sized root>large-sized root> small-sized root>middle-sized root. The RLD and the length of middle-sized, small-sized root and fine-sized root gradually decent for a given soil layer increase, while the SRL and the length of large-sized root first ascent and then decent. In horizontal direction, the length of large-sized root and RLD reduced gradually, the length of middle-sized root and small-sized root and RLD first reduced and then increased, and the length of fine-sized root first increased and then decreased. The SRL of large-sized root and middle-sized root first increased and then decreased, that of small-sized root gradually decreased, and that of fine-sized root increased gradually.The order of primary nutrient content of root in Miluo plantations was:Ca>N >K>Mg>P, and that of micronutrient was:Fe>Mn>Cu>Zn>Ni>Pb>Cd. In horizontal direction, N content in large-sized root first increased and then decreased, P and K increased gradually, and Ca, Mg and Fe showed a decease-increase-decrease change pattern. Generally, the remaining micronutrients showed a decline tendency. In middle-sized root, N content decreased gradually, K, Ca and Mg increased, and P and micronutrients first increased and then decreased. In small-sized root, N content gradually declined, P, K, Cu, Mn and Cd first declined and then increased, and Ca, Zn and Ni first increased and then decreased. In fine-sized root, the content of N and K decreased gradually, Mg increased gradually, Ca and Fe first increased and then decreased, and other nutrient elements did not show apparent change pattern. In vertical direction, N content in large-sized root first decreased and then increased with the increase of soil depth, P, K, Ca, Fe, Mn and Ni first increased and then descent, Mg and Cu gradually declined, and Zn, Cd and Pb increased gradually. In middle-sized root, the nutrient contents each showed a complicated change pattern. In small-sized root, the contents of N, P, Ca, Mg, Cu, Ni, Pb and Cd basically increased and then declined, K decreased gradually, Zn and Mn first decreased and then increased. In fine-sized root, K content first increased and then descent, Mg exhibited a reversal change pattern, Cu increased gradually, and Zn, Mn, Cd and Pb declined.
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