Nutrient Biogeochemical Cycling In Cunninghamia Lanceolata Plantation Of Different Forest Ages

The aim of this study is to explorer the law of biogeochemical cycle of nutrient in Chinese fir plantation at different ages.In this study,based on the basic data of 25years of continuous measurement of Chinese fir forest plantation in Huitong County,the internal nutrient cycle in Chinese fir is considered as a link of the biogeochemical cycle of stand nutrients,and the nutrient balance model including the internal nutrient cycle is expanded on the basis of traditional nutrient balance model.Based on the growth cycle of branches and leaves,the existing biomass(including tree stem,bark,branches,leaves,roots,etc.)in the stand at the beginning of a certain age segment is estimated,which is still viable at the end of this age segment marked as"("which means for"is expressed as"still viable biomass").A soil nutrient uptake model was established to estimate the effect of tree nutrient internal circulation on soil nutrient uptake.According to the growth cycle of branches and leaves,how to estimate the nutrients returned from the dead branches and leaves in a certain age segment and those that do not belong to the soil nutrients absorbed by trees in a certain age segment,the nutrient biogeochemical cycles of different ages of Chinese fir plantation were studied.The results are as follows.1)In the 8-11 year old Chinese fir forest,31.33 kg/hm~2of nutrients were transported from the external"still viable biomass".The rate of nutrient transfer into"still viable organs"(if the whole tree is still viable biomass,use"still viable biomass"expression,if the tree is still viable an organ,use"still viable organ"expression,the same below):leaves>branches>skins>stems>roots.The nutrient transfer from"still viable biomass"was 25.90～29.15 kg/hm~2at each age segment after 11 years of stand.The rate of outward nutrient transfer from the"still viable organ"was:stem>root>branch>leaf>bark.The nutrient transfer direction of"still living biomass"is determined by the nutrient demand and utilization efficiency of trees in different growth and development stages.The rate of nutrient transfer in organs is related to the physiological function of organs.For the transfer rate of nutrients transported from the external organ,the higher the function of the organ,the greater the nutrient transfer rate into the organ,and the higher the function of the organ,the smaller the nutrient transfer rate of the organ.2)In the Chinese fir forest with 1 to 25 years of life,3.04～31.89kg/hm2.a was transferred to the living tissue annually before the death of the branches and leaves,and most of the nutrients were transferred before the death of the leaves.The transfer amount of N,P and K elements is greater than that of Ca and Mg.There was no significant difference in the nutrient transfer rate of Chinese fir branches and leaves before dying(P>0.05),and the effect of forest age on the nutrient preservation ability of Chinese fir branches and leaves was not significant3)The nutrient accumulation at different ages was 470.58～1838.83 kg/hm~2.At any age of the forest,the nutrient accumulation of each organ was in the order of leaf>branches>bark>trunk>roots,and the amount of nutrient accumulation in each organ was in the order of N>Ca>K>Mg>P.Nutrient accumulation increased with forest age.The nutrient accumulation of each organ and the variation trend with the age of the forest were mainly controlled by the productivity,nutrient concentration and growth characteristics of each organ at different growth stages.Some of the nutrients that are recycled are stored in the tree in the form of recycled material.The contribution of different forms of nutrient internal circulation to the nutrient accumulation in the stand was significantly higher than that of the"still viable biomass"before the death of branches and leaves.4)The average annual nutrient return was 10.20～101.31 kg/hm2.Leaf and branch return accounted for 60.32%～100%and 0～39.68%,respectively.The amount of returned nutrient elements,Ca and N accounted for more than two-thirds.The amount of nutrient returned increased with the increase of forest age.Different organs of trees have different growth cycles,and some of the nutrients returned in a certain age segment of Chinese fir forest are not returned by the dead of the branches and leaves produced in the stand of the current age segment,but by the branches and leaves produced in the stand of the previous age segment.The transfer of nutrients to living tissues before the death of tree organs can reduce the amount of nutrients returned.5)The average annual absorption of soil nutrients in Huitonghua fir forest at different ages ranged from 82.05 to 160.72kg/hm2 a,and the amount of soil nutrients absorbed was N>Ca>K>Mg>P.Reusing nutrients from branches and leaves before dying to living tissues can reduce the uptake of soil nutrients by trees.In the stage of rapid growth from young trees to trees,new nutrients need to be added to the"still viable biomass",which has the effect of increasing the amount of soil nutrient absorption.The effect of nutrient reuse from"still viable biomass"in reducing soil nutrient uptake at each age segment after stand closure.In terms of reducing the amount of soil nutrient uptake,the transfer of nutrients from branches and leaves to living tissues before dying was more important than that from"still living biomass".The soil nutrients absorbed by stands at different age segments were not only controlled by the productivity of different organs with different nutrient contents,but also affected by the transfer of nutrients to living tissues before the death of branches and leaves and the new absorption or transfer of nutrients from"still living biomass".6)In the nutrient biogeochemical cycle of Huitong Chinese Fir forest,the utilization coefficients of each nutrient element decreased with the increase of forest age,indicating that the utilization efficiency of nutrient element increased with the increase of forest age.The utilization efficiency of Mg and N elements in 1-11 years old Chinese fir forest was higher than K,P and Ca.After stand closure,the nutrient use efficiency of N,P and K was higher than that of Ca and Mg.The internal nutrient circulation of trees can improve the efficiency of nutrient utilization.7)In the nutrient biogeochemical cycle of Chinese fir forest,the nutrient cycle coefficient increased with the increase of age,indicating that the efficiency of nutrient return increased with the increase of tree growth and development stage.The nutrient cycling coefficients of N,P and K were less than those of Ca and Mg,indicating that the utilization time of N,P and K was longer than that of Ca and Mg during the growth of Chinese fir.The internal nutrient circulation of trees can increase the nutrient return efficiency of stands.8)In this study,the tree nutrient internal cycle was taken as an intermediate link of forest nutrient biogeochemical cycle,and the nutrient balance model including nutrient internal cycle was extended on the basis of the traditional nutrient balance model.The extended model overcomes the neglect of plant nutrient cycling in forest nutrient biogeochemical cycling in traditional models.It can reflect the nutrient utilization characteristics and circulation of trees at different forest ages.The extended model can be applied to nutrient cycle analysis of various forest types.