Stump Biomass Of Eucalyptus Plantations And Physical,Chemical And Structural Component Characteristics Of Stumps

Guangxi has huge stump biomass due to logging of Eucalyptus plantations every year.The decomposition and nutrient return of stumps are very important for the fertility maintenance of Eucalyptus plantations and the improvement of sustainable management.However,the distribution of stump biomass in Eucalyptus plantations is still unclear,the biomass changes with the stump decomposition and the relationship with the physical,chemical and structural components of stumps are still unknown.This will further hinder our quantification of nutrient biomass per unit area of Eucalyptus plantations and accurate assessment of the role of stumps in nutrient cycling and soil fertility maintenance in Eucalyptus plantations.To this end,this study set up plots under the Eucalyptus urophylla×E.grandis forests planted from 2015 to 2021.We investigated the biomass of Eucalyptus stumps decaying for 6,5,4,3,2,1 and 0 years,respectively,and its distribution with decomposition time,decay class and diameter class,and excavated the stump samples to determine the physical,chemical and structural component characteristics for clarifying the relationship between these factors and stump biomass.The research results can provide scientific reference for the elucidation of the biomass and decomposition characteristics of stumps in Eucalyptus plantations,the in-depth quantification of nutrients in stumps,and the accurate understanding of the role of stumps in nutrient cycling and soil fertility maintenance in Eucalyptus plantations.The main results are as follows:(1)In this study,the initial stump biomass in Eucalyptus plantation was 32.47 t/hm~2,but decreased to 12.32 t/hm~2 with decomposition.The biomass for roots always accounted for more than 80%of the total biomass.The stumps of decay class I and II were dominant in the early stage(0-1 years)of stump decomposition.The stumps of decay class III,IV and V dominated in the middle and late stages(2-6 years).In the early stage of decomposition,the biomass of stumps with a diameter of 15.0-20.0 cm was dominant.In the middle and later stages of decomposition,the biomass of stumps with a diameter of≤15.0 cm was dominant.(2)The biomass loss of aboveground piles of Eucalyptus was faster than that of underground roots.The biomass loss constants were 0.34 and 0.19 for Eucalyptus piles and roots,respectively.The time required for 50%and 95%of loss of pile biomass was 2.04 years and 8.81 years,respectively,while that of root biomass was about 3.57 and 15.44 years.(3)The density,cellulose and hemicellulose of stumps decreased with the increase of decomposition time,while the moisture content and water holding capacity showed the opposite trend.The root density increased with the increase of decomposition time,the water and hemicellulose contents also showed the opposite trend.The water holding capacity,cellulose and lignin contents of roots did not change significantly with the increase of decay class and decomposition time.With the continuous decomposing of stumps,the organic carbon(OC)content of piles and roots and the total nitrogen(TN)content of piles showed an"enrichment-release"trend,the total nitrogen content of roots and total phosphorus(TP)contents of piles and roots showed a"release"trend.As decomposition,both piles and roots increased in C:N,C:P,N:P,and lignin:N.With the increase of decay class,the residual rate of pyrolysis substances of piles increased,that of roots did not change significantly.But the maximum pyrolysis rates of piles and roots of the same decay class were different.(4)Pile and root biomasss were significantly positively correlated with hemicellulose content,and significantly positively and negatively correlated with density,respectively.Hemicellulose contents of piles and roots were significantly negatively correlated with decomposition time,while density of piles and roots was significantly negatively and positively correlated with decay class,respectively.