Changes Of Fine Root Respiration And Fine Root Decomposition After Degraded Succession Of Primary Korean Pine Broad-leaved Forest

Forest ecosystem plays a very important role in the carbon cycle of terrestrial ecosystem.In the process of forest degradation and succession under human disturbance,soil organic carbon turnover will be affected by the decomposition of aboveground and root litter.Fine root,as an important driving factor of soil carbon turnover,plays an irreplaceable role in the biogeochemical cycle of forest ecosystem.Fine root respiration and fine root decomposition are the main ways of carbon exchange and nutrient absorption.In this study,the primary Korean pine broad-leaved forest(PK)and the middle and young secondary broad-leaved forest(SF)formed by 60 years of clear-cutting in Liangshui National Nature Reserve of Heilongjiang Province were selected as the research sites.The purposes are to compare the changes of fine root biomass,fine root respiration and fine root decomposition in order to provide basis for the scientific evaluation of carbon turnover level of the two forests.At the same time,the coupling relationship between soil physical and chemical factors,soil microbial community structure,enzyme activity and fine root decomposition were established based on structural equation model(SEM).The main results are as follows:1.In the whole growing season,the average fine root biomass in 0-20cm soil layer of PK and SF was 28.91 t·hm-2 and 40.26t·hm-2,respectively,and PK was 28.19%lower than that of SF.The proportion of fine root biomass in 0-10cm of soil surface of the two forests was 64.37%and 60.50%of 0-20cm soil layer,respectively.It meant that there were more fine root biomass in 0-10cm soil layer of the two forests.2.In the whole growing season,the PK(2.52)was significantly higher than that of SF(2.21)in terms of the ratio of living/dead root biomass(p<0.05),and the fluctuation degree of the variation curve of living and dead fine root biomass of PK was stronger than that of SF,which reflected that the fine roots of primary Korean pine forest were in the process of rapid growth,death and turnover.3.The fine root respiration rate(Rr)of PK was lower than that of SF during the whole growing season,and the difference between them was extremely significant(p<0.001).Rr of the two forests were positively correlated with soil temperature(p<0.001),and the temperature sensitivity index(Q10)of PK was significantly higher than that of SF<0.05).In the context of global warming,higher temperature will have a greater impact on the fine root respiration in the primary Korean pine forest and release more CO2 into soil through fine root respiration.4.Pearson correlation and linear regression analysis showed that there was a significant positive linear correlation between Rr and living fine root biomass,total soil organic carbon,soil soluble organic carbon and soil microbial biomass carbon(p<0.05).Stepwise regression further showed that the synergistic effect of the living fine root biomass and soil microbial biomass carbon could explain 75%of the changes in Rr(p<0.001,R2=0.752).This means that fine root respiration after degradation succession of primary Korean pine forest is more easily affected by underground biomass and soil microbial biomass.5.Fine roots in the PK and SF both decomposed more rapidly and lost more mass in the period of early stage(from May 2016 to September 2016),while they decomposed relatively slowly in the period of late stage(from October 2016 to June 2017).After one year decomposition,the fine root mass remaining of PK(45%)was significantly lower than that of SF(56%)(p<0.01).At the same time,according to the Olson’s model(negative exponential equation),the annual decomposition rate coefficient(k)of fine root(0.60)was significantly higher in the PK than that of SF(0.43),and the time of fine root mass decomposition of 50%and 95%was significantly lower than that of SF(p<0.05).6.During the whole decomposition period(one year),the chemical composition mass remaining(%)except K in SF was significantly higher than that of PK(p<0.05),which meant that the turnover time of main fine root chemical composition in the secondary broad-leaved forest was significantly longer than that in the primary Korean pine forest.Among them,in the process of fine root decomposition,the fine root C decomposition dynamic curve of SF was slower than PK,the decomposition was slow and chemical composition mass remaining was higher,indicating that the release rate of fine root C in the SF was slower than that in the PK.Overall,it reflects that the primary Korean pine forest has a stronger ability to return soil nutrients through fine root decomposition.7.During the whole decomposition period,except for soil actinomycetes,the total phospholipid fatty acid content(PLFAs)of soil microorganisms,the PLFAs of bacteria,Gram-positive bacteria,Gram-negative bacteria and fungi and the ratio of fungi to bacteria were significantly higher in PK than those of SF(p<0.001).The PLFAs of each group of soil microorganisms of the two forest types reached the maximum in August 2016(except Gram-positive bacteria).Simultaneously,there were significant differences in soil microbial diversity index and microbial community structure between the beginning of decomposition and the end of decomposition.And the soil microbial diversity index and soil microbial community biomass were significantly higher in general in PK than those of SF(p<0.05).Non-metric multi-dimensional scale analysis(NMDS)also proved that there was obvious differentiation of soil microbial communities in the two forest types at the beginning and end of fine root decomposition.The results showed that fine root decomposition had an important effect on the structure of soil microbial community.8.During the whole decomposition period,except for cellulase activity,forest type and decomposition period had extremely significant effects on the other five soil enzyme activities(p<0.001).In addition to catalase activity,the peak value of enzyme activity appeared in September and October 2016.Except for phenoloxidase and leucine aminopeptidase activities,the mean enzyme activity of PK was significantly higher than that of SF(p<0.05).The activities of cellulase,polyphenol oxidase and acid phosphatase of the two forest types were all significantly different at the beginning and end of fine root decomposition(p<0.05).9.The construction of the structural equation model(SEM)was to transform the variables(fine root substrate quality,soil physical and chemical properties,soil microbial PLFAs biomass and enzyme activity)which had a nonlinear relationship with the fine root decomposition into the linear relationship through the generalized additive model.Then the important significant factors affecting fine root decomposition were screened out by stepwise regression,which were fine root substrate quality(potassium and lignin),soil physical and chemical properties(soil bulk density,soil total nitrogen,soil C/N ratio)and soil microbial properties(fungal PLFAs biomass,actinomycete PLFAs biaomass,soil phenoloxidase and phosphatase activities).The variation partition analysis showed that soil microbial properties had the most significant effect on fine root decomposition,followed by fine root substrate quality,and finally soil physical and chemical properties.Based on SEM analysis,it was found that forest type can not only directly affect fine root decomposition,but also indirectly affect fine root decomposition by changing substrate quality,soil microbial properties and soil physico-chemical factors.Among them,fine root chemical composition(potassium and lignin)had the most direct influence on fine root decomposition,and soil fungi and actinomycetes PLFAs biomass had more important influence on fine root decomposition than other functional groups.Eventually,many factors(soil physicochemical factors and soil microbial community)would affect fine root decomposition through soil phenoloxidase and acid phosphatase activities.