| Cunninghamia lanceolata is one of the main fast-growing timber species in southern China.With the improvement of people’s living standards,the market demand for large-diameter timber of C.lanceolata has been increasing,and the efficient cultivation of large-diameter timber has become a hot topic in the research of C.lanceolata cultivation.Meanwhile,the continuous increase in global temperatures and greenhouse gas emissions have profoundly affected the atmospheric carbon balance.Forest ecosystem is the largest carbon reservoir in terrestrial ecosystem,and forest carbon sequestration has become an important means to effectively deal with global climate change.Increasing carbon fixation in forest ecosystems through rational afforestation and forest management is one of the main means of implementing emissions reduction.Therefore,how to create C.lanceolata artificial forests that have dual functions of producing large-diameter timber and carbon sequestration has become a technical difficulties to be addressed in the management of C.lanceolata artificial forests in the new era.In the process of cultivating large-diameter timber of C.lanceolata,constructing a mixed-age forest structure of conifers and broadleaved trees through underplanting native broadleaved tree species is one of the main technical measures.To accurately evaluate the carbon sequestration capacity of C.lanceolata forests with underplanted broadleaved species and to reveal the soil carbon sequestration mechanism of the forest,this study selected a 43-year-old C.lanceolata artificial forest in Yong’an State-owned Forest Farm in Fujian Province as the research object.Using randomized block test design,three management models(T1 selective logging,T2 selective logging and interplanting Castanopsis hystrix,T3 no selective logging and no interplanting)were designed,and based on the measurement of ecosystem biomass and carbon storage and its allocation pattern of the three management models,the content of active organic carbon(AOC)components and the trend of AOC/SOC changes in the soil of the three management models were further analyzed.In addition,the differences in microbial residue carbon(MRC)indicators and their contribution to soil organic carbon(SOC)turnover were investigated,and using soil macro-genome sequencing technology,the differences in the abundance of microbial carbon cycling-related functional genes in the soil of the three management models were explored.Furthermore,the relationship between AOC components,MRC indicators,microbial carbon cycling functional gene abundance,and environmental factors was analyzed.The research results provide scientific basis for the scientific management and exploitation of carbon sequestration potential of C.lanceolata artificial forests.The main research results are as follows:(1)The average height,average diameter at breast height,and average biomass of C.lanceolata in the T1 and T2 treatments were higher than those in the T3 treatment.The biomass allocation of C.lanceolata in all treatments showed a pattern of trunk>root>leaf>branch.In the large-diameter timber C.lanceolata plantation,both the biomass of the tree layer and the ecosystem showed a pattern of T3>T2>T1,with the ecosystem biomass in the T3 treatment significantly higher than that in the T1 treatment(P<0.05).The biomass of the understory vegetation and litter layer in the large-diameter timber C.lanceolata plantation showed a pattern of T3>T1>T2,with the biomass of the understory vegetation in the T2 treatment significantly lower than that in the T1 and T3 treatments(P<0.05).The biomass of the understory vegetation in all treatments was significantly higher in the underground part than in the aboveground part(P<0.05).Therefore,the selective harvesting treatments(T1,T2)can increase the growth of C.lanceolata,but at the same time reduce the biomass per unit area of the stand.The selective logging and interplanting C.hystrix(T2)can to some extent accelerate the decomposition rate of litter,resulting in a lower litter layer biomass.(2)There were no significant differences in the carbon content of the understory vegetation and litter layer among the three treatments(P>0.05).The carbon content of the aboveground part of the understory vegetation in all treatments was significantly higher than that of the underground part(P<0.05).With the increase of soil depth,the soil carbon content in all treatments showed a decreasing trend,and the carbon content of the 0-20 cm soil layer was significantly higher than that of the 60-100 cm soil layer(P<0.05).In the 0-10 cm soil layer,the soil carbon content in the T2 treatment was significantly higher than that in the T1 and T3 treatments(P<0.05);in the 10-40 cm soil layer,the soil carbon content in the T2 treatment was significantly higher than that in the T1 treatment(P<0.05);in the 40-100 cm soil layer,there was no significant difference in soil carbon content among the three treatments(P>0.05).Therefore,the selective logging and interplanting C.hystrix(T2)can increase the soil carbon content in the 0-40 cm soil layer of the stand.(3)The carbon storage distribution of each organ of C.lanceolata in the three treatments was as follows:trunk>root>leaf>branch.The carbon storage of the ecosystem in the large-diameter C.lanceolata plantation showed a pattern of T2>T3>T1,with the carbon storage of the T2 treatment significantly higher than that of the T1 treatment(P<0.05).The carbon storage allocation pattern of the three treatments in the forest ecosystem all showed a pattern of soil layer>tree layer>understory vegetation layer>litter layer.The carbon storage of the tree layer in the large-diameter C.lanceolata plantation showed a pattern of T3>T2>T1.The carbon storage of the understory vegetation layer and the litter layer in the large-diameter C.lanceolata plantation showed a pattern of T3>T1>T2,consistent with the pattern of biomass.With the increase of soil depth,the soil carbon storage of the three treatments all showed a decreasing trend,and the proportion of carbon storage in the 0-60cm soil layer accounted for 72.93%-80.53%of the total soil layer.In the 0-60 cm soil layer,the soil carbon storage of the T2treatment was significantly higher than that of the T1 and T3treatments(P<0.05),while there was no significant difference in soil carbon storage among the three treatments in the 60-100 cm soil layer(P>0.05).It can be seen that the selective logging and interplanting C.hystrix(T2)can enhance the carbon storage capacity of the soil layer and the ecosystem of the forest stands.(4)The distribution of organic carbon in soil particles,soluble organic carbon(DOC),labile organic carbon(ROOC),and microbial biomass carbon(MBC)in the three treatments all exhibit a certain degree of aggregation.The POC and DOC content of the three treatments decrease gradually with soil depth.The MBC content of the T2 treatment soil first decreases and then increases with increasing soil depth.The DOC/SOC and MBC/SOC ratios of the three treatments increase with increasing soil depth.There was no significant difference in POC and MBC content among the three treatments(P>0.05).In the 10-40 cm soil layer,the ROOC content of the T2 treatment was higher than that of the T1 and T3 treatments.In the 0-100 cm soil layer,the DOC content of the T2 treatment was higher than that of the T1 and T3treatments.In the 60-100 cm soil layer,the DOC/SOC of the T1and T2 treatments was higher than that of the T3 treatment.All AOC components were negatively correlated with soil bulk density(P<0.01)and positively correlated with soil total nitrogen(TN),SOC,and available nitrogen(P<0.01).Redundancy analysis showed that soil bulk density was the main driving factor for changes in AOC component content in the 0-10 cm,20-40 cm,and60-100 cm soil layers,while soil total phosphorus(TP)was the main driving factor for changes in AOC component content in the10-20 cm soil layer,and soil TN was the main driving factor for changes in AOC component content in the 20-40 cm soil layer.Therefore,the logging treatment(T1,T2)led to a decrease in the stability of organic matter in the surface soil,but the logging plus C.hystrix planting treatment(T2)enhanced the migration of DOC to deeper soil layers,which was conducive to the storage of DOC and ROOC in the soil.(5)The total amount of soil amino sugars,Glu N,Mur N,Gla N,MRC_F,and MRC_B,showed a decreasing trend with increasing soil depth in three forest management treatments.Soil amino sugars,Glu N,Gla N,and MRC_F were highly enriched in the surface layer.In the 0-10 cm soil layer,the content of soil amino sugars,Glu N,Mur N,Gal N,and MRC_F was highest in T2,followed by T1 and T3,and the difference was significant(P<0.05).In the 40-60 cm soil layer,the content of soil amino sugars,Glu N,Mur N,Gla N,MRC_F,and MRC_B was highest in T1 and T2,followed by T3,and the difference was significant(P<0.05).In the 0-40 cm soil layer,Glu N/Mur N was highest in T2,followed by T1 and T3.In the 20-40 cm soil layer,MRCB/SOC was lowest in T2 and highest in T1and T3,and the difference was significant(P<0.05).The contributions of MRC_F and MRC_B to SOC were stronger in the 20-60 cm soil layer.All MRC indicators were strongly positively correlated with soil TN and SOC(P<0.01).Redundancy analysis showed that TP was the main driving factor for changes in MRC indicators in the 0-10 cm and 40-60 cm soil layers,while TN was the main driving factor for changes in MRC indicators in the 10-20 cm soil layer,and soil bulk density was the main driving factor for changes in MRC indicators in the 20-40 cm soil layer.Therefore,the logging plus C.hystrix planting treatment(T2)can increase the content of soil MRC in the surface layer of C.lanceolata plantations and promote the accumulation of fungal dead residues in the surface soil.(6)Based on the CAZy and KEGG databases,the composition and abundance of carbon cycling functional genes in soil microorganisms showed significant differences among the different treatments(P<0.05).The main carbohydrate enzymes screened in the three treatments were glycoside hydrolases(mainly GH57)and glycosyltransferases(mainly GT2).The relative abundance of GT2 was significantly higher in T1 than in T2 and T3,while the relative abundance of GH57 was significantly higher in T2 than in T1 and T3(P<0.05).The top 10 KO genes in terms of relative abundance among the screened genes in the three treatments were:K02274,K03520,K00174,K03518,K00175,K03519,K00244,K00169,K00170,K00016.The carbon metabolism functional gene K03518 up-regulated in the soil microorganisms in T1 showed a significant negative correlation with soil nutrients and DOC(P<0.05).The carbon fixation functional gene K00244 up-regulated in the soil microorganisms in T1 showed a significant negative correlation with soil moisture content and SOC(P<0.05).The carbon metabolism functional genes K01648 and K03520 up-regulated in the soil microorganisms in T2 showed significant positive correlations with soil available nitrogen,ROOC,MRC,and available phosphorus(P<0.05).In addition,soil DOC,ROOC and MRC were significantly correlated with the abundance of some soil carbon cycle functional genes(P<0.05).Redundancy analysis showed that soil moisture content was the main driving factor for changes in the abundance of carbon cycling-related functional genes in the 0-10 cm and 20-60 cm soil layers,while soil available phosphorus was the main driving factor for changes in the abundance of carbon cycling-related functional genes in the 10-20cm soil layer.Therefore,it can be seen that selective logging and understory planting of C.hystrix(T2)can promote the decomposition of litter and affect the metabolic turnover of soil AOC and MRC by changing the abundance of some carbon cycle functional genes and increasing the activity of hydrolase. |
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