Return Patterns Of Litter Recalcitrant Components In Typical Subalpine Coniferous Forests In Western Sichuan

Litterfall is the key link between the aboveground and underground processes of the forest ecosystem,and it is an important carrier of the material cycle of ecosystem.Meanwhile,as the main sources of soil organic matter,the return of its refractory components is the main endogenous factor that regulates the formation of soil organic matter in forests.Different types of forests may result in different return amounts and rhythms of recalcitrant components of litterfall due to the differences in species composition,but lack of attention.As an important water conservation area in the upper reaches of the Yangtze River and an important part of the national ecological security barrier,the alpine forests in Western Sichuan have important strategic significance.However,affected by low temperature and frequent geological disasters,the soil in this area is shallow and grows slowly.Therefore,the carbon cycle with litter as a carrier is the key approach to the formation of regional soil organic matter.Therefore,the typical forest of Abies faxoniana and Picea asperata forest in the alpine area of Western Sichuan were taken as the research objects,and the production and seasonal dynamics,the return characteristics of recalcitrant components of different types of litterfall were continuously monitored for 4 years.（1）The total litter production of Picea asperata forest was 3547.3 kg hm^-2 yr^-1,and of Abies faxoniana forest was 5377.0 kg hm^-2 yr^-1.Foliar litter and branch litter were the main components of litter,accounting for 57.4%-61.4%,and 17.7%-26.4%of total litter productions,respectively,while the litters of flowers,fruit and seed,bark and epiphytes accounted for less.In Picea asperata forest,the productions of total litter,foliar litter and branch litter were bimodal and the peaks occurred in April or May and October.Moreover,bark litter and flower and fruit litter were unimodal,while epiphytic litter was stable.In Abies faxoniana forest,the productions of total litter,foliar litter,branch litter,bark litter,flower and fruit litter and epiphytic litter were annually bimodal and the peaks occurred in May and October.In the same forest,the production of foliar litter in tree species was higher than that of foliar litter in shrub species,and evergreen species was higher than that in deciduous species,and coniferous species was higher than that in broadleaved species.（2）Significantly affected by litter components,the alkali-soluble carbon of foliar litter in Abies faxoniana forest and its contribution to organic carbon were the highest,followed by reproductive litter such as flowers and fruits.Meanwhile,reproductive litter was the highest in Picea asperata forest.The alkali-soluble carbon in litter of Picea asperata forest was higher in July,while that of Abies faxoniana forest was lower in July.The alkali-soluble carbon contents of tree species,evergreen species or coniferous species were higher than that of shrub species,deciduous species or coniferous species,respectively.Moreover,in Picea asperata forest,the alkali-soluble carbon of tree species,evergreen species and coniferous species were relatively stable during the study period,while shrub species,deciduous species and broad-leaved species were higher in August.In Abies faxoniana forest,the alkali-soluble carbon contents of all kind of foliar litter were higher from May to June.（3）TheΔlogK values of reproductive organs and foliar litter were lower while the A600/C value of which were higher.Furthermore,theΔlogK and E4/E6 values of foliar litter in functional types in Picea asperata forest were lower in August,while theΔlogK and E4/E6 values of which in Abies faxoniana forest were higher in August.Moreover,in the same forest,theΔlogK and E4/E6 values of shrub species,deciduous species or broadleaved species was lower than that of tree species,evergreen species or coniferous species while A600/C value was higher.The aromatic structures of recalcitrant substances in different kind of foliar litter in Picea asperata forest were more stable in August,showing the opposite characteristics of Abies faxoniana forest.（4）Effected by litter components,the ASE of foliar litter in the same forest were significantly lower than those of other litter components.In Picea asperata forest,The ASE content of litterfall was lower from July to August,while that in Abies faxoniana forest was higher from August to November.The ASE content of foliar litter in tree species,evergreen species and coniferous species in Picea asperata forest were relatively stable,while those in shrub species,deciduous species and broadleaved species were higher in September.The ASE content of foliar litter in functional types in Abies faxoniana forest were relatively stable during the study period.The AUR content of foliar litter was significantly lower than that of branch litter and reproductive organs litter.In Picea asperata forest,the content of AUR in litter components were higher in August,while the AUR of litter components in Abies faxoniana forest were relatively stable.（5）The alkyl C and aromatic C contents of foliar litter were generally higher than those of branch litter,while the content of O-alkyl C of branch litter was higher than that of branch litter.In Picea asperata forest,the alkyl C in foliar litter and banch litter were higher from July to August.Meanwhile,In Abies faxoniana forest,the aromatic C of foliar litter was higher in July,while which of branch litter was relatively stable in the study period.Moreover,the alkyl C content of foliar litter in rosa was higher than that in the other species,and the contents of O-alkyl C and aromatic C of which were lower than those in the other species.Overall,the alkyl C and O-alkyl C contents of fir（Abies faxoniana）and spruce（Picea asperata）were stable.However,the alkyl C content of foliar litter in willow was higher in August in Picea asperata forest,while of which was stable in Abies faxoniana forest.（6）The alkali-soluble carbon return of litterfall in Picea asperata forest was 840.3kg hm^-2 yr^-1,and the alkali-soluble carbon return of which in Abies faxoniana forest was 994.3 kg hm^-2 yr^-1,and foliar litter and branch litter were the main litter components.Moreover,the return of alkali-soluble carbon showed a dynamic change similar to that of litter production.In addition,the contribution of alkali-soluble carbon to organic carbon in Picea asperata forest was higher than that in Abies faxoniana forest,and the return of alkali-soluble carbon in foliar litter and branch litter in Picea asperata forest greatly contributed to the organic carbon return in April and July,respectively,while it was June and April,respectively in Abies faxoniana forest.In summary,foliar litter was the main source of litter and its refractory components input,the peak period of litterfall,particularly.Meanwhile,compared to other litterfall components,the aromatic structures of the recalcitrant components in reproductive litter were more stable,and the recalcitrant carbon groups of which were more abundant,from July to August particularly.The results above are of great significance for in-depth understanding of the the role and significance of litterfall in the carbon cycle of subalpine coniferous forests in western Sichuan.