| Shrubs are animportant component of forest ecosystem, of which litter decomposition plays essential role in maintaining soil fertility, carbon sequestration and nutrient cycling. However, the role of shrub litter decomposition in the forest ecosystem would change with gap formation. Meanwhile, the redistribution of forest gaps on sunlight and precipitation may alter the decomposition environments and the structure and function of microbial community, and then manipulate the shrubs litter decomposition processes inside and outside of gaps in different critical periods. As yet, little attention has been paid to the shrub litter decomposition and its changes with gap positions. In order to understand the effects of alpine forest gaps on mass loss, nutrients release and the degradation of lignin, cellulose and phenol during shrub foliar litter decomposition, a field litterbag experiment was conducted in an alpine fir (Abies faxoniana) forest of the eastern Tibent Plateau which was affected by significant seasonal snow cover in winter. Litterbags with dwarf bamboo (Fargesia nitida) and willow (Salix paraplesia) fresh leaf litter were placed on the forest floor from the gap center to the closed canopy. The litterbags were sampled at five critical periods as decomposition proceeded (snow-formation period, snow-cover period, snow-melting period, early growing season and later growing season). The results were expected to provide some information for management of the alpine forest ecosystems.(1) Over the 2-year decomposition period, the both shrub litter species displayed higher mass loss rate in the gap centers. Compared with other locations, results of exponential regression (Olson model) showed that the 50% and 95% decay time for dwarf bamboo litter in the gap center have shorten 0.44 and 1.89 year, respecitively; and it for willow litter in the gap center have shorten 0.11and 0.49 year, respecitively.(2) Over the 2-year experiment, the release rate of carbon, nitrogen and phosphorus and the degradation rate of lignin, cellulose and phenol in the two foliar litter species decreased from the gap center to the closed canopy during the snow-formation period, snow-cover period, snow-melting period and the later growing season of the two year, whereas increased from the gap center to the closed canopy during the two early growing season (Although foliar litter nitrogen and phosphorus in all positions displayed enrich process during the first early growing season and the second snow-melting period, but the enrich value increased and decreased from the gap center to the closed canopy during the first early growing season and the second snow-melting period, respectively).The percentage of the carbon, nitrogen and phosphorus release rate and the cellulose and phenol degradation rate during the first winter compared to the entire 2-year period accounted for important proportion for both foliar litter species, whereas the lignin degradation for dwarf bamboo and willow litter mainly occurred during the first year and the second year, respectively.(3) Foliar litter C:N, C.P, Lignin/N and Lignin/P decreased and N:P, Lignin/Cellulose and Lignin/Phenol increased for the both speices as the decomposition proceeded. Compared to gap positions, there was an increase in C:N?Lignin/N and Lignin/Cellulose of the two species from the gap center to the closed canopy and a decrease in C:P, N:P, Lignin/P and Lignin/Phenol of the two species from the gap center to the closed canopy.(4) The MBC, MBN, MBP, fungal and bacterial gene copies of the two species decreased from the gap center to the closed during the snow-formation period, the snow-cover period, the snow-melting period and the later growing season of the two year, whereas an increase was observed from the gap center to the closed during the two early growing season,while this tendency was more obvious for MBC, fungal and bacterial gene copies. There was a decreasing trend in MBC/MBN, MBC/MBP, MBN/MBP and Fungus/Bacteria from the gap center to the closed canopy during most decomposition period. During the two snow-formation periods, the two snow-cover periods, the two snow-melting periods and the two later'growing seasons, the Richness index and Shannon-Wiener index of fungus and bacteria in the both species decreased and the Simpson index of fungus and bacteria in the both species increased from the gap center to the closed canopy, while this tendency was reversed during the two early growing season.(5) A total of 14 fungal classes were detected among gap positions and the ten decomposition periods, they were:Ascomycetes, Dothideomycetes, Leotiomycetes, Agaricomycetes, Saccharomycetes, Eurotiomycetes, Sordariomycetes, Pezizmycetes, Hyphpmycetes, Tremellomycetes, Insecta, Oomycetes, Filasterea and Mesomycetozoea; in addition, the Ascomycetes, Dothideomycetes, Leotiomycetes and Agaricomycetes accounted for most in the fungal communities in each specific period and gap position. A total of 10 bacterial classes were detected among gap positions and the ten decomposition periods, they were:Bacteroidetes, Flavobacterial, Sphingobacteria, Alphaproteobacteria, Betaproteobacteria, Epsilonproteobacteria, Gammaproteobacteria, Actinobacteria, Fusobacteria and Unknown; in addition, the Bacteroidetes, Flavobacterial, Sphingobacteria and Alphaproteobacteria accounted for most in the bacterial communities in each specific period and gap position.(6)Foliar litter mass loss rate, carbon, nitrogen and phosphorus release rate and lignin and cellulose degradation rate in the two snow-cover periods and the phenol degradation rate in the first snow-cover period of both species were positively correlated with the average temperatre, the snow cover depth, MBC, fungal and bacterial gene copies. Foliar litter carbon release rate and cellulose degradation rate in the two early growing seasons, phenol degradation rate in the first early growing season, nitrogen and phosphorus release rate in the second early growing season of both species and the mass loss rate in the second early growing season of willow litter were significant negatively correlated with the average temperature. In addtion, there were significant correlation between litter mass loss rate, nutrient release rate and lignin, cellulose and phenol degradation rate with other investigated factors such as microenvironmantal factors and litter substrate quality during some specific period.Conclusively, the alpine forest gaps manipulate the solar radiations and precipitation of the forest floor, creating heterogeneous microenvironment and microbial communities, which promoted the wintertime and annual litter mass loss, carbon, nitrogen and phosphorus release and lignin, cellulose and phenol degradation. It implied that shrub litter decomposition may be inhibited under the scenarios ofgaps vanishing as forest regenerated. Therefore, further characterize the role of shrubs in the material cycles and energy transformation in forest ecosystems and its response to forest gaps regeneration can not only farther the undertanding on forest material cycles and energy flux processes but also provide scientific theory on sustainable forest management. However, this research just focus on the effects of alpine forest gap positions on the decomposition of two dominant shrub species and its microbial comunities dynamics. The relationship between gaps regeneration and the growing of understory plants and litter output, the linkage between litter decomposition and soil organic carbon sequestration need further research. |
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