青藏高原东缘高山森林-苔原交错带土壤微生物生物量碳、氮和可培养微生物数量的季节动态

Seasonal dynamics in soil microbial biomass carbon and nitrogen and microbial quantity in a forest-alpine tundra ecotone,Eastern Qinghai-Tibetan Plateau,China

为了了解青藏高原东缘高山森林-苔原交错带土壤微生物的特征和季节变化,研究了米亚罗鹧鸪山原始针叶林、林线、树线、密灌丛、疏灌丛和高山草甸土壤微生物生物量碳(MBC)、氮(MBN)和可培养微生物数量的季节动态。结果表明,植被类型和季节动态对MBC、MBN和微生物数量都有显著影响。不同时期的微生物在各植被类型间分布有差异,植物生长季初期和生长季中期,树线以上群落的MBC高于树线下的群落,而到生长季末期恰恰相反,暗针叶林、林线和树线的MBC显著升高,各植被之间MBC的差异减小;微生物数量基本上也是以树线为界,树线以下群落土壤微生物数量显著低于树线以上群落,其中密灌丛的细菌数量最高;可培养微生物数量为生长季末期>生长季初期>生长季中期。生长季末期真菌数量显著增加,且MBC/MBN最高。统计分析表明,MBN与细菌、真菌、放线菌数量存在显著的相关关系,而MBC仅与真菌数量存在显著相关关系(p<0.05)。植物生长季末期大量的凋落物输入和雪被覆盖可能是微生物季节变异的外在因素,而土壤微生物和高山植物对有效氮的竞争可能是微生物季节变异的内在因素。植物生长季初期对氮的吸收和土壤微生物在植物生长季末期对氮的固定加强了高山生态系统对氮的利用。气候变暖可能会延长高山植物的生长季,增加高山土壤微生物生物量,加速土壤有机质的分解,进而改变高山土壤碳的固存速率。

Aims The forest-alpine tundra ecotone is one of the most conspicuous climate-driven ecological boundaries.However,dynamics of soil microbial biomass and quantity during different stages of the growing season in the ecotone remain unclear.Our objective was to understand the temporal and spatial variations of microbial biomass and quantity to explore the main drivers in the ecotone.Methods We collected soil samples in a forest-alpine tundra ecotone（dark-conifer forest,timberline,treeline,dense shrub,sparse shrub and alpine meadow） during early,mid and late growing season（EGS,MGS and LGS）.The number and species composition of soil microorganisms were determined by means of the plate count method.Soil microbial biomass carbon（MBC） and nitrogen（MBN） were measured by the chloroform fumigation leaching method.Important findings Vegetation and seasonality significantly influence MBC,MBN and microbial community structure.Microbial biomass distribution among vegetation types was different in the three stages of the growing season.MBC above treeline was higher than below during EGS and MGS.The MBC of dark-conifer forest,timberline and treeline during LGS was significantly increased,and MBC differences among different vegetation types decreased.There were significant differences in measured soil microbial quantity between above-and below-treeline vegetation types;bacteria of dense shrub were highest among vegetation types.The amount of cultivated microorganisms was LGSEGSMGS.The ratio of MBC to MBN was the highest and the quantity of fungi increased largely late in the growing season.Statistical analysis showed that there were significant correlations between MBN and bacteria,fungi and actinomyces quantity,while only MBC and fungi quantity weresignificantly correlated（p 〈0.05）.Litter input and snow cover late in the growing season were external factors of microbial seasonal variation.Soil microbes and alpine plants competing for nitrogen may be internal factors.Plant nitrogen absorption early in the growing season and microorganisms＇ nitrogen fixation late in the growing season enhanced the alpine ecosystem＇s nitrogen fixation and utilization.Climate warming may extend the growing season of alpine plants,increasing the alpine soil microbial biomass,and accelerate the decomposition of soil organic matter,which may change soil carbon sequestration rates in the alpine ecosystem.