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

Abstract

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 aboveand below-treeline vegetation types; bacteria of dense shrub were highest among vegetation types. The amount of cultivated microorganisms was LGS>EGS>MGS. 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 were 刘洋等: 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 383 doi: 10.3724/SP.J.1258.2012.00382 significantly 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.