MICROBIAL ECOLOGY

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Microbial distribution

The microbial community in alpine meadow ecosystem, essentially all the functional and systematic groups, is similar to that in soils of other ecosystem.

Microbial numbers estimated by dilution plate counting

Significant seasonal changes of microbial numbers were observed in surface horizon.  The numbers of bacteria, actinomycetes and fungi were appeared a peak value from mid July to early September.  It was not obvious seasonal change in other horizons.  The microbial numbers are very low during the cold season, which most bacteria are the spore-forming bacteria.

The numbers of bacteria, actinomycetes and fungi were the highest in 0 cm~10 cm of soil and those decreased gradually with the increase of soil depth. It is because the root biomass in 0 cm~10 cm covers about 84.35% of plant belowground biomass. On the other hand, available phosphorus, available nitrogen and organic carbon in surface horizon are higher than those in other horizons.

The different vegetation types are located at the different habits.  In general, the vegetation types that change with the gradient of the soil humidity are Kobresia tibetica swamp meadow, Potentilla fruticosa shrub and Kobresia humilis meadow.  Microbial distribution also changes with the soil humidity in the different vegetations. With the decrease of soil humidity, the bacterial number decreases, while the fungal number increases generally.

Microbial biomass estimated by fluorescein microscopic counting

The highest biomass of bacteria was appeared in August.  The bacterial biomass in alpine scrubby meadow soil is increased by nearly 21.9% compared with that in alpine meadow soil, while that in the soil of degraded meadow was diminished about 22.6%.

The fungal biomass consists of hyphal and spore biomass.  The hyphal biomass was found during the period from June to September, and the major peak appeared in August.  The fungal hypha was not found during the period from October to the May of next year.  The spore biomass in the warm season is lower than that in the cold season.  The spore biomass in soil covers about 79.7% of the fungal biomass.  The fungal biomass decreased with the increase of soil depth, declined by 48.3% in 10 cm~20 cm and by 66.6% in 20 cm~40 cm, whereas that in the degraded meadow soil was reduced about 11.7%.

The correlate analysis showed that soil temperature and soil humidity are the main limiting factors for microbial biomass.

Decomposition

Although alpine meadow ecosystem contains most physiological and taxonomic groups of microorganisms, the range of activities of these microorganisms is limited by adequate soil temperature and moisture.

The cellulose decomposition rate was presented a significant seasonal dynamics. It was the highest (58.39%~74.19%) within the period of July and August. It was only about 1.89% during the cold season. The cellulose decomposition in alpine scrubby meadow soil was significantly higher than that in alpine meadow soil. It was correlated with the soil temperature and the number of the cellulose decomposing microorganisms.

The decomposition rates of plant root, litter and animal feces were 26.55%~27.13%, 20.34%~22.95% and 4.84%~9.29% after 30 days, whereas those were only 2.16%~7.47%, 1.6%~5.92% and 1.79%~4.71% from 120 to 150 days respectively. Their losses of weight correlated mainly with temperature and relative humidity, rather than with other climate factors.

Nitrogen metabolic microorganisms

Most of nitrogen metabolic microorganisms are located in the surface layer and decrease sharply with depth.  In the surface layer, the dominant microbes are usually denitrifying bacteria, followed by anaerobic nitrogen fixing, ammonifying and nitrifying bacteria. Aerobic nitrogen fixing bacteria were not found in the soil of alpine meadow ecosystem.  The nitrogen fixation in alpine meadow ecosystem is mainly accomplished by anaerobic nitrogen fixing bacteria.

Both ammonification and nitrification are the highest in 0 cm~10 cm soil depth.  Denitrification and nitrogen fixation in 10 cm~20 cm horizon are higher than those in other soil horizons, in which the partial pressure of oxygen is lower than that in surface horizon, and the number of microorganisms is more than that in below horizon.

Denitrification in the degraded meadow soil is higher than that in the natural meadow soil.  One of the most important factors, which affect the grassland degradation, is the invasion of Ajania tenuifolia.   The number and activity of denitrifying bacteria are stimulated strongly by the volatile oil of A. tenuifolia.  The increase of denitrification causes the further loss of available nitrogen in soil, and leads grassland to further degeneration.

Phosphorus metabolic microorganisms

The insoluble phosphate, which is not directly available to plants usually, comprises around 95% to 99% of the total phosphate. Many soil microorganisms transform both inorganic and organic phosphates into soluble phosphate, which is a fertilizer nutrient for plants.  The numbers of phosphate-solubilizing bacteria were decreased rapidly with the increase of soil depth.  The numbers of phosphate-solubilizing bacteria were rather low in the soil of forb meadow.   The phosphorus transformation activity in the surface horizon was higher than that in other soil horizons. It is correlated with the soil temperature and organic matter. The activity of organic phosphorus transformation is higher than that of inorganic in soil.

Although the phosphorus transformation activity is not very low in the soil of alpine meadow ecosystem, the amount of available soil phosphorus is only 2.6 ppm ~ 5.1 ppm. The main reason is that the transformed available phosphate reacts with CaO or MgO to form insoluble Ca₃(PO₄)₂ or Mg₃(PO₄)₂ in alkaline soil.

Armillaria luleovirens in alpine meadow

STAFF

Wenbo Jiang    Associate Professor

Qilan Wang    Assiociate Professor

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