Soil Microbial Properties Of Different Degraded Grassland At Ch’i-lien Mountains

This paper selected alpine grassland of the Ch’i-lien Mountains, located at the northeastern margin of Qinghai-Tibetan plateau, as study plot. Through investigating the characteristics of plant community and soil nutrient, then on the basis of the National Standards of “Parameters for degradation, sandification and salification of rangelands”(GB19377-2003), three degraded grasslands: lightly degraded grassland(LDG), moderately degraded grassland(MDG), and severely degraded grassland(SDG) were identified. Then the index of soil microbial activity on grassland were measured, including the population of three main types of soil microbes, soil microbial species diversity, soil microbial biomass carbon and nitrogen(SMBC, SMBN), soil respiration, microbial respiratory quotient(q CO2) and microbial quotient(q MBC), the population of soil microbes involved in soil nitrogen cycle, and eventually four important enzymes for chemical processes in soil- invertase, urease, phosphatase and catalase. The author was intended to explain soil microbial characteristics of different degraded grassland at alpine region, and the main results obtained as below.1. Characteristics of grassland vegetation and soil properties were both changed by grassland degradation and seasonal variation. Vegetation coverage, height and aboveground grass yield in summer of LDG(96%, 43 cm and 1396 kg/hm2) were the highest among the three different degraded grasslands; the lowest appeared in winter of SDG(2%, 2cm and 47 kg/hm2). Characteristics of soil nutrients were similar with vegetation. Soil organic matter, soil total nitrogen, soil total phosphorus of LDG(13.11%、0.43%、0.16%) were the highest and the lowest showed at SDG(6.20%、0.39%、0.10%). The highest and lowest soil bulk density(0.95、0.61g/cm3) were just the opposite and emerged at SDG and LDG. And the changes of other soil nutrients in three grasslands were not identical, the highest soil available potassium and nitrate nitrogen appeared in MDG, and ammonium nitrogen appeared in LDG. All of this three soil nutrients did not decrease with the aggravation of degradation degree.2. The three main types(bacteria, fungi and actinomycetes) of alpine grassland soil microbial population decreased with the aggravation of degradation degree, like LDG > MDG > SDG, and the performance in two seasons were summer > winter. The biggest value of population of bacteria, fungi and actinomycetes(2.49×106, 5.90×103 and 1.98×105 cfu/g) appeared in the summer of LDG, and the smallest value(1.15×106、0.52×103�'� 0.34×105 cfu/g) in the winter of SDG. Meanwhile, there was obvious vertical spatial differentiation in different degradation grassland, and the number of the upper soil was higher than that of the lower soil, and the maximum was found in the 0~10cm soil layer. The diversity of soil bacteria was in fluctuation with the degree of degradation and the seasonal variation.3. From the 16 S r DNA clone library analysis, the bacterial diversity was relatively high; the dominant species of LDG and MDG were Proteobacteria, in addition to Firmicutes, Actinobacteria, and Acidobacteria etc. Comparably, in SDG the population of Proteobacteria was greatly reduced, but dominant with Actinobacteria and Acidobacteria. However, the soil bacterial composition of MDG and LDG was not significant varied, but there was a significant effect of SDG on both the bacterial composition and the dominant species.4. The microbial index involved in soil carbon cycling in alpine grassland ecosystem was different between different degradation degree, plant growing season(summer) and non growing season(winter), the soil microbial biomass carbon, microbial quotient all decrease with the aggravation of degradation degree, the maximum value was in summer of LDG(485.40mg/kg, 6.18mg/g), the minimum in winter of SDG(154.45mg/kg, 0.56mg/g). The changes of microbial respiration quotient were different, the maximum and minimum(0.24%, 0.14%) occurred in winter of SDG and LDG. Meanwhile, soil microbial biomass carbon, microbial quotient demonstrated obvious vertical spatial differentiation in different degradation grassland, with soil depth increasing the value diminished, the maximum(685.80 mg/kg, 4.35 mg/g) appeared in 0~10 cm soil layer of LDG. But there was no corresponding spatial variation between microbial respiratory quotient and soil layer, whose minimum value(0.08%, 0.1%) appeared in 0~10 cm soil layer of MDG and SDG, respectively.5.The microbial index associated soil nitrogen cycling in alpine grassland ecosystem was influenced differently by the grassland degradation degree and seasonal variation, soil microbial biomass nitrogen, population of soil nitrogen functional microbial group all decreased with the aggravation of degradation degree in both seasons correspondingly. Except denitrification bacteria, the maximum value of soil microbial biomass nitrogen, ammonifying bacteria, aerobic nitrogen fixing bacteria, anaerobic nitrogen fixing bacteria and nitrifying bacteria were in summer of LDG(31.01mg/kg, 4.89×105cfu/g, 1.59×105cfu/g, 6.23×104cfu/g and 3.50×103cfu/g), and the minimum value in winter of SDG(7.35mg/kg, 2.26×105cfu/g, 2.87×104cfu/g, 0.66×104cfu/g and 0.66×103cfu/g). Meanwhile, soil microbial biomass nitrogen, nitrogen functional microbial group demonstrated obvious vertical spatial differentiation in different degradation grassland, with soil depth increasing the value diminished, the maximum(24.97mg/kg, 5.42×105cfu/g, 1.29×105cfu/g, 5.61×104cfu/g, 2.62×103cfu/g, 6.95×103cfu/g) appeared in 0~10 cm soil layer.6. Soil enzyme of alpine grassland decreased with the aggravation of degradation degree in plant growing season and non-growing season respectively. Except catalyst, the maximum value of invertase, urease and phosphorase were in summer of LDG(6.10mg/g.d, 0.64mg/g.d and 0.0082mg/g.d), and the minimum value in winter of SDG(0.72mg/g.d, 0.22mg/g.d and 0.0049mg/g.d) in addition to the degree of degradation of grassland with invertase deeper change more obvious than the amount of the other three enzymes decreased, but the difference was not significant. Description of soil chemical processes in grassland degradation, not only material flows decreased, the rate and intensity of reaction there is a corresponding decrease change. Except urease, the activity of other three enzymes demonstrated obvious vertical spatial differentiation, the maximum(2.49mg/g.d, 0.44 mg/g.d and 1.23ml/g.h) appeared in 0~10 cm soil layer.7.Through the overall analysis of soil microbial properties in grassland ecosystem, and combined with the vegetation characteristics and soil properties, the author considers that the microorganism has a high performance of sensitivity on the environment change, and it is importance that microorganism associates with the environment factor closely. Therefore, it is suggested that soil microbial activity index should be considered in the evaluation and classification of degradation of natural grassland.