Variations In Soil Bacterial Community From Soil Samples Along The Altitude Gradient At The Upper Reaches Of The Heihe River, Northwestern China

The upper reaches of Heihe River （96°42′～102°04′E，37°45′～42°40′N） located atmiddle segment of Qilian Mountains, its ability of water conservation sustains irrigationfarming in the middle reaches and helps maintaining a stable ecosystem in the downstream.Vegetation plays a key role in water conservation in the upper reaches of Heihe River. Soilmicroorganisms can improve soil quality and supply plants with nutrients. There have beenmany researches focused on hydrology and vegetation in the region, but few has beenreported on soil microbiology.In this study, soil samples were collected below different vegetation types along analtitude gradient in the upper reaches of Heihe River. Culture-dependent method andmolecular biological techniques were applied to analyze soil bacterial community. Fourkinds of mediums （PYGV, R2A, ENB and Czapek） were used to culture bacteria, colonyforming units （CFUs） were counted. The isolates were identified via16S rDNA sequenceanalysis. Species and cultivable numbers of the isolates were used to calculate the Shannonindex for each sampling sites. The results showed that the numbers of cultivable bacteriaranged from4.6×10⁶·g^-1soil to37.0×10⁶·g^-1soil and significantly decreased with theincreasing of altitude. Based on16S rDNA sequence analysis,26species affiliated to19genera that belonged to7phylum were detected. Among which, Agreia pratensis,Mucilaginibacter ximonensis, Cryobacterium psychrophilum and Arthrobacter oxydanswere dominant species. Relative abundance of Cryobacterium psychrophilum significantlyincreased at higher elevations （3602m⁴130m）while the one of Agreia pratensis decreasedwith increasing of altitude. Diversity of cultivable bacteria increased and then decreasedalong the altitude gradient. Shannon index ranged from1.6to2.2, reaching its max at3205m, while Simpson index varied from0.7to0.8and reached its max at3602m.Redundancy analysis indicated that CFUs of cultivable bacteria was principally affected byaltitude （r=－0.911, P=0.004） while diversity correlated with both vegetation types and soil physicochemical properties.Culture-dependent method, clone library and Q-PCR were applied to analyzecommunities of nitrogen-fixing bacteria in each sampling sites. The results demonstratedthat numbers of cultivable nitrogen-fixing bacteria ranged from1.0×10⁶·g^-1soil to29.9×10⁶·g^-1soil while Shannon index of which varied from0.2to1.4, all of themdecreased along the altitude gradient and positively correlated with soil pH. According tonifH gene clone library,16genera of nitrogen-fixing bacteria were detected, affiliating toBradyrhizobium sp., Sinorhizobium sp., Burkholderia sp., Dechloromonas sp.,Herbaspirillum sp., Methylocystis sp., Rhizobium sp., Azospirillum sp., Thiocapsa sp.,Derxia sp., Ectothiorhodospira sp., Mesorhizobium sp., Klebsiella sp., Ensifer sp.,Methylocella sp. and Peseudomonas sp.. They all belonged to Proteobacteria. Copynumbers of nitrogenase gene （nifH） ranged from0.4×10⁷copies·g^-1soil to6.9×10⁷copies·g^-1soil while diversity of which varied from1.5to2.8. Correlation analysis showedthat nifH gene diversity negatively correlated with underground biomass of vegetation （r=－0.859, P=0.013） while nifH gene copies positively correlated with aboveground biomassand cover degree （r=0.846, P=0.016; r=0.803, P=0.030）, which was also affected by soilavailable K.