Background Soil microbial community structure and distribution characteristics can be used as highly sensitive indicators reflecting environmental changes.The Qilian Mountain is an important ecological security barrier for western China.However, the distribution characteristics of soil microbial community structure and its influencing factorsin this area were rarely reported.

Methods Soil samples were collected from four soil layers(0-10, 10-20, 20-30 and 30-40 cm) in different vegetation zones along an altitudinal gradient, i.e., 2 830 m(E1), 2 860 m(E2), 2 890 m(E3), 2 900 m(E4) and 2 920 m(E5) in Qilian Mountains.Soil microbial community composition and structure among the five elevation gradients were detected using the phospholipidfatty acid(PLFA) labeling method, and their physicaland chemical properties were determined.

Results 1) In total, 31 PLFAs were detected from the soil of the five altitudinal gradients, and the total PLFA(28)and content((770.55±23.77) nmol/g) in E3 soil were significantly higher than those in other elevations(P < 0.05). The PLFA biomarkers with high content were 15:0 iso, 16:00, 18:1 w7c, 18:1 w9c and 16:0 10-methyl. The bacteria were the most dominant microorganisms, while the abundance of protozoa was the lowest in these soils.2)Except for protozoa, the PLFA biomarkers' contents of soil microorganisms in different vegetation zones showed a trend of E3>E2>E4>E5>E1.With the increase of soil layers, the contents of PLFA biomarkers tended to decrease in E1, E2 and E3 soil, while increase in E4 and E5 soil.3)Principal component analysis showed that the explanatory variances of the two principal components related to soil microbial PLFA community structure were 57.35% and 35.73%, respectively, which basically distinguished the microbial community characteristics at different altitudes.Correlation analysis of the principal component score with 31 PLFA biomarkers showed that in 27 PLFA biomarkers were significantly correlated with PC1 score and 18 PLFA biomarkers were significantly correlated with PC2 score(P < 0.05).The redundancy analysis showed that the first and second ordination axes accounted for 89.45% and 5.04%, respectively.Soil microbial communities were negatively correlated with pH, clay and silt volume fractions, while positively correlated with soil water content, sand volume fraction, total carbon(TC), total nitrogen(TN) and C/N(P < 0.05).

Conclusions Variations in soil water content, soil TC and TN contents resulted from altitude gradients could have a strong influence on soil microbial community diversity.