Changes in soil microbial community during shrub encroachment process in the Inner Mongolia grassland of northern China

Abstract This study attempted to elucidate the characteristics of the soil microbial community during the grass-to-shrub transition process and to identify underlying factors influencing changes in the soil microbial community with increasing intensity of shrub encroachment. In this study, evaluations were conducted on three states (grass-dominated, grass-shrub mixed and shrub-dominated states) of the shrub-encroached grassland. In each state, the landscape consists of shrub patches and grass patches, therefore the soils were sampled under the shrub patches and the grass patches respectively. The composition and biomass of the soil microorganisms were quantified by the phospholipid fatty acid (PLFA) method. The results indicated that as shrub encroachment increased, the contents of soil total PLFAs, total bacteria, fungi, actinomycetes and G− bacteria increased under the shrub patches. And the contents of total PLFAs, total bacteria, fungi, actinomycetes, and G− bacteria first decreased and subsequently increased under the grass patches from grass-dominated state to shrub-dominated state. The soil PLFAs were mostly distributed in the soil depth 40 cm under the shrub patches but were primarily in the soil layer 20 cm under the grass patches. Soils under the shrub patches had greater contents of total PLFAs, total bacteria, actinomycetes and Gram-positive bacteria (G+) than did soils under the grass patches. Soil water and belowground biomass were significantly correlated with the concentrations of total PLFAs, bacteria, fungi, G+, G−, actinomycetes, G−/G− and fungi/bacteria (F/B). Additionally, the aboveground biomass (AB) of C. microphylla L. of the shrub patches and AB of S. krylovii of the grass patches were both significantly correlated with the concentration of G−/G+. There were significant positive correlations between the belowground biomass (BB) and total PLFAs, bacteria, G+ bacteria, G− bacteria and actinomycetes. The contents of soil sand and silt, organic matter, total N, total P, available P and NO−3-N were significantly correlated with the concentrations of total PLFAs, bacteria, fungi, G+, G−, actinomycetes, and G−/G+. The findings of this study highlight that shrub encroachment significantly affect the microorganism biomass and the community composition. The inherent characteristics of the plants and the soil physicochemical properties are the main factors for the change of microbial community during the processes of shrub encroachment.