Changes in microbial-community structure with depth and time in a chronosequence of restored grassland soils on the Loess Plateau in northwest China

Microbial-community structure is closely associated with plant-community composition. The objective of this study was to evaluate the effects of depth and revegetation time on the microbial-community structure of restored grassland soils of the Loess Plateau of China. Microbial-community structure at 0-10cm (depth 1) and 10-20cm (depth 2) of eight sites of a grassland chronosequence with revegetation time ranging from 1 to 78 y was determined using phospholipid fatty acid (PLFA) analysis. Except for the youngest site microbial-community structure at the two depths varied distinctly with actinomycetes and vesicular arbuscular mycorrhiza as the most important discriminators. Total PLFA content decreased with depth and increased with revegetation time. Protozoa were the only functional group whose proportion did not change with depth. Their relative abundance tended to decrease with increasing revegetation time. The microbial community at all sites was bacteria-dominated with Gram-negative bacteria representing the largest proportion. Principal-component and cluster analysis revealed that microbial-community structure in the surface soil (depth 1) of the older sites (23-78 y of revegetation) was rather similar which may be due to only small effects of the plant species present on the soil environment. Differences in microbial-community composition at 0-20cm depth of the eight sites were partly related to variations in the physico-chemical characteristics. Relations to organic C, alkali-extractable N, pH, and available P were found. Revegetation of former agricultural sites on the Loess Plateau improves the soil status and leads to shifts in microbial-community structure more pronounced with depth than time.