Objectives Planting grasses in orchard improve soil fertility and structure. Hence, we studied soil biological properties and microbial structures as affected by grass species in orchard.

Methods The field experiment was conducted at the Mazhuang experimental base in Tai'an City, Shandong Province, China. The test grass species were White clover (Trifolium pratense), Pith grass (Bouteloua gracilis), Brucella (Eragrostis trichodes), Curving thrush (Eragrostis curvula) and Canadian Elymus (Elymus canadensis). The five grass species were seeded in 2017 between the apple rows while no grass planting was taking as control. The microbial community and enzyme activities in soil were determined in three consecutive years.

Results 1) A total of 31459 bacterial operating taxa (or 1019 OTUs) and 5021 fungal operating taxa (or 108 OTUs) were obtained from all the soil samples. The average Shannon, Simpson and Chao1 indices of bacteria diversity in the grass planting soils were 4.61%, 7.01% and 27.44% respectively higher than that of control. At the phylum classification level, Proteobacteria, Acidobacteria, Actinobacteria, Gemmatimonadetes, Planctomycetes, Chloroflexi, Bacteroidetes and Verrucomicrobia were the dominant bacteria, accounting for more than 95% of the respective treatment. Ascomycota, Basidiomycota, Glomeromycota and Zygomycota were the dominant fungi, with their relative abundance accounting for over 80% of the respective treatment. Curving thrush increased soil bacteria abundance and diversity, but Brucella decreased the relative abundance of germ cell phylum, Phytophthora phylum and Curvulariaviridis. All the five grasses increased the relative abundance of Basidiomycota; Pith grass decreased the relative abundance of Ascomycota. Pith grass, Curving thrush and Canadian elymus increased the relative abundance of glomus. 2) The soil microbial population showed increasing trend with increase in years of the planted grasses, and the microbial population in three-year-old thrush soil was significantly higher than that in CK. 3) The activities of soil urease, phosphatase and catalase in the three years were all significantly higher than those of CK. Thrush, Canadian elymus and Curving thrush increased the soil organic carbon and soluble organic carbon contents. 4) The results of correlation analysis showed that the soil total organic carbon and enzyme activities were positively (P<0.05) correlated with the number of microorganisms.

Conclusions Curving thrush and Canadian elymus planting significantly increased the soil total and soluble organic carbon contents, soil enzyme activities, microbial populations, and changed the community of bacteria and fungi composition to different extents as the grass growing year increased. The other three grasses did not show such good effect. So, Curving thrush and Canadian elymus have great promotion value.