Intensive citrus plantations suppress the microbial profiles of the β-glucosidase gene

Increases in the input of chemical fertilizer and rapid decline of soil organic matter have increased serious demand for sustainable agriculture. Cellulose is an important component of soil organic matter, which strongly affects global carbon cycle and crop yields in the agroecosystem. β-glucosidases (BG) convert cellobiose to glucose, providing carbon for soil microorganisms and plants. However, little is known about the structure of β-glucosidase gene (GH3)-harboring microbial community and its functions in response to long-term intensive cropping with frequent fertilization. In this study, quantitative PCR (qPCR) and high-throughput sequencing were used to determine the abundance of bacterial and fungal GH3 gene as well as the diversity of GH3-harboring community in intensified citrus orchards of different ages (5-year, 15-year and 30-year), or under different management practices (straw application on the surface, peanut planting between rows and control) for 15 years. The adjacent natural forest and grass soils were used as reference to investigate the effect of intensive citrus plantation on soil BG enzyme activity and the GH3 gene. The results revealed that citrus plantation significantly reduced soil GH3 gene abundance, especially for the bacterial GH3 gene. The higher abundance of bacterial GH3 gene than that of fungal GH3 gene in the soil highlighted the key role of bacterial GH3 gene in regulating soil BG enzyme activity. Among the reservoirs of the GH3 gene, Proteobacteria, Acidobacteria, Firmicutes and Actinobacteria were the main bacterial phyla, while Ascomycota was the only dominant fungal phylum. Citrus plantation caused soil bacterial community varied from Proteobacteria-dominant to Acidobacteria-dominant and reduced BG enzyme activity. The higher diversity of the bacterial community indicated that bacteria play a dominant role in the conversion of cellobiose to glucose. Citrus plantation and plantation age strongly influenced the diversity and structure of β-glucosidase gene-harboring community. A significant decline occurred in bacterial diversity along with increasing citrus plantation age. The community diversity exhibited a clear land-use specific pattern, as indicated by the obviously different community structure between the forest soil and citrus soil. Among the detected environmental factors, SOC, bacterial β-glucosidase gene abundance and the community diversity were the most important factors controlling soil BG enzyme activity. These results indicate that citrus plantation with long-term fertilization suppresses the abundance of the β-glucosidases gene, and the bacterial β-glucosidase gene contributes more to the BG enzyme activity than the fungal gene.