关键词: 油松-辽东栎混交林, 微生物残体碳, 磷脂脂肪酸, 碳降解基因, 微生物CAZyme家族

Abstract:

Objective: In this study, the Pinus tabuliformis-Quercus wutaishanica mixed forest in the Huanglong Mountain region of Shaanxi Province was taken as the research object, with pure stands of P. tabuliformis and Q. wutaishanica serving as controls. This study analyzed the variation characteristics of soil microbial source carbon (living biomass carbon and necromass carbon) under different stand types, and investigated the impact of P. tabuliformis-Q. wutaishanica mixed forests compared to the pure forests on soil microbial source carbon accumulation, aiming to elucidate the key microbial functional mechanisms driving these changes, and to identify the environmental factors regulating this process. Method: The phospholipid fatty acid (PLFA) method was used to determine microbial live biomass carbon content, and the amino sugar method was used to determine microbial necromass carbon content. Microbial metagenomic sequencing was employed, annotating against the Carbohydrate-Active Enzymes (CAZyme) database and the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, to obtain and analyze the characteristics of microbial carbon-decomposition functional genes. Mantel test was conducted to explore the interactive relationships among carbon-decomposition functional genes, soil nutrients, and microbial derived carbon. Based on the distributional characteristics of the data, both parametric and nonparametric tests were applied to assess significant differences among groups and perform multiple comparisons. All statistical analyses and data visualizations were performed using R v4.2.1. Result: 1) Compared with pure P. tabuliformis and pure Q. wutaishanica stands, the mixed forests significantly increased the microbial PLFA biomass carbon content of bacteria (Gram-positive bacteria: 37.5%–54.9%), fungi (ascomycota and basidiomycota: 35.7%–42.5%; arbuscular mycorrhizal fungi: 23.7%–95.8%; zygomycota: 57.8%–89.9%), and total microorganisms (37.7%–55.1%). 2) The mixed forests significantly enhanced the content of bacterial necromass carbon (45.0%–56.7%) and total microbial necromass carbon (20.2%–28.4%), and increased the contributions of bacterial necromass carbon (44.3%–59.9%), fungal necromass carbon (15.3%–30.6%), and total microbial necromass carbon (19.3%–34.7%) to the soil organic carbon pool. 3) The relative abundance of CAZyme genes involved in the decomposition of plant-derived carbon (hemicellulose, cellulose, and lignin) and bacterial-derived carbon (peptidoglycan) in the P. tabuliformis-Q. wutaishanica mixed forests were intermediate between those in pure P. tabuliformis and pure Q. wutaishanica forests, while the relative abundance of chitin-degrading CAZyme genes was higher than in pure forests. KEGG functional genes related to carbon degradation exhibited a similar trend. 4) Bacterial necromas carbon content showed significant positive correlations with the PLFA biomass carbon content of actinobacteria (R = 0.65, P< 0.01), firmicutes (R = 0.60, P< 0.01), and Gram-negative bacteria (R = 0.67, P< 0.01). Fungal necromass carbon content was significantly positively correlated with the PLFA biomass carbon content of ascomycota and basidiomycota (R = 0.53, P< 0.05), arbuscular mycorrhizal fungi (R = 0.63, P< 0.05), and zygomycota (R = 0.74, P< 0.01). Both microbial PLFA biomass carbon and necromass carbon contents were significantly influenced by soil available phosphorus (AP) content. The fungal-to-bacterial PLFA biomass carbon ratio and the fungal-to-bacterial necromass carbon ratio were both significantly positively correlated (P< 0.01) with the relative abundance of CAZyme genes decomposing plant-derived carbon and bacterial-derived carbon, respectively. Conclusion: The mixture of P. tabuliformis and Q. wutaishanica can significantly increase the contents of bacterial, fungal, and microbial PLFA biomass carbon and necromass carbon, and also enhance the contribution of bacterial, fungal, and microbial necromass carbon to the soil organic carbon pool. In the mixed forests, the relative abundance of CAZyme genes and the absolute abundance of carbon-degrading functional genes are intermediate between those in pure P. tabuliformis and pure Q. wutaishanica forests. Both the fungal-to-bacterial PLFA biomass carbon ratio and the fungal-to-bacterial necromass carbon ratio are influenced by the relative abundance of CAZyme genes and the absolute abundance of carbon-degrading functional genes. Soil phosphorus availability is identified as a key factor regulating the accumulation of microbial PLFA biomass and necromass carbon.

Key words: Pinus tabuliformis-Quercus wutaishanica mixed forests, microbial necromass carbon, phospholipid fatty acids (PLFAs), carbon degrading genes, microbial CAZyme family