Exogenous substrate quality determines the dominant keystone taxa linked to carbon mineralization: Evidence from a 30-year experiment

Organic amendments stimulate carbon (C) mineralization by affecting the soil nutrient content, soil organic carbon (SOC) chemistry, and microbial community structure, especially keystone taxa. Exogenous substrates' quality is expected to affect the relative importance of these biotic and abiotic factors in C mineralization. However, little evidence from long-term studies has been found, and the underlying mechanisms remain unclear. Therefore, this study investigated how exogenous substrate quality affects the changes in keystone taxa and their relative contribution to C mineralization in comparison with soil nutrient content and SOC chemistry in a long-term field experiment. Six fertilization treatments from a 30-year field experiment were selected. To investigate the impact of substrate quality, the six treatments were sorted into three groups: non-organic-amended group, including no fertilization and inorganic N fertilizer; green manure-amended groups, including full amount of green manure input and 50% green manure plus 50% inorganic N fertilizer; and wheat straw-amended group, including full amount of wheat straw input and 50% wheat straw plus 50% inorganic N fertilizer. Our results confirmed that the relative importance of keystone taxa on C mineralization varied according to the exogenous substrates’ quality. In the non-organic-amended group, the keystone taxa belonged to Betaproteobacteriales, and the keystone Betaproteobacteriales module, from guided network analysis, was associated with higher C mineralization. In the wheat straw-amended treatments, the keystone taxa Chaetomiaceae module had a dominant influence on C mineralization. In contrast, in the green manure-amended treatments, the keystone taxa in Subgroups 4 and 6 of Acidobacteria and Alphaproteobacteria had a limited impact on the C mineralization rate, whereas the increased aromatic C component was an important explanatory variable associated with C mineralization. In conclusion, this study confirmed that exogenous organic substrate quality affects the occurrence of keystone microbial taxa linked to soil C mineralization.