Priming and balance of soil organic carbon differ with additive C:N ratios and long-term green manuring

The input of organic C strongly alters the magnitude and direction of the microbial mineralization of soil organic matter (SOM), a phenomenon known as the “priming effect” (PE). The C:N ratios of additives are expected to affect the intensity of PE as it balances the C and N requirements of microbial growth. Growing of green manure (GM) crops in agricultural system strongly regulates PE intensity as it enhances the level and stability of SOM. However, the driving factors of additive C:N ratios and long-term GM crops on PE remain unclear. We addressed this knowledge gap by performing a 92-day incubation of 10-year summer fallow-wheat and GM-wheat soils by adding mixtures of 13C-labelled glucose and ammonium sulfate differing in the C:N ratio (15 vs. 50). The PE was increased by 148–288 % due to the high C:N ratio of the added mixtures, indicating that the quality of input exerted effects on the PE across the two soils. The PE of green manured (GMd) soil was increased by 23 % (p < 0.05) compared with that of summer fallow soil, as microorganisms produce extracellular enzymes such as β-glucosidase and leucine aminopeptidase to co-metabolize SOM. Nevertheless, compared with the summer fallow soil, 26 % more glucose-C was sequestered in GMd soil to compensate for C loss. We propose a conceptual model of “N mining” and “co-metabolism” to explain the effect of additive C:N ratio on PE in the soils under different GM practices. The “N mining” is the main cause of PE when the additive C:N ratio is high, and the “co-metabolism” becomes the dominant factor in long-term GMd soil with high SOM content and stability. Our findings demonstrate the importance of long-term incorporation of GM-driven changes in organic C inputs and SOM content and stability in regulating PE and soil C dynamics. Understanding the C dynamics under long-term GM practices contributes to formulate optimized agricultural strategies for promoting C sequestration and accurately predict soil C dynamics in the future.