Root exudates can greatly modify microbial activity and soil organic
matter (SOM) mineralization. However, the mechanism of root exudation
and its stoichiometric ratio of C/N controlling upon paddy soil C
mineralization are poorly understand. In this study, we used a mixture
of glucose, oxalic acid, and alanine as root exudate mimics, employing
three C/N stoichiometric ratios (CN6, CN10, and CN80) to explore the
underlying mechanisms involved in C mineralization. The input of root
exudates enhanced CO2 emission by 1.8–2.3-fold than that of the
control. Artificial root exudates with low C/N ratios (CN6 and CN10)
increased the metabolic quotient (qCO2) by 12% over those obtained at
higher stoichiometric ratios (CN80 and C-only), suggesting a relatively
high energy demand for microorganisms to acquire organic N from SOM by
increasing N-hydrolase production. The stoichiometric ratios of enzymes
(β-1,4-glucosidase to β-1,4-N-acetyl glucosaminidase) promoting organic
C degradation compared to those involved in organic N degradation showed
a significant positive correlation with qCO2; the stoichiometric ratios
of microbial biomass (MBC/MBN) were positively correlated with carbon
use efficiency. This suggests that root exudates with higher C/N ratios
entail an undersupply of N for microorganisms, triggering the release of
N-degrading extracellular enzymes. This in turn decreases SOM
mineralization, implying the C/N ratio of root exudates to be a
controlling factor. Our findings show that the C/N stoichiometry of root
exudates controls C mineralization by the specific response of the
microbial biomass through the release of C- and N-releasing
extracellular enzymes to adjust for the microbial C/N ratio.
Different root exudates C/N ratios accelerate CO2 emission from paddy soil