Root exudates with low C/N ratios accelerate CO2 emissions from paddy soil

Abstract

AbstractRoot exudates can significantly modify microbial activity and soil organic matter (SOM) mineralization. However, how root exudates and their C/N stoichiometric ratios control rice field (paddy) soil C mineralization is poorly understood. This study used a mixture of glucose, oxalic acid, and alanine as root exudate mimics for three C/N stoichiometric ratios (CN6, CN10, and CN80) to explore the underlying mechanisms involved in SOM mineralization. The input of root exudates enhanced CO2 emissions by 1.8–2.3‐fold that of soil with only C additions (C‐only). Artificial root exudates with low C/N ratios (CN6 and CN10) increased the metabolic quotient (qCO2) by 12% over those with 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 increase of stoichiometric ratios of C‐ to N‐hydrolase [β‐1,4‐glucosidase to β‐1,4‐N‐acetyl glucosaminidase (NAG)] promoted SOM degradation compared to those involved in organic C‐ and N‐degradation, which had a significant positive correlation with qCO2. The stoichiometric ratios of microbial biomass were positively correlated with C use efficiency, indicating root exudates with higher C/N ratios provide an undersupply of N for microorganisms that trigger the release of N‐degrading extracellular enzymes. Our findings showed that the C/N stoichiometry of root exudates controlled SOM mineralization by affecting the specific response of the microbial biomass through the activity of C‐ and N‐releasing extracellular enzymes to adjust the microbial C/N ratio.