Effects of simulated root exudate C:N stoichiometry on key microbial taxa and microbial respiration

Abstract

Plant roots release carbon (C) and nitrogen (N) compounds into the rhizosphere, thereby enhancing nutrient availability and promoting plant growth. However, the influence of root exudate stoichiometry on soil C cycling is not well understood. This study added root exudate mimics to soil collected from a Robinia pseudoacacia plantation (35 years old). The experiment lasted 110 days and root exudate mimics included water (control), C alone, N alone, and combinations of C and N (C:N ratios of 10, 50 and 100). The cumulative CO₂ flux significantly differed among C:N10, C:N50, C:N100 and CK groups (P < 0.05) and cumulative CO₂ flux significantly increased with increasing exudate C:N ratio (P < 0.05). Meanwhile, bacterial Shannon index decreased with increasing exudate C:N, and increased with increasing incubation time. Permutational Multivariate Analysis of Variance (PERMANOVA) of pairwise distances between bacterial communities indicated that significant differences in bacterial composition over time and in response to exudate inputs. Furthermore, network analysis showed that network complexity increased with increasing root exudate C:N ratio. In module hubs of the bacterial network, the relative abundance of Acidobacteriota and Proteobacteria decreased with increasing exudate C:N ratio, but Actinobacteriota showed an opposite trend. Actinobacteriota and Acidobacteriota were regarded as k-strategists, and Proteobacteria as r-strategists. These results suggested different responses to exudate input by key bacterial taxa with the same life strategy. Finally, partial least squares path analysis showed that root exudate stoichiometry regulated soil respiration via key bacterial taxa and enzyme activity. Our study provides fundamental information on the effects of root exudates on microbial communites, microbial metabolism, and soil C pool stability.