Figure legends
Fig. 1. CO2 efflux rates (a) and cumulative CO2 (b) over the 45-day incubation period. The control represents no addition of artificial root exudates to soil; C-only represents addition of C substrates glucose and oxalic acid only; CN6, CN10, and CN80 represent addition of C substrates glucose and oxalic acid as well as the N substrate alanine at different C/N stoichiometries of CN6, CN10, and CN80, respectively. Different letters indicate significant differences among stoichiometric ratios at the end of cumulative CO2 emission (one-way ANOVA and LSD test;P < 0.05 level). Error bars show standard errors (n = 4).
Fig. 2. Metabolic quotient of soil microbial biomass (qCO2) (a) and carbon use efficiency (CUE) (b) at days 3, 12, and 45 during the 45-day incubation period. C-only represents addition of C substrates glucose and oxalic acid only; CN6, CN10, and CN80 represent addition of C substrates glucose and oxalic acid as well as the N substrate alanine at different C/N stoichiometries of CN6, CN10, and CN80, respectively. Different letters and capital letters indicate significant differences among stoichiometric ratios and sampling periods, respectively (one-way ANOVA and LSD test; P< 0.05 level). Error bars show standard errors (n = 4).
Fig. 3. Activity of extracellular enzymes β-1,4-glucosidase (BG), β-1,4-xylosidase (XYL), and β-1,4-N-acetyl-glucosaminidase (NAG) at days 3, 12, and 45 during the 45-day incubation period. C-only represents addition of C substrates glucose and oxalic acid only; CN6, CN10, and CN80 represent addition of C substrates glucose and oxalic acid as well as the N substrate alanine at different C/N stoichiometries of CN6, CN10, and CN80, respectively. Different letters and capital letters indicate significant differences among stoichiometric ratios and sampling periods, respectively (one-way ANOVA and LSD test; P< 0.05 level). Error bars show standard errors (n = 4).
Fig. 4. Relationships between metabolic quotient (qCO2) and soil enzyme stoichiometry (BG/NAG) (a), qCO2 with microbial biomass stoichiometry (MBC/ MBN) (c); carbon use efficiency (CUE) with soil enzyme stoichiometry (BG/NAG) (b), CUE with microbial biomass stoichiometry (MBC/ MBN) (d). Abbreviations are BG, β-1,4-glucosidase; NAG, β-1,4-N-acetyl glucosaminidase. The dots represent data from all treatments in the 45-day incubation period.
Fig. 5. Structural equation model of the multivariate effects of C/N ratios, leaching water sample DOC-L/NO3--L ratios and DOC-L/NH4+-L ratios, soil sample DOC/NO3- ratios and DOC/NH4+ ratios, microbial biomass stoichiometry, and enzyme stoichiometry on the CO2emission. Microbial biomass stoichiometry represents the ratios of microbial biomass C to microbial biomass N; enzyme stoichiometry represents the average ratios of the activity of β-1,4-glucosidase to β-1,4-N-acetyl glucosaminidase and the activity of β-1,4-xylosidase to β-1,4-N-acetyl glucosaminidase. The solid lines indicate positive path coefficients; dashed lines, negative. R2 values indicate the proportion of variance explained by each variable contribution to the CO2 emission from soil application with substrates. The numbers and the width of the arrows indicate the standardized path coefficients.
Fig. 6. Standardized effect of stoichiometric C/N ratios, leaching water sample DOC-L/NO3--L ratios and DOC-L/NH4+-L ratios, soil sample DOC/NO3- ratios and DOC/NH4+ ratios, enzyme stoichiometry, and microbial biomass stoichiometry on CO2 emission.
Fig. 7. Schematic diagram emphasizing the importance of the stoichiometric control of plants by their release of root exudates of different N contents in the flooded rice-paddy soil system. C and N circles represent artificial root exudates; DOC-W represents dissolved organic C from water solution; SOM represents the soil organic matter; microbial biomass stoichiometry represents the C to N ratios of microbial biomass; enzyme stoichiometry represents the ratios of the activity of β-1,4-glucosidase to β-1,4-N-acetyl glucosaminidase and the activity of β-1,4-xylosidase to β-1,4-N-acetyl glucosaminidase. The solid lines represent the transportation of microbial active pathways from stoichiometric controls of root exudates.