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.