Increased plant productivity and decreased microbial respiratory C loss by plant growth-promoting rhizobacteria under elevated CO₂

Abstract

Increased plant productivity and decreased microbial respiratory C loss can potentially mitigate increasing atmospheric CO₂, but we currently lack effective means to achieve these goals. Soil microbes may play critical roles in mediating plant productivity and soil C/N dynamics under future climate scenarios of elevated CO₂ (eCO₂) through optimizing functioning of the root-soil interface. By using a labeling technique with (13)C and (15)N, we examined the effects of plant growth-promoting Pseudomonas fluorescens on C and N cycling in the rhizosphere of a common grass species under eCO₂. These microbial inoculants were shown to increase plant productivity. Although strong competition for N between the plant and soil microbes was observed, the plant can increase its capacity to store more biomass C per unit of N under P. fluorescens addition. Unlike eCO₂ effects, P. fluorescens inoculants did not change mass-specific microbial respiration and accelerate soil decomposition related to N cycling, suggesting these microbial inoculants mitigated positive feedbacks of soil microbial decomposition to eCO₂. The potential to mitigate climate change by optimizing soil microbial functioning by plant growth-promoting Pseudomonas fluorescens is a prospect for ecosystem management.

Figure 1. Plant C pool size per pot (a) and biomass C:N ratio (b).

Control: ambient CO₂ and without bacteria addition; B: ambient CO₂ and with bacteria addition; eCO₂: elevated CO₂ and without bacteria addition; B + eCO₂: elevated CO₂ and with bacteria addition. Error bars show standard error of the mean (n = 6). The same letters denote non-significant differences between treatments (P > 0.05).

Figure 2. Linear relationships of plant C:N ratio with root surface area (a) and total C:N enzyme ratio (b) across all treatments.

Control: ambient CO₂ and without bacteria addition; B: ambient CO₂ and with bacteria addition; eCO₂: elevated CO₂ and without bacteria addition; B + eCO₂: elevated CO₂ and with bacteria addition.

Figure 3. δ¹⁵N value in plant biomass (a), and linear relationships of plant δ¹⁵N value with rhizosphere priming effects (RPEs) (b) and total root length (c) across all treatments.

Control: ambient CO₂ and without bacteria addition; B: ambient CO₂ and with bacteria addition; eCO₂: elevated CO₂ and without bacteria addition; B + eCO₂: elevated CO₂ and with bacteria addition. Error bars show standard error of the mean (n = 6). The same letters denote non-significant differences between treatments (P > 0.05).

Figure 4. Plant-derived C inputs to soil (a) and specific microbial respiration (b).

Control: ambient CO₂ and without bacteria addition; B: ambient CO₂ and with bacteria addition; eCO₂: elevated CO₂ and without bacteria addition; B + eCO₂: elevated CO₂ and with bacteria addition. Error bars show standard error of the mean (n = 6). The same letters denote non-significant differences between treatments (P > 0.05).