The Exudation of Surplus Products Links Plant Functional Traits and Plant-Microbial Stoichiometry

Julian Cardenas,Eva Kaštovská,Fernando Santa

doi:10.3390/land10080840

Julian Cardenas, Eva Kaštovská + Show 1 more

Open Access

https://doi.org/10.3390/land10080840

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Abstract

The rhizosphere is a hot spot of soil microbial activity and is largely fed by root exudation. The carbon (C) exudation flux, coupled with plant growth, is considered a strategy of plants to facilitate nutrient uptake. C exudation is accompanied by a release of nutrients. Nitrogen (N) and phosphorus (P) co-limit the productivity of the plant-microbial system. Therefore, the C:N:P stoichiometry of exudates should be linked to plant nutrient economies, plant functional traits (PFT) and soil nutrient availability. We aimed to identify the strongest links in C:N:P stoichiometry among all rhizosphere components. A total of eight grass species (from conservative to exploitative) were grown in pots under two different soil C:nutrient conditions for a month. As a result, a wide gradient of plant–microbial–soil interactions were created. A total of 43 variables of plants, exudates, microbial and soil C:N:P stoichiometry, and PFTs were evaluated. The variables were merged into four groups in a network analysis, allowing us to identify the strongest connections among the variables and the biological meaning of these groups. The plant–soil interactions were shaped by soil N availability. Faster-growing plants were associated with lower amounts of mineral N (and P) in the soil solution, inducing a stronger competition for N with microorganisms in the rhizosphere compared to slower-growing plants. The plants responded by enhancing their N use efficiency and root:shoot ratio, and they reduced N losses via exudation. Root growth was supported either by reallocated foliar reserves or by enhanced ammonium uptake, which connected the specific leaf area (SLA) to the mineral N availability in the soil. Rapid plant growth enhanced the exudation flux. The exudates were rich in C and P relative to N compounds and served to release surplus metabolic products. The exudate C:N:P stoichiometry and soil N availability combined to shape the microbial stoichiometry, and N and P mining. In conclusion, the exudate flux and its C:N:P stoichiometry reflected the plant growth rate and nutrient constraints with a high degree of reliability. Furthermore, it mediated the plant–microbial interactions in the rhizosphere.

Highlights

The rhizosphere, i.e., the soil directly impacted by the activity of roots [1,2], is one of the most important zones for biogeochemical cycles in terrestrial ecosystems
The N concentration and respective stoichiometric ratios were most variable in the plant biomass and soil solution, whereas the P concentration and related elemental ratios were most variable in root exudates and microbial biomass
The nutrient uptake was proportional to the plant biomass

Summary

Introduction

The rhizosphere, i.e., the soil directly impacted by the activity of roots [1,2], is one of the most important zones for biogeochemical cycles in terrestrial ecosystems. It closely connects the following two main actors: plants and soil microorganisms. Enhanced microbial activity feeds back to plants through changes in nutrient availability [7] and plant development [8,9]. This complexity represents a major challenge for the understanding of ecological processes

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