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Modelling root exudation and plant-microbe interactions under CO<sub>2</sub> fertilization in a mature forest
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DOI：10.5194/bg-23-4529-2026.png)
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Abstract
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Abstract. Root exudation; defined as plant labile carbon (C) allocation from fine roots into soils; is a substantial yet often overlooked pathway of the terrestrial C cycle. Root exudation is expected to increase under rising atmospheric CO2; yet its consequences for soil C and nutrient cycling remain poorly constrained. Additional labile C input may stimulate microbial growth and increase soil C storage; but microbial nutrient acquisition could offset this through enhanced decomposition of soil organic matter. Here; we implement a dynamic representation of root exudation; driven by plant surplus C and nutrient limitation; in the microbial-explicit terrestrial biosphere model QUINCY-JSM. We evaluate the effects of elevated CO2 (eCO2) on root exudation and on microbial C; nitrogen (N); and phosphorus (P) cycling using observations from the Eucalyptus Free Air CO2 Enrichment (EucFACE) experiment in a soil P-impoverished forest. In this experiment; eCO2 increased gross primary productivity (GPP) and soil respiration; but more than half of additional GPP under eCO2 could not be assigned to measured biomass production or autotrophic respiration; and was likely allocated belowground. With the explicit implementation of root exudation; the model predicted that eCO2 increases belowground C allocation by 20 % and microbial growth by 14 %; but has a limited effect on soil C storage. Root exudation increased by 30 %; but more than half of this additional input was directly respired by microbes. Thus; root exudation gives a possible explanation for the unmeasured fraction of plant C allocation; effectively closing the gap between enhanced GPP and increased heterotrophic respiration under eCO2 in the experiment. Although increased C input through root exudation stimulated microbial growth; microbes partially met their higher nutrient demand through a 9 % higher decomposition and increased mineralization of organic matter; which negated the build-up of microbial necromass. Our study highlights the importance of root exudation as a key pathway in vegetation C allocation under eCO2 and identifies microbial responses to this flux as a key modulator of soil C sequestration in nutrient-limited forests; thereby guiding further research regarding plant-microbe interactions.
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