Glucose as a surrogate for root exudates overestimates greenhouse gas emissions from anoxic soils

Glucose is frequently used as a surrogate for root exudates in priming studies because it plays a central role in plant metabolism. Glucose-induced priming can accelerate the decomposition of organic matter via co-metabolism, thereby enhancing the release of greenhouse gases such as CO₂, N₂O, and CH₄. However, despite Glucose’s widespread use as a proxy, actual root exudates are far more complex and further include organic acids, phenolic compounds, and nitrogen-containing molecules. Many studies fail to capture this complexity, particularly functions related to mineral dissolution, nutrient acquisition, and microbial interactions.

We hypothesized that glucose, compared to plant-derived exudates, leads to disproportionately high soil respiration and methanogenesis, thereby overestimating carbon decomposition and associated biogeochemical processes. To test this hypothesis, we collected thawed permafrost mineral soil from Abisko, Sweden. Permafrost soils store nearly twice as much carbon as is currently present in the atmosphere, thus, they represent a critical component of the global carbon cycle. The soil was incubated in anoxic microcosms and amended with four different exudate mixtures at environmentally realistic concentrations: glucose; a more complex carbon mixture composed of sugars and organic acids without nitrogen; the same complex carbon mixture with nitrogen-containing glycine; and exudates derived from graminoid plants obtained from thawed permafrost soil in Abisko. Within days of amendment, plant-derived and nitrogen-containing exudates resulted in lower CO₂ emissions than glucose and the nitrogen-free mixture, highlighting a key role of nitrogen in diversifying microbial metabolism. The effects on CH₄ emissions were even more pronounced than those on CO₂: glucose and the nitrogen-free mixture produced significantly higher CH₄ emissions compared to plant-derived and nitrogen-containing exudates.

Together, our results suggest that artificial mixtures of sugars, organic acids, and nitrogen-containing compounds should be preferentially used in priming studies to better reflect the complexity of root exudation.