Organic acid exudation can enhance carbon storage and regulate nutrient cycling in permafrost soils

Warming is leading to shrub expansion across the Arctic, with unknown consequences for permafrost soil carbon stocks. Root exudation, especially of organic acids, may lead to high soil carbon losses by releasing soil organic matter (SOM) from mineral bonds and by stimulating microbial decomposition via rhizosphere priming. Our understanding of root exudate effects primarily rests on incubations with high substrate additions that correspond to root exudation over several growing seasons. Using short-term sterile and inoculated incubations, we investigated the concentration dependence of abiotic and biotic effects of organic acids on carbon and nutrient cycling in permafrost soils. We compared traditionally high concentrations equal to 1% of soil organic carbon, with realistic, 18 times lower concentrations that match observed root exudation of the common tundra shrub Betula glandulosa . We found that only high concentrations abiotically released mineral-bound carbon and phosphate whereas low concentrations improved binding of surrounding SOM. In microbial presence, abiotically released carbon was respired but overall did not induce positive priming. Highly concentrated acids increased potential microbial stoichiometric nutrient demand resulting in a median net decrease in phosphate and ammonium by -42% and -47% (interquartile range: -45 to -35% and -57 to -25%, respectively). By contrast, low concentrated acids increased median nitrate availability by +1123% (+1053 to +1196%). Both acid concentrations substantially contributed to soil carbon storage via sorption and microbial incorporation of acids and SOM as well as negative priming. Median carbon storage was equivalent to half of total soil respiration (34 to 69%) in low-acid treatments and exceeded it by 28% in high-acid treatments (121 to 139%). We here provide evidence that organic acid effects on soil carbon and nutrient cycling are concentration-dependent, can have positive effects on carbon storage, and that laboratory experiments simulating root exudation should be aligned with the exudation rates of living plants.