Does a shift in vegetation type in high-latitude soils enhance soil organic matter destabilization from mineral-organic associations by organic acid exudation?

Soil organic matter bound to soil minerals contribute to long-term soil carbon and nutrient sequestration by protecting organic matter from rapid microbial decomposition. However, the binding between minerals and organic matter can be weakened by plant root exudates enhancing the potential for additional CO₂ emissions. Root exudates contain low molecular weight organic acids that might promote carbon and nutrient release by stimulating microbial decomposition of soil organic matter (priming) or directly by weathering of soil minerals. The vulnerability of mineral-associated organic matter is particularly significant at higher latitudes where global warming is accelerating the thaw of carbon-rich permafrost soils and where changes in vegetation distribution due to warming can already be observed. Differences in root exudation among plant types and their effect on soil carbon and nutrient cycling remain however unknown. Here, we explore differences in root exudation between functionally different tundra plants to assess how the release of specifically organic acids contributes to soil organic matter destabilization in the Arctic. We analyzed total carbon, organic acids as well as other primary metabolites in root exudates from Betula glandulosa, Alnus viridis, and Eriophorum vaginatum with liquid chromatography–mass spectrometry and showed that exudation rates differed significantly between B. glandulosa and E. vaginatum. Organic acids contributed less than 2% to total organic carbon exudation and measured exudation rates were much lower than typically simulated in laboratory incubations that test organic acid effects on soils. These observations question to what extent previous laboratory findings describe processes relevant in natural systems. Based on our observational data, we designed a soil incubation experiment comparing how priming and soil mineral destabilization by organic acids influence carbon, nitrogen and phosphorus cycling in the mineral-associated organic matter fraction (MAOM) and the intact bulk soil. Organic acid mixtures at two concentrations were compared: A commonly applied concentration corresponding to 1% of soil organic carbon (SOC) and a lower concentration representing seven days of observed organic acid exudation (0.001% of SOC). Preliminary data emphasize that the more realistic low acid treatment did not stimulate microbial CO₂ production compared to the control without acid addition while in contrast the high acid treatment led to an overstimulation of microbial CO₂ production of about 80-100%. We will connect these observations to data on CO₂ sources and changes in soil nitrogen and phosphorus, to assess the impact of changes in vegetation distribution on mineral-bound organic matter in thawing permafrost soils.