Arctic willow (Salix polaris) exudation as a driver of microbial activity and soil formation in the high arctic tundra

Abstract

Colonization by pioneer plants, among which the arctic willow (Salix polaris) is one of the most important, accelerates soil development after deglaciation. This is achieved through the increased input of organic matter from plant biomass and the exudation of low molecular mass organic compounds (LMMOA), predominantly organic acids, which facilitate mineral dissolution and nutrient release. These exudates support microbial activity and contribute to the formation of soil organic matter. While there is quite a lot of data on the exudation and acceleration of microbial activity in the rhizosphere of various plants, similar data concerning arctic plants, including willow, are scarce. Furthermore, there is a lack of data on the effect of C, N, P root stoichiometry on nutrient content in exudates and the rhizosphere microbiome during soil succession after deglaciation. In this study, we analysed various habitats of high-arctic tundra in Petuniabukta (Billefjorden, Svalbard), representing different stages of vegetation development. Our objectives were (i) to assess soil and rhizosphere carbon and nutrient content and availability, as well as microbial biomass CNP; (ii) to evaluate the rhizosphere effect on nutrient availability and the microbiome of arctic willow; and (iii) to measure root and exudation CNP and quality, primarily LMMOA, in arctic willow from the studied habitats. The exudates released to deionised water were analysed for LMMOA and inorganic anions (ion chromatography) as well as the total content of C and N. The plants roots were analysed for CNP content. Soil chemical properties (e.g. pH, organic C, total and exchangeable content of elements, water extractable PO₄³⁻) and microbial parameters (microbial biomass and quantity of bacteria and fungi) were assessed in both rhizosphere and bulk soils, with the rhizosphere effect calculated accordingly. The most abundant LMMOA species in willow exudates were lactate, acetate, formate, malate and citrate, followed by pyruvate, quinate and oxalate, collectively representing approximately 2% of the total exuded C. The rhizosphere effect of willows on nutrient availability and microbial parameters was the most significant at sites with early soil development and diminished with increasing vegetation cover. A link was observed between nitrogen and phosphorus exudation and plant root stoichiometry. These trends underscored the essential role of root exudation in overcoming microbial nutrient limitations during early soil development, particularly in sites with lower nitrogen availability by reducing the soil C/N ratio.