Increasing susceptibility to priming of soil N mineralisation across a subarctic tundra‐forest ecotone under N fertilisation

Climate change is driving shrub encroachment and treeline advance in the Arctic, increasing inputs of plant‐derived carbon (C) to soil. These inputs could enhance or reduce microbial decomposition of soil organic matter (SOM) (‘priming’), affecting the soil C balance. In nitrogen (N)‐poor subarctic soils, priming may arise due to selective microbial N‐mining.

We investigated the susceptibility of soils to priming across a subarctic ecotone (four sites: birch forest, treeline, shrub‐dominated tundra, tundra heath). Fertilisation with N and phosphorus (P) in the field for 2 years was used to test how nutrient availability interacted with plant communities to modulate microbial responses to simulated rhizosphere input. Soils were incubated with either ¹³ C‐glucose (simulating rhizosphere input) or water (control) for 1 week under controlled laboratory conditions to assess priming effects on soil C mineralisation, soil N mineralisation and microbial growth.

Soil and foliar nutrient concentrations tended to decline from forest to tundra, reflecting shifts in vegetation composition and associated plant–soil‐microbial feedbacks. Soil C mineralisation was initially primed most strongly in tundra (+130%) and P‐fertilised treeline soils (+150%), but these effects largely disappeared within 24 h, resulting in no change to the overall soil C balance. In all soils, in the absence of added glucose, we found evidence for selective microbial use of N‐rich organic matter, suggesting high microbial demand for N. However, counter to expectation, glucose addition did not generally strengthen selective microbial N‐mining, likely because microbial use of N‐rich SOM was already near its stoichiometric limit (set by the lowest possible C:N ratio of organic compounds). This was with the exception of N‐fertilised soils, where glucose addition primed soil N mineralisation (up to +250%), resulting in intensified selective microbial N‐mining, with an effect size that tended to strengthen towards the forest. Microbial growth peaked 1 day after glucose addition and was then sustained, with stronger responses in fertilised soils, consistent with alleviated nutrient limitation.

We show that in subarctic soils, priming of N mineralisation is regulated by N availability, suggesting contrasting feedbacks depending on soil nutrient status. In more N‐poor tundra soils, labile C inputs may slow N cycling and further reduce N availability. In comparably fertile treeline and forest soils, labile C inputs may instead accelerate N cycling, increasing inorganic N availability to plants.