Long-term plant-microbe interactions weaken the rhizosphere priming effect in tundra systems

Ecosystem priming is a critical process contributing to the carbon balance of tundra soils. On one hand, plant exudation of labile organic compounds can stimulate microbial activity inducing the decomposition of more complex organic matter, resulting in soil carbon loss. On the other hand, the efficient processing of plant exudates, and stabilization of microbial necromass in soils, can increase soil carbon stocks, reducing CO₂ emissions to the atmosphere. The divergence between positive and negative priming depends on ecosystem stoichiometry, microbial trait distribution, climate, and non-linear interactions between plants and microbial activity. Here we employ a mechanistic model, ecosys, to examine the role of microbial trait distribution and plant-microbe interactions in determining priming effects on tundra soil carbon stocks. The ecosys model represents distinct functional guilds of bacteria (e.g., heterotrophic decomposers, nitrifiers) and fungi (e.g., mycorrhizae and saprotrophs), and the diversity within, as a function of their traits, including carbon use efficiency (CUE). We examine the role of priming in short- and long-term experiments. We initially benchmarked the ecosys model to well-studied sites in the North American Arctic and explore how diversity in microbial CUE regulates soil carbon stocks under different priming conditions (e.g., a single application of labile carbon vs. semi-continuous exudation conditions) over the course of one year. We then scale up these simulations to the whole of Alaska and examine how plant-microbe interactions alter the priming effect over centennial time scales, with and without warming. We generally observed the attenuation of the priming effect contingent upon elevated nutrient concentrations under warming, which reduced plant exudation to soils. We will discuss these results, and how microbial traits influence the long-term balance of soil carbon in tundra ecosystems.