The response of tundra and boreal forest ecosystem functions are of great interest for estimating the impact of climate change on cold-climate regions. This study focuses on the warming responses of tundra and boreal forest soil saprotrophic microbial functions and tests their non-linearity and the existence of threshold levels of warming. We hypothesized that the functions would become less efficient but more acquisitive with warming, and drastically so above the threshold, leading to higher soil C loss. Functions and variables related to soil C, nitrogen (N), and phosphorus (P) dynamics were tested, such as enzyme production, soil respiration, soluble organic C and inorganic NP, microbial biomass CNP, microbial CN absorption, and microbial carbon use efficiency. This was done by sampling tundra, tree line, and boreal forest soils from Abisko Sweden, and incubating them under five conditions ranging from current to +9 ℃ warming conditions within climate chambers. Non-linear shifts of functions consistent with the hypothesis were observed, such as the decrease of microbial biomass CN ratios, and the increase of dissolved organic C and microbial CN uptake. These results support the need for incorporating non-linearity and thresholds into future studies of cold climate ecosystem warming responses.