Bibliografisk note

Funding Information:
This research was supported by funds from the Academy of Finland (grant numbers 319952 , 316401 ), Helsinki Institute of Life Science (HiLIFE) (a HiLIFE Fellow Grant to K.K covering e.g. the salary of N.M.) and Maj and Tor Nessling foundation (personal grant to O.-M.S.) We thank Marjut Wallner for her assistance with laboratory analysis. We would also like to sincerely thank staff at the Taita Research Station, University of Helsinki , for their support and assistance while undertaking the fieldwork. Three anonymous reviewers are gratefully acknowledged.

Funding Information:
We found that along our gradient SOM nutrient availability (as indicated by extracellular enzyme activity and qM) and soil pH were more strongly correlated with microbial 18O-CUE than soil stoichiometric controls. This finding was similar to a study from a wide latitudinal gradient in western Siberia by Takriti et al. (2018). There, the range of soil C:N ratios was wider than in our study, but still SOM quality and pH were stronger determinants for substrate use efficiency than stoichiometry. Other studies have also demonstrated a strong link between SOM quality and microbial CUE (Manzoni, 2017; Soares and Rousk, 2019). These results support the view that soil C:N ratios alone are not always good predictors of microbial CUE (Sinsabaugh et al., 2016). However, other studies have demonstrated that the effect of N-availability becomes more evident in soils with comparable SOM properties (e.g. Silva-Sánchez et al., 2019; Soares and Rousk, 2019).Based on our variance partitioning analyses, labile-C degradation related enzyme activities were more associated to microbial growth on SOM, whereas nutrient availability of the substrate, as reflected by chitinase and phosphatase activities, was positively associated to microbial 18O-respiration. Soil C:N ratio correlated negatively with 18O-CUE and positively with 18O-respiration along the altitude gradient. Soil C:N ratio contributed to explaining the observed variation both in microbial respiration and growth. A recent meta-analysis by Hu et al. (2022) indicates that N addition causes increases in microbial 18O-growth rate, and thus increases in 18O-CUE estimates, which may have been the case in also our gradient. Our results from variance partitioning also support the microbial carbon pump theory where labile C is easily taken up by the microbes and transformed into microbial biomass through microbial growth, whereas modification of recalcitrant lignin compounds and scavenging SOM for nutrients causes CO2 emissions from soil (Liang et al., 2017). However, some of these CO2 fluxes can also originate from extracellular enzyme activity, and thus may not be directly correlated with microbial growth and microbial CUE. We suggest that the low 18O-CUE in higher elevations of our gradient indicates greater need of microbes to allocate energy for nutrient acquisition related enzyme production than for growth compared to microbes at low elevations.This research was supported by funds from the Academy of Finland (grant numbers 319952, 316401), Helsinki Institute of Life Science (HiLIFE) (a HiLIFE Fellow Grant to K.K covering e.g. the salary of N.M.) and Maj and Tor Nessling foundation (personal grant to O.-M.S.) We thank Marjut Wallner for her assistance with laboratory analysis. We would also like to sincerely thank staff at the Taita Research Station, University of Helsinki, for their support and assistance while undertaking the fieldwork. Three anonymous reviewers are gratefully acknowledged.

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