Soil profile connectivity can impact microbial substrate use, affecting how soil CO<sub>2</sub> effluxes are controlled by temperature

Abstract. Determining controls on the temperature sensitivity of heterotrophic soil respiration remains critical to incorporating soil-climate feedbacks into climate models. Most information on soil respiratory responses to temperature come from laboratory incubations of isolated soils, and typically subsamples of individual horizons. Inconsistencies between field and laboratory results may be explained by labile C or N priming supported by cross-horizon exchange – an indirect effect of quantifying microbial temperature response within intact soil profiles. Here we assess the role of soil horizon connectivity, by which we mean the degree to which horizons remain layered and associated with each another as they are in situ, on microbial C and N substrate use and its relationship to the temperature sensitivity of respiration. We accomplished this by exploring changes in C : N, soil organic matter composition (via amino acid composition and concentration, and nuclear magnetic resonance spectroscopy), and the δ13C of respiratory CO2 during incubations of organic horizons collected across boreal forests in different climate regions where soil C and N composition differ. The experiments consisted of two treatments: soil incubated (1) with each organic horizon separately, and (2) as a whole organic profile, permitting cross-horizon exchange of substrates during the incubation. The soils were incubated at 5 °C and 15 °C for over 430 days. Enhanced microbial use of labile C-rich, but not N-rich, substrates were responsible for enhanced, whole-horizon respiratory responses to temperature relative to individual soil horizons. This impact of a labile C priming mechanism was most emergent in soils from the warmer region, consistent with these soils' lower C bioreactivity relative to soils from the colder region. Specifically, cross-horizon exchange within whole soil profiles prompted increases in mineralization of carbohydrates and more 13C-enriched substrates and increased soil respiratory responses to warming relative to soil horizons incubated in isolation. These findings highlight that soil horizon connectivity can impact microbial substrate use in ways that affect how soil effluxes of CO2 are controlled by temperature. The degree to which this mechanism exerts itself in other soils remains unknown, but these results highlight the importance of understanding mechanisms that operate in intact soil profiles – only rarely studied – in regulating a key soil-climate feedback.