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. Determining controls on the temperature sensitivity ofheterotrophic soil respiration remains critical to incorporatingsoil–climate feedbacks into climate models. Most information on soilrespiratory responses to temperature comes from laboratory incubations ofisolated soils and typically subsamples of individual horizons.Inconsistencies between field and laboratory results may be explained bymicrobial priming supported by cross-horizon exchange of labile C or N. Suchexchange is feasible in intact soil profiles but is absent when soils areisolated from surrounding depths. Here we assess the role of soil horizonconnectivity, by which we mean the degree to which horizons remain layeredand associated with each other as they are in situ, on microbial C and N substrateuse and its relationship to the temperature sensitivity of respiration. Weaccomplished this by exploring changes in C : N, soil organic mattercomposition (via C : N, amino acid composition and concentration, and nuclearmagnetic resonance spectroscopy), and the δ13C of respiratoryCO2 during incubations of organic horizons collected across borealforests in different climate regions where soil C and N compositions differ.The experiments consisted of two treatments: soil incubated (1) with eachorganic horizon separately and (2) as a whole organic profile, permittingcross-horizon exchange of substrates during the incubation. The soils wereincubated at 5 and 15 ∘C for over 430 d. Enhancedmicrobial use of labile C-rich, but not N-rich, substrates were responsiblefor enhanced, whole-horizon respiratory responses to temperature relative toindividual soil horizons. This impact of a labile C priming mechanism wasmost 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 promptedincreases in mineralization of carbohydrates and more 13C-enrichedsubstrates and increased soil respiratory responses to warming relative tosoil horizons incubated in isolation. These findings highlight that soilhorizon connectivity can impact microbial substrate use in ways that affecthow soil effluxes of CO2 are controlled by temperature. The degree towhich this mechanism exerts itself in other soils remains unknown, but theseresults highlight the importance of understanding mechanisms that operate inintact soil profiles – only rarely studied – in regulating a keysoil–climate feedback.

biogeosciences

Soil profile connectivity can impact microbial substrate use, affecting how soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; effluxes are controlled by temperature