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. Developing a more mechanistic understanding of soil respiration ishampered by the difficulty in determining the contribution of differentorganic substrates to respiration and in disentangling autotrophic-versus-heterotrophic and aerobic-versus-anaerobic processes. Here, we use arelatively novel tool for better understanding soil respiration: theapparent respiration quotient (ARQ). The ARQ is the amount of CO2 producedin the soil divided by the amount of O2 consumed, and it changesaccording to which organic substrates are being consumed and whether oxygenis being used as an electron acceptor. We investigated how the ARQ of soilgas varied seasonally, by soil depth, and by in situ experimental warming(+4 ∘C) in a coniferous-forest whole-soil-profile warmingexperiment over 2 years. We then compared the patterns in ARQ to those ofsoil δ13CO2. Our measurements showed strong seasonalvariations in ARQ, from ≈0.9 during the late spring and summer to≈0.7 during the winter. This pattern likely reflected a shift fromrespiration being fueled by oxidized substrates like sugars and organicacids derived from root and root respiration during the growing season tomore reduced substrates such as lipids and proteins derived from microbialnecromass during the winter. This interpretation was supported by δ13CO2 values, which were lower, like lipids, in thewinter and higher, like sugars, in the summer. Furthermore,experimental warming significantly changed how both ARQ and δ13CO2 responded to soil temperature. Wintertime ARQ and δ13CO2 values were higher in heated than in control plots,probably due to the warming-driven increase in microbial activity that mayhave utilized oxidized carbon substrates, while growing-season values werelower in heated plots. Experimental warming and phenology change the sourcesof soil respiration throughout the soil profile. The sensitivity of ARQ tothese changes demonstrates its potential as a tool for disentangling thebiological sources contributing to soil respiration.

Using respiration quotients to track changing sources of soil respiration seasonally and with experimental warming