Using Respiration Quotients to Track Changing Sources of Soil Respiration Seasonally and with Experimental Warming

Abstract. Developing a more mechanistic understanding of soil respiration is hampered by the difficulty in determining the contribution of different organic substrates to respiration and in disentangling autotrophic versus heterotrophic and aerobic versus anaerobic processes. Here, we present a relatively novel tool for better understanding soil respiration: the apparent respiration quotient (ARQ). ARQ is the amount of CO 2 produced in the soil divided by the amount of O 2 consumed and it changes according to which organic substrates are being consumed and whether oxygen is being used as an electron acceptor. We investigated how the ARQ of soil gas varied seasonally, by soil depth, and by experimental warming in situ in a coniferous forest whole-soil-profile warming experiment over two years. We then compared the patterns in ARQ to those of soil δ 13 CO 2 . Our measurements showed strong seasonal variations in ARQ from ≈ 0.9 during the late spring and summer to ≈ 0.7 during the winter. This pattern likely reflected a shift from respiration being fueled by oxidized substrates like sugars and organic acids derived from root and root respiration during the growing season to more reduced substrates such as lipids and proteins derived from microbial necromass during the winter. This interpretation was supported by δ 13 CO 2 values, which were relatively depleted, like lipids, in the winter and more enriched, like sugars, in the summer. Furthermore, wintertime ARQ was higher in warmed (+4 °C) than in control plots, probably due to an increase in the use of more oxidized carbon substrates with warming. Our results demonstrate that soil ARQ shows strong seasonal patterns in line the phenology of carbon inputs and patterns in soil δ 13 CO 2 , verifying ARQ as a tool for disentangling the biological sources contributing to soil respiration.