Abstract
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 use a relatively novel tool for better understanding soil respiration: the apparent respiration quotient (ARQ). The ARQ is the amount of CO2 produced in the soil divided by the amount of O2 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 in situ experimental warming (+4 ∘C) in a coniferous-forest whole-soil-profile warming experiment over 2 years. We then compared the patterns in ARQ to those of soil δ13CO2. 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 δ13CO2 values, which were lower, like lipids, in the winter 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 may have utilized oxidized carbon substrates, while growing-season values were lower in heated plots. Experimental warming and phenology change the sources of soil respiration throughout the soil profile. The sensitivity of ARQ to these changes demonstrates its potential as a tool for disentangling the biological sources contributing to soil respiration.
Highlights
Despite making extensive measurements of soil respiration (Bond-Lamberty and Thomson, 2010), scientists lack methods to disentangle the processes underlying, and substrates contributing to, soil respiration, which hampers predictions of terrestrial carbon cycle responses to global change (Phillips et al, 2017)
The seasonal range in apparent respiration quotient (ARQ) from ≈ 0.9 during the growing season to ≈ 0.7 during the winter may reflect a shift in the molecules fueling respiration from more oxidized substrates like sugars and organic acids derived from roots in the summer to more reduced substrates in the winter such as lipids and proteins derived from microbial necromass
For the first time, annual patterns in the soil ARQ and how ARQ is affected by experimental warming
Summary
Despite making extensive measurements of soil respiration (Bond-Lamberty and Thomson, 2010), scientists lack methods to disentangle the processes underlying, and substrates contributing to, soil respiration, which hampers predictions of terrestrial carbon cycle responses to global change (Phillips et al, 2017). Mechanistic uncertainty surrounding soil respiration is partly responsible for the 1000 Pg spread in model predictions of end-of-century terrestrial carbon– climate feedbacks (Friedlingstein et al, 2013). Heterotrophic respiration, which has increased globally over the past 3 decades (Bond-Lamberty et al, 2018), is itself the sum of various processes using different sources of energy. Microbes consume different organic substrates depending on what molecules are accessible and whether the microbes are living in the rhizosphere or bulk soil, and mi-.
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