Soil CO2 constrain and distinction of root respiration and microbial activity by soil CO2 and CH4 profile

Abstract:

Profiles of soil pore space CO₂ and CH₄ concentrations are rarely reported, especially from the same soils, yet are important for a number of applications. First, quantifying the component of respired CO₂ in the soil pore spaces improves paleosol-based paleo-atmospheric CO₂ estimates. Second, profiles can be used to estimate the average depth of biological activity (e.g. respiration and CH₄ oxidation). Third, CH₄ profiles, by identifying microbial activity, may help distinguish root/rhizosphere respiration from microbial decomposition. Here, we report soil CO₂ and CH₄ profiles measured at the Semi-Arid Climate Observatory and Laboratory (SACOL) on the Chinese Loess Plateau (CLP) at Lanzhou University, Gansu, China. Soil parent material on the site is mainly Quaternary aeolian loess and classifies as an Entisol.

Soil respired CO₂ (S(z) = soil CO₂ – atmospheric CO₂) is the most uncertain variable required to reconstruct ancient atmospheric CO₂ concentrations from paleosol carbonates. Our direct soil pore space CO₂ measurements show that S(z) values varied from ~100ppmV during the spring to ~2200ppmV during the summer. S(z) average 390 ± 30ppmV during May before the summer monsoon begins when soil temperature is increasing, soil water content is at a minimum and pedogenic carbonate may be forming. This value lies in the range of S(z) values previously estimated for surface Inceptisols (300 ± 100ppmV, Breecker 2013) and is lower than Pleistocene CLP paleosols (Da et al.,2015) in similar parent material. Our direct measurements of soil pore space CO₂ thus support these previous independent S(z) estimates.

We also investigate the average depth of CH₄ oxidation and soil respiration, which range from 3-10cm and at least 20cm, respectively, using the shapes of soil gas profiles. Fitting observed soil CO₂ and CH₄ profiles with a production-diffusion model show that the average depth of CH₄ oxidation was always at least 10 cm shallower than the average depth of respiration. This is likely the result of deeper average depths of root respiration than microbial activity. If soil specific relationships between methane oxidation and microbial respiration can be developed, then soil CH₄ profiles might help quantify the rate and depth of root respiration.