Dark Fixation of Carbon Dioxide in an Agricultural Soil

Greenhouse gas production, diffusion and consumption in a soil profile under maize and wheat production

“…Microbial dark CO 2 fixation is a process reported to decrease with depth in temperate soil (Akinyede et al, 2020), increase with greater CO 2 concentration (Spohn et al, 2020) and be optimal close to 25 • C (Nel and Cramer, 2019). The fluxes we measured agree with the range measured (0.2 -4.8 mg CO 2 m -2 h − 1 ) in a short (30 mins) incubation of agricultural surface soil by Shimmel (1987) and at the lower end of the range (2.8 -36.5 mg CO 2 m -2 h − 1 ) of longer (21 d) temperate field and forest topsoil incubations (Santruckova et al, 2005). The net fluxes measured in this incubation will be a balance between the production and fixation of CO 2 .…”

Flow of CO2 from soil may not correspond with CO2 concentration in soil

“…Although the microbial production of CO 2 can generate a negative feedback on microbial respiration, CO 2 can also be directly metabolized by microbes 1 , 18 , which would remove CO 2 from the soil air and thereby reduce the overall CO 2 flux from the soil. A similar mechanism is well known for methane, which is produced by methanogens and consumed by methane oxidizers 16 .…”

“…The soil is an important global carbon (C) pool because it contains two- to three-times more C than the atmosphere 1 . The CO 2 flux from the soil (i.e., soil respiration) represents an important part of the global C cycle because it supplies most of the CO 2 that moves from terrestrial ecosystems to the atmosphere 2 , 3 .…”

“…Studies have shown, however, that in alkaline soils CO 2 can be absorbed in the soil in such extend that in extreme causes result in negative flow of CO 2 from the soil 17 . Moreover, soil microbes can metabolize CO 2 via “heterotrophic fixation” 1 , 18 and that an increase in soil CO 2 concentration can reduce microbial respiration and thus CO 2 input to the soil 19 . If these mechanisms are quantitatively important, they may cause substantial differences among soil “spots” (by spot, we mean the area assessed by a CO 2 sensor as described in the Methods) in the relationship between the CO 2 flux from the soil and the soil CO 2 concentration and may even cause the relationship to be negative namely in case when soil diffusion reduces (e.g.…”