Global warming is believed to be closely associated with the increasing concentration of atmospheric CO₂. Reducing CO₂ emission and enhancing soil carbon sequestration are promising ways to curb global warming. Microbially mediated atmospheric CO₂ reduction by autotrophic bacteria, which are estimated to be from 0.6 to 4.9 Pg C yr^–1, contributes a substantial part to the sequestration of CO₂ in terrestrial ecosystems.

Autotrophic bacteria mainly fix CO₂ via the Calvin-Benson-Bassham (CBB) cycle. Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO), the enzyme catalyzes the rate-limiting step in CBB cycle, exists in four distinct holoenzyme forms (I, II, III, and IV). Form I RuBisCO, which is encoded by the cbbL gene, is the most abundant CO₂ fixation enzyme recognized in photoautotrophic and chemolithoautotrophic bacteria. Soils are complex systems that differ in their physical and chemical properties. The changes in physical and chemical conditions associated with soil depth, such as light, substrate and nutrient availability, may influence the community composition of autotrophic bacteria and the CO₂ fixation processed mediated by autotrophs.

Combining the ¹⁴CO₂ continuous labeling and culture-independent molecular techniques, the research group from the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) investigated the distribution patterns of assimilated ¹⁴C (as14C-SOC) and the abundance and activity of CO₂ fixing bacteria at different soil depths (0–1, 1–5, 5–17 cm).

The researchers found that the recovery of ¹⁴C significantly decreased with increasing distance from the soil surface over the 110 days (P<0.05). The majority of the fixed ¹⁴C was concentrated at 0-1cm depth with a mean concentration of 573.2 ± 177.1 mg kg–1, whereas no ¹⁴C was detected in 5-17cm depth in one quarter of the investigated soils, which suggested that the bacteria CO₂ fixation was dominated in 0-1cm depth and the assimilated ¹⁴C can delivered down to deep soil depths. Moreover, higher amount of assimilated ¹⁴C were recovered in paddy soils than in upland soils, irrespective of the distance from the soil surface.

The research also showed that the activity and abundance of CO₂ fixation bacteria decreased with increasing soil depth. Significant positive correlations were found between RubisCO activity, cbbL genes copy numbers and the ¹⁴C concentration in 0-1cm depth (P<0.01), which highlighted the importance of phototrophs in CO₂ fixation in surface soils, due to the fact that light is restricted to the top few millimeters of the soil profiles.

The research was supported financially by the National Natural Science Foundation of China (41271279, 41090283, 41301275), International cooperation and regional science and technology of Hunan Province (2013WK4009), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, the CAS/SAFEA International Partnership Program for Creative Research Teams (KZCX2-YW-T07; 20100491005–8).

The study entitled“Changes in bacterial CO₂ fixation with depth in agricultural soils” has been published in Volume 98, August 2013 of Applied Microbiology and biotechnology. More details could be found at http://link.springer.com/article/10.1007/s00253-013-5179-0#page-1.