Changes in bacterial CO2 fixation with depth in agricultural soils

Area of Science:

• Soil Science
• Microbial Ecology
• Biogeochemistry

Background:

• Soil organic carbon (SOC) is crucial for soil health and climate regulation.
• Understanding carbon distribution and microbial roles in different soil types is essential for effective land management.

Purpose of the Study:

• To quantify and compare the distribution of labeled carbon ((14)C) in soil organic carbon (SOC), microbial biomass (MBC), and dissolved organic carbon (DOC) between paddy and upland soils.
• To investigate the role of phototrophs and other microorganisms in carbon fixation within different soil profiles.

Main Methods:

• Soils were incubated with (14)CO2 to trace carbon fixation.
• Distribution of (14)C-SOC, (14)C-MBC, and (14)C-DOC was analyzed at various depths (0-1, 1-5, and 5-17 cm).
• Analysis of RubisCO activity and cbbL gene abundance, followed by phylogenetic analysis of cbbL genes.

Main Results:

• Paddy soils exhibited significantly higher (14)C-SOC concentrations (mean 1,180.6 mg kg(-1) at 0-1 cm) compared to upland soils.
• (14)C incorporation into MBC and DOC was substantially greater in paddy soils than in upland soils across all measured depths.
• The majority of fixed (14)C-SOC, RubisCO activity, and cbbL gene abundance were concentrated in the 0-1 cm soil layer, indicating the importance of surface soil processes.

Conclusions:

• Paddy soils are more effective at sequestering carbon compared to upland soils, particularly in the surface layer.
• Phototrophic microorganisms, alongside chemoautotrophic bacteria, are key players in CO2 fixation in surface soils, as evidenced by gene abundance and activity.
• The distribution of carbon fixation potential spans across soil depths, involving diverse bacterial communities like Rhodopseudomonas palustris and Bradyrhizobium japonicum.