Microbial phototrophic fixation of atmospheric CO2 in China subtropical upland and paddy soils.

Autotrophic microorganisms, which can fix atmospheric CO 2 to synthesize organic carbon, are numerous and widespread in soils. However, the extent and the mechanism of CO 2 fixation in soils remain poorly understood. We incubated five upland and five paddy soils from subtropical China in an enclosed, continuously 14 CO 2 -labeled, atmosphere and measured 14 CO 2 incorporated into soil organic matter (SOC 14 ) and microbial biomass (MBC 14 ) after 110 days. The five upland soils supported dominant crops soils (maize, wheat, sweet potato, and rapeseed) in the region, while all paddy soils were cultivated in a regime consisting of permanently-flooded double-cropping rice cultivation. The upland and paddy soils represented typical soil types (fluvisols and ultisols) and three landforms (upland, hill, and low mountain), ranging in total carbon from low (<10 g kg −1 soil organic carbon) to medium (10–20 g kg −1 ) to high (>20 g kg −1 ). Substantial amounts of 14 CO 2 were fixed into SOC 14 (mean 20.1 ± 7.1 mg C kg −1 in upland soil, 121.1 ± 6.4 mg C kg −1 in paddy soil) in illuminated soils (12 h light/12 h dark), whereas no 14 C was fixed in soils incubated in continuous darkness. We concluded that the microbial CO 2 fixation was almost entirely phototrophic rather than chemotrophic. The rate of SOC 14 synthesis was significantly higher in paddy soils than in upland soils. The SOC 14 comprised means of 0.15 ± 0.01% (upland) and 0.65 ± 0.03% (paddy) of SOC. The extent of 14 C immobilized as MBC 14 and that present as dissolved organic C (DOC 14 ) differed between soil types, accounting for 15.69–38.76% and 5.54–18.37% in upland soils and 15.57–40.03% and 3.67–7.17% of SOC 14 in paddy soils, respectively. The MBC 14 /MBC and DOC 14 /DOC were 1.76–5.70% and 1.69–5.17% in the upland soils and 4.23–28.73% and 5.65–14.30% in the paddy soils, respectively. Thus, the newly-incorporated C stimulated the dynamics of DOC and MBC more than the dynamics of SOC. The SOC 14 and MBC 14 concentrations were highly significantly correlated ( r = 0.946; P < 0.0001). We conclude that CO 2 uptake by phototrophic soil microorganisms can contribute significantly to carbon assimilation in soil, and so warrants further future study. [ABSTRACT FROM AUTHOR]