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

Abstract

Autotrophic microorganisms, which can fix atmospheric CO2 to synthesize organic carbon, are numerous and widespread in soils. However, the extent and the mechanism of CO2 fixation in soils remain poorly understood. We incubated five upland and five paddy soils from subtropical China in an enclosed, continuously 14CO2-labeled, atmosphere and measured 14CO2 incorporated into soil organic matter (SOC14) and microbial biomass (MBC14) after 110days. 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 (<10gkg−1 soil organic carbon) to medium (10–20gkg−1) to high (>20gkg−1). Substantial amounts of 14CO2 were fixed into SOC14 (mean 20.1±7.1mgCkg−1 in upland soil, 121.1±6.4mgCkg−1 in paddy soil) in illuminated soils (12h light/12h dark), whereas no 14C was fixed in soils incubated in continuous darkness. We concluded that the microbial CO2 fixation was almost entirely phototrophic rather than chemotrophic. The rate of SOC14 synthesis was significantly higher in paddy soils than in upland soils. The SOC14 comprised means of 0.15±0.01% (upland) and 0.65±0.03% (paddy) of SOC. The extent of 14C immobilized as MBC14 and that present as dissolved organic C (DOC14) 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 SOC14 in paddy soils, respectively. The MBC14/MBC and DOC14/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 SOC14 and MBC14 concentrations were highly significantly correlated (r=0.946; P<0.0001). We conclude that CO2 uptake by phototrophic soil microorganisms can contribute significantly to carbon assimilation in soil, and so warrants further future study.