The Turnover Of Photosynthetic Carbon Of Soybean And Relevant Bacterial Community Characteristics In The Mollisols

This study aimed to investigate the turnover of the photosynthetic carbon in Mollisols and the bacterial characteristics. We identified the critical level of 13 C enrichment in the substrate for the effective separation of 13C-DNA. Thereafter, DNA-SIP and Illumina pyrosequencing methods were used to identify the bacterial community in the rhizosphere that incorporated the soybean plant-derived carbon(C) and compared these bacterial communities in two Mollisols differing in soil organic carbon(SOC) content, quantify the contribution of the soybean residue-C to the SOC pools in the two Mollisols, and compare the bacterial community structure between the soils after the soybean residues were amended, and discriminate the decomposition processes of different soybean residues, including the leaf, stalk, and root, and finally quantify the amount of C incorporated from these residues into SOC pools and examine the dynamic shift of the soil bacterial community compositions after soybean residue were added into the soil. The results were as follows:(1) Using the stable-isotope probing(SIP) technology to detect the critical level of 13 C for the separation of 13C-DNA, the result showed that the detective level of 13C-DNA could be reduced to 2 atom% 13 C of glucose(1.30 atom% 13 C in DNA extract), while the ideal level was 10 atom% 13 C glucose(2.25 atom% 13 C in DNA).(2)The soybean was labelled with 13CO2, and then the stable isotope probing(SIP) and pyrosequencing analysismethods were used to identify the bacterial communities that use the plant-derived carbon. The result showed that the C flow from the plant to bacterial communities in the rhizosphere was soil-specific. Some of genera such as Aquincola, Massilia, Amycolatopsis and Dechloromonas had greater relative abundances in the low-Corg soil than in the high-Corg soil. However, Enhydrobacter, Propionibacterium and Staphylococcus exhibited the opposite trend. These differences also reflected in the number of OTUs affiliated to these genera. Specific species in these genera were likely to be associated with the fate of plant-fixed C in the soil.(3)A 150-day incubation experiment was conducted with different 13C-labelled residues of soybean, i.e., leaf, stalk, and root, incorporated into a Mollisol. The leaves had the highest decomposition rate. At the end of the incubation, cumulative respiration reached 7.76 mg CO2-C g-1 in the leaf-incorporated soil, but only 5.98 and 5.51 mg CO2-C g-1 were recorded for the stalk- and root-amended soils. Furthermore, similar trends were found for the microbial biomass C and dissolved organic C. Different residue sources greatly affected the residue-derived C incorporation in the SOC fractions, resulting in a ranking of root>stalk>leaf. The root-derived C incorporation values were 49.5, 17.2, and 5.0 g residue C kg-1 in the coarse particulate organic C(POC), fine POC, and mineral-associated C(MOC) fractions, respectively, which were significantly higher than those for the stalk- and leaf-derived C. These results indicate that C input from roots can play an important role in C stability in this Mollisol by incorporating more C in the POC and MOC.(4) The diversity of the bacterial community and structure of several abundant bacterial taxa were altered by the incubation time and the type of the residues. In this study, abundances of the genera Chitinophaga, Bacillus, Streptomyces, Niastella and Nonomuraea were positively responded to the residues treatment. Among them, the extent increase of Niastella was increased with the incubation time. However, the Bacillus and Streptomyces had the opposite trend. Different types of residues added into the soil resulted in a different change of the bacterial communities. Chitinophaga, Bacillus and Streptomyces had the greatest increase for the leaf- stalk- and root-amended soil, respectively.(5) A 150-day incubation was conducted with 13C-labelled residues of soybean incorporated two types of Mollisols with different SOC content. The results showed that soybean residue amendment increased C sequestration across SOC pools in Mollisols. The residue-derived C retentions to the coarse particulate organic C(POC), fine POC and mineral-associated C(MOC) fractions in the SOC-rich soil were 4.8-, 4.0- and 1.7-fold over the SOC-poor soil, respectively. However, as residue was amended, the primed CO2 from native SOC was stronger in the SOC-rich soil, compared to the SOC-poor soil, indicating an accelerated C exchange rate in the SOC pools of this soil. Application of residue shifted the bacterial community favoring the genera of Xanthomonas, Sphingomonas and uncultured-Gemmatimonadaceae, while Xanthomonadales, Rhizobium, Steroidobacter and unclassified-Proteobacteria in the SOC-poor soil were favored. The CCA analysis further proved that the association of bacterial community with the fresh C input into SOC pools. For example, Proteobacteria and Gemmatimonadetes had much higher percentage in soil microaggregates that mainly comprise fine POC. This indicates a close relationship between bacterial community composition and the environmental variables that mainly were SOC associated.