The Effect Of Elevated CO₂ Concentration On The Turnover Of Photosynthetically Fixed Carbon And Relevant Bacterial Community Characteristics In Mollisols

Elevated atmospheric CO₂ concentration?eCO₂?likely increases the capability of photosynthesis in many plant species,and alters the efflux of photosynthetically-fixed carbon?C?into the rhizosphere.The changes in quantity and quality of rhizodeposits in response to eCO₂ may fundamentally influence soil microbial activity and community composition,because rhizodeposits supply nutrients and energy resources to microbes in the rhizosphere.As soil microbes play great roles in soil C cycling,the mechanisms about how they metabolize the plant-derived C will be essential to understand the soil C stability in future climate change.This study aimed to investigate the effect of eCO₂ concentration on the turnover of photosynthetically-fixed C and the relevant changes of bacterial community characteristics in the Mollisols.To explicit the differences of bacterial community diversity and structure under different CO₂ concentrations,stable isotope probing?SIP?coupled with Illumina high throughput sequencing approaches were applied to study the bacterial community that incorporated the soybean plant-derived C in the rhizosphere in Mollisols.We applied ¹³C-labelled crop residues into Mollisols to elucidate the impact of different crop residues derived from eCO₂ on soil organic C?SOC?content and stability,and to estimate the residue-derived C retained in particulate organic C?POC?fractions,as well as the changes of bacterial community that metabolize the residue-derived C in soil.This study revealed the turnover of residue-C in Mollisols and the bacterial community structure in response to eCO₂.The results are as follows:1,Stable isotope probing?SIP?technique was used to separate bacterial taxa that metabolized soybean photosynthetically-fixed C under eCO₂,and then applied the ¹³C-DNA fractions for sequencing.The results showed that the richness and diversity of bacterial community that utilized soybean plant derived-C in the rhizosphere of Mollisols were significantly lowered under e CO₂ concentration.Elevated CO₂decreased the abundances of genera that are ranked as r-strategies microbes,such as generaPseudarthrobacter,Gaiellales_uncultured,Gemmatimonasand Acidobacteriaceae_uncultured,but significantly increased the abundances of some genera that are capable to degrade complex substrate,like Novosphingobium,Acidimicrobiales_uncultured,Bacillus,Flavisolibacter and Schlesneri in the rhizosphere.The changes of bacterial community succession in the rhizosphere soil were likely attributed to the alteration of plant photosynthetically fixed-C under eCO₂.2,By continuously amending the eCO₂-derived residue of soybean to two Mollisols differing in SOC content?SOC-rich and SOC-poor soils?for 3 years,we found that eCO₂ significantly increased the amount of residue returned back to soil,and efficiently increased the C content in soil POC fractions?i.e.coarse POC and fine POC fractions?.However,we did not find significant increase of SOC content in two soils.Compared to no-residue control,residue return significantly increased the C content in coarse POC,fine POC and MOC fractions in the SOC-rich soil,but only increased the C content in MOC fraction in the SOC-poor soil.The result of this study showed that,plant residue derived from eCO₂ could efficiently increase the C stability by promoting the physical protection of POC structure in C-rich soil.3,¹³CO₂-labelled soybean shoot residues derived from aCO₂ and eCO₂ were amended to Mollisols for 28 days.Soil total DNA was subjected to high-throughput sequencing,and DNA extracted on the 7^th and 28^th incubation time was applied to ultracentrifugation and DNA in the heavy fractions were sequenced as well.Results showed that soybean residues from different CO₂ concentrations had no effect on total bacterial community structure,while the bacterial community structure changed over the incubation time.The community structure of bacteria that metabolized soybean residue C was significantly different between different CO₂ treatments on the 28^th day after incubation.4,A 200-day incubation was conducted with the amendment of ¹³C-labelled wheat residue that were produced under eCO_2.Soil respiration was monitored during the incubation time.The results showed that soil amended with wheat eCO_2-derived shoot residue exhibited the largest soil cumulative respiration,which indicated a greatest microbial activity rate in this treatment.Wheat residues derived from different CO₂concentration had different influences on SOC content.The amendment of eCO_2-derived wheat shoot residues was much more efficient in the C sequestration,compared to the amendment of wheat shoot residues derived from aCO₂.However,no significant differences were found in SOC content in the Mollisols that were amended with wheat root residue derived from aCO₂ and eCO₂.In this study,we found that the enrichment of soil SOC content after the amendment of wheat shoot-residue were primarily through the increase of C content in the coarse POC fraction.5,When wheat residue derived from different CO₂ concentrations were applied to soil,soil bacterial community would be greatly altered due to the wheat residue types and incubation time.The PCoA analysis proved that the bacterial community structure significantly differed between soils amended with wheat shoot residue derived from aCO₂ and eCO_2.However,the amendment of root residue derived from different CO₂concentrations imposed no significant changes on soil bacterial community structure.PCoA results showed that bacterial community in wheat shoot residue amended soil were more similar to those in root amendment soils,which indicated that the decomposition of eCO₂-derived wheat shoot residue was similar with root residues.