土壤质地对自养固碳微生物及其同化碳的影响

自养微生物可同化大气中的CO2并将其转化为土壤有机碳,对提高农田土壤的碳吸收和碳储存有重要意义,然而土壤质地对自养固碳微生物功能种群及其同化碳的影响机制还不清楚.本研究选取亚热带地区同一母质发育而成的两种质地水稻土壤(壤质黏土和砂质黏壤土),通过14C-CO2连续标记技术结合室内模拟培养实验,探讨土壤质地对自养微生物同化碳(14C-SOC)、自养微生物截留碳(14C-MBC)和自养微生物可溶性碳(14C-DOC)的影响.以固碳功能基因(cbb L基因)作为指示基因,结合PCR和克隆测序技术,分析不同质地土壤自养固碳微生物群落结构和多样性的差异.结果表明,壤质黏土14C-SOC、14C-MBC和14C-DOC平均含量分别为133.81、40.16和8.10 mg·kg-1,均显著高于砂质黏壤土14C-SOC(104.95 mg·kg-1)、14C-MBC(33.26 mg·kg-1)和14C-DOC(4.18 mg·kg-1)平均含量(P0.05),说明土壤质地显著影响了土壤自养微生物碳同化量以及自养微生物同化碳在土壤中的转化.稀疏曲线、细菌cbb L基因文库覆盖度以及多样性指数分析结果显示壤质黏土固碳细菌群落多样性高于砂质黏壤土.系统发育分析表明,壤质黏土细菌cbb L基因序列与Rhodoblastus acidophilus、Blastochloris viridis、Thauera humireducens、Mehylibium sp.、Variovorax sp.等具有一定的同源性,而砂质黏壤土cbb L基因序列主要与根瘤菌和放线菌同源.可见,土壤质地对自养固碳微生物群落结构和多样性产生了深刻的影响,壤质黏土中较高的黏粒含量、土壤养分含量和阳离子交换量可能有利于维持更高的自养固碳微生物多样性和活性,从而导致不同质地土壤自养微生物碳同化量及其转化存在显著差异.

Effects of Soil Texture on Autotrophic CO2 Fixation Bacterial Communities and Their CO2 Assimilation Contents

Autotrophic bacteria can assimilate atmospheric carbon dioxide (CO₂) and convert CO₂ into organic carbon. The CO₂ fixation by autotrophic bacteria is important for the improvement of carbon sequestration in agricultural soils. However, the effect of soil texture on autotrophic CO₂ fixation bacteria and their CO₂ fixation capacity is still unknown. Here, two paddy soils with different textures (loamy clay soil and sand clay loam soil) were incubated with continuous ¹⁴C-CO₂ in a glass chamber. The two soils were developed from the same parent. At the end of 110 days incubation, the ¹⁴C-CO₂ incorporated in soil organic carbon (¹⁴C-SOC), microbial biomass carbon (¹⁴C-MBC) and dissolved organic carbon (¹⁴C-DOC) were measured to explore the effects of soil texture on the autotrophic bacterial CO₂ fixation rates. The effect of soil texture on the composition and diversity of autotrophic CO₂ fixation bacterial community was investigated using cloning and sequencing of the cbbL gene, which encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) in the Calvin cycle. The results showed that the average contents of ¹⁴C-SOC, ¹⁴C-MBC and ¹⁴C-DOC were 133.81, 40.16 and 8.10 mg·kg^-1 in loamy clay soil, respectively, which were significantly higher than their corresponding contents in sand clay loam soil (P<0.05). This suggested that soil texture not only affected the amounts of autotrophic bacteria CO₂ fixation but also had an effect on the transformation of microbial assimilated ¹⁴C in soil. The cbbL gene libraries of two soils were significantly different as revealed by libshuff analyses (P<0.05). Phylogenetic analysis showed that cbbL sequences from the loamy clay soil were closely affiliated with known cultures such as Rhodoblastus acidophilus, Blastochloris viridis, Thauera humireducens, Mehylibium sp.and Variovorax sp., whereas these sequences belonging to the sand clay loam soil were related to branching lineages originating from Rhizobiales and Actinomycetales.Rarefaction curve, clone library coverage and diversity index analysis based on bacterial cbbL clone libraries indicated that the loamy clay soil had higher cbbL gene diversity compared to the sand clay loam soil. These results suggested that soil texture had a pronounced effect on the composition and diversity of autotrophic CO₂ fixation bacterial communities. The higher clay content, nutrient availability and cation exchange capacity may stimulate the growth and activity of autotrophic bacteria, and result in the higher amounts of ¹⁴C in loamy clay soil. These data broaden the understanding and knowledge of mechanisms of microbial carbon fixation and their influencing factors in agricultural soils.