关键词: 固氮微生物, 水稻秸秆, 葡萄糖, 氮, 紫云英, nifH

Abstract:

【Objective】This study was to reveal the regulating roles of carbon (C) sources (rice straw vs. glucose) and nitrogen (N) addition in soil diazotroph community during growth of Chinese milk vetch (Astragalus sinicus L.), which is crucial for the management of crop residue and mineral fertilizer application in Chinese milk vetch - rice rotation system in southern China. 【Method】A pot experiment was conducted with seven treatments, including CK (no exogenous C and N addition), rice straw (Rs) plus various rates of N inputs (Rs, RsN₁ and RsN₂, corresponding to C/N ratios of 66, 25 and 13, respectively), and glucose (Glc) addition plus different N rates (Glc, GlcN₁, and GlcN₂) with same C quantity and C/N ratios in Rs-included treatments. Soils were sampled during the fast-growing phase of Chinese milk vetch, and destined for characterization of nifH gene marker and diazotroph community by using the Illumina Miseq PE300 sequencing and PCR techniques.【Result】Soil C/N ratios were similar between the CK and the treatments with straw or glucose addition alone, while tended to decrease with increasing N input, and significant decrease was observed in the GlcN₂ relative to CK. Regarding to the available nutrients, comparable soil NO₃ ^--N contents were observed under treatments of CK, Rs, and RsN₁, but it was significantly increased by 60% under the RsN₂ treatment. Compared to the CK, Glc-included treatments increased soil NO₃ ^--N content by 35%-79%. There were limited variations of soil available phosphorous (P) content under the CK and Rs-included treatments. 16%-24% decrease of soil available P content was found in the Glc-included treatments than that under CK, but not affected by N rates. The copy number of nifH gene ranged from 80.4×10 ⁶—140.5×10 ⁶ g ^-1soil under all treatments. Compared to the CK, nifH gene copy number tended to increase under treatments with both Rs and Glc addition, while a downward trend was observed with increasing N inputs. Exogenous C and N addition resulted in an overall decrease of diazotroph α-diversity than that under the CK. The responses of diazotroph α-diversity to N supply differed between C sources (Rs vs. Glc). Compared to Rs alone, RsN₁ and RsN₂ had less observed species (decreased by 6%-11%) and Chao 1 index (decreased by 13%-15%), however, GlcN₁ and GlcN₂ enhanced α-diversity to some extent relative to Glc alone. PCoA showed that diazotroph community structure was clustered into different groups depending upon C sources, and was marginally affected by N inputs. Bradyrhizobium was the most abundant genus in all treatments, and its relative abundance was significantly reduced by C and N addition in comparison with CK, however, the magnitude of reduction was obviously less in Rs-included treatments than in Glc-included treatments (12.3%-19.7% vs. 31.6%-40.5%). In contrasting to Bradyrhizobium, the relative abundance of the second most dominant genus (Geobacter) was markedly increased by C addition relative to the CK, with greater magnitude observed in Glc-included vs. Rs-included treatments (by 170%-270% vs. 25.0%-54.6%, respectively). Meanwhile, Multivariate regression tree analysis, RDA, and Mantel analysis revealed that the diazotroph abundance, diversity and community structure were closely associated with soil NO₃ ^--N and available P concentrations. 【Conclusion】The results suggested that effects of N supply on soil diazotroph abundance, diversity and structure were regulated by C sources or the C availability of rice straw and glucose amendments. Meanwhile, the resulted differences of soil available P availability by various C additions might be a key driving factor of reshaping soil diazotroph community during Chinese milk vetch growth.

Key words: soil diazotroph, rice straw, glucose, nitrogen, Chinese milk vetch, nifH