Elucidating the impact of different carbon/nitrogen ratio on soil biochemical properties and rhizosphere bacterial communities of flue-cured tobacco plants

Biochar and nitrogen fertilizer are frequently used together to enhance the soil fertility and crop productivity. This study elucidated, how combined application of biochar and nitrogen fertilizer influence the soil biochemical properties, rhizosphere bacterial communities, and biomass accumulation of tobacco plants through pot and field experiments. The experiments were performed using biochar (0, 600, and 1200 kg/ha) and nitrogen fertilizer (105 and 126 kg/ha) under different carbon/nitrogen (C/N) ratios as follows: B0N105, B600N105, B1200N105, B0N126, B600N126, and B1200N126. The results confirmed that the soil organic carbon pool increased with the increase of biochar dosage under different treatments. The soil C/N ratio was found significantly higher under a high dose of biochar (1200 kg/ha) and low nitrogen rate (105 kg/ha), while decreased under high dose of biochar (1200 kg/ha) and high nitrogen rate (126 kg/ha). Soil nitrate nitrogen and total nitrogen contents, soil enzymatic activities, and biomass accumulation of tobacco plants significantly increased under high dosage of biochar and nitrogen (B1200N126). Analysis of rhizosphere bacteriome through amplicon sequencing of 16S rRNA revealed that the structure, diversity, and composition of rhizosphere bacterial communities dramatically changed under different dosages of C/N ratios. Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes, and Acidobacteria were highly abundant bacterial phyla in the rhizosphere of tobacco plants under different treatments. Results of co-occurrence network analysis displayed a fewer negative correlation among rhizosphere bacterial communities under high dosage of biochar and nitrogen (B1200N126) than other treatments, which showed a less competition for resources among microbes. In addition, a redundancy analysis further proved a significant positive correlation among soil microbial biomass of carbon and nitrogen, soil biochemical properties, and high dosage of biochar and nitrogen (B1200N126). Thus, we conclude that high dosage of biochar (1200 kg/ha) under high application rate of nitrogen fertilizer (126 kg/ha) enhanced the biomass accumulation of tobacco plants by improving the soil biochemical properties and activities of rhizosphere bacterial communities.

生物炭（Biochar）与氮肥常被配合施用以提升土壤肥力与作物生产力。本研究通过盆栽与田间试验，阐明了生物炭配施氮肥如何影响土壤生化特性、根际细菌群落及烟草植株生物量积累。试验设置了不同施用量的生物炭（0、600、1200 kg/ha）与氮肥（105、126 kg/ha），形成6组碳氮比（C/N）处理组合，分别为B0N105、B600N105、B1200N105、B0N126、B600N126及B1200N126。结果表明，不同处理下土壤有机碳库均随生物炭施用量的增加而升高。当施用高剂量生物炭（1200 kg/ha）配施低量氮肥（105 kg/ha）时，土壤C/N比显著升高；而当高剂量生物炭配施高量氮肥（126 kg/ha）时，土壤C/N比则有所降低。高用量生物炭配施氮肥（B1200N126）处理下，土壤硝态氮、全氮含量、土壤酶活性及烟草植株生物量积累均显著提升。通过16S核糖体RNA（16S rRNA）扩增子测序对根际细菌群落进行分析后发现，不同C/N比施用量下，根际细菌群落的结构、多样性与组成均发生显著改变。不同处理下烟草根际土壤中丰度较高的细菌菌门包括变形菌门（Proteobacteria）、拟杆菌门（Bacteroidetes）、放线菌门（Actinobacteria）、厚壁菌门（Firmicutes）与酸杆菌门（Acidobacteria）。共现网络分析结果显示，与其他处理相比，高用量生物炭配施氮肥（B1200N126）处理下的根际细菌群落间负相关关系更少，表明微生物间的资源竞争程度更低。此外，冗余分析进一步证实，土壤微生物碳、氮生物量、土壤生化特性与高用量生物炭配施氮肥（B1200N126）之间存在显著正相关关系。综上，本研究认为，高用量氮肥（126 kg/ha）配施高剂量生物炭（1200 kg/ha）可通过改善土壤生化特性及根际细菌群落活性，提升烟草植株的生物量积累。