Variation of bacterial community structure and the main influencing factors in Eum-orthic Anthrosols under different fertilization regimes

Objectives The fertility change in nutrient contents and microbial structure of farmland after long-term fertilization was studied in this paper, which would provide theoretical base for reasonable fertilization practice to maintain a stable and healthy soil ecosystem.

Methods The investigated farmland was from a 28-years’ long-term fertilization experiment in the “National Monitoring Base of Soil Fertility and Fertilizer Efficiency on Loess Soil” in Yangling Demonstration Zone, Shaanxi Province, where the cropping system was winter wheat-summer fallow and without irrigation. The soil samples were collected from three of the experiment treatments, namely, no fertilization control (CK), pure chemical fertilizer (NPK, N–P₂O₅–K₂O＝135–108–67.5 kg/hm²) and chemical fertilizer and manure combination (MNPK, 70% of N from cattle manure). After harvest of wheat, soil samples of 0–20 cm in depth were collected. The soil nutrient contents, water content, microbial biomass C and N contents were determined. The DNA of genome in soil samples were extracted and detected by 1% agarose gel electrophoresis, and sequenced afterwards. The Shannon, ACE and Chao1 indices were calculated.

Results Compared with CK, the NPK and MNPK significantly increased soil organic carbon, total nitrogen, microbial biomass carbon, microbial biomass nitrogen, nitrate nitrogen and ammonium nitrogen, while significantly reduced soil pH. The copy number of bacterial genes were 6.69 × 10⁹ – 16.46 × 10⁹ per gram of dry soil for the three treatments, and those in NPK and MNPK treatments were 77% and 146% significantly higher than that in CK. The Shannon diversity index of soil bacteria of MNPK was significantly higher, while Simpson index were significantly lower than those of CK and NPK treatments, and there was no significant difference between NPK and CK. The bacterial richness index (Chao1 index and ACE index) and the uniformity index were not significantly different among the 3 treatments. A total of 35 groups of bacteria were obtained at the level of phylum, in which Actinobacteria, Proteobacteria, Acidobacteria and Chloroflexi were the main dominant ones (relative abundance > 10%), and accounted for 80.1%–81.7% of the total bacteria phyla. Compared with CK, MNPK significantly reduced the relative abundance of Actinomycetes (F = 5.845, P < 0.05) and increased that of Bacteroides (F = 4.461, P < 0.05). There was no significant difference for other phyla of bacteria among the 3 treatments. The results of redundancy analysis showed that the soil bacterial community structures were significantly different between no fertilization (CK) and fertilization (NPK and MNPK), and MNPK had a greater impact than NPK on soil bacterial community composition. The importance of soil physical and chemical properties affecting the bacterial communities was as follows: nitrate nitrogen > soluble organic carbon > pH > ammonium nitrogen > organic carbon > soil water content.

Conclusions In the rain-fed farmland in Eum-orthic anthrosol area of Guanzhong, the bacteria population, diversity and richness are significantly improved in soils under long-term balanced chemical fertilization and combination of organic and inorganic fertilization. Combination of organic and inorganic fertilizers could increase the bacteria diversity that is more efficient in increasing the abundance of Bacteroides and decreasing that of Actinomycetes, which represents more stable and healthy in soil ecosystem.