Reduced Chemical Fertilizer Combined With Bio-Organic Fertilizer Affects the Soil Microbial Community and Yield and Quality of Lettuce

Abstract

Reducing chemical fertilizers in combination with bio-organic fertilizers can limit the use of chemical fertilizers while maintaining soil fertility. However, the effects of combined fertilization on soil chemical properties, microbial community structure, and crop yield and quality are unknown. Using high-throughput sequencing, we conducted field experiments using lettuce plants subjected to five fertilization treatments: chemical fertilizer with conventional fertilization rate (CK), chemical fertilizer reduction by 30% + 6,000 kg ha^-1 bio-organic fertilizer (T1), chemical fertilizer reduction by 30% + 9,000 kg ha^-1 bio-organic fertilizer (T2), chemical fertilizer reduction by 40% + 6,000 kg ha^-1 bio-organic fertilizer (T3), and chemical fertilizer reduction by 40% + 9,000 kg ha^-1 bio-organic fertilizer (T4). Compared with CK, the T1-T4 had significantly higher soil pH and soil organic matter (SOM) and showed increased richness and diversity of the bacterial community, and decreased richness and diversity of the fungal community. Principal coordinate analysis evidenced that the bacterial and fungal communities of CK and T1-T4 were distinctly separated. The Kruskal-Wallis H-test demonstrated that the fungal community was more sensitive than the bacterial community to chemical fertilizer reduction combined with bio-organic fertilizer. Among the soil chemical parameters measured, only TN (total nitrogen) was significantly correlated with bacterial and fungal community composition. The T1 and T2 increased lettuce yield. Moreover, T1-T4 characterized reduced nitrate content and increased levels of soluble sugars and vitamin C in lettuce. Overall, the combined application of reduced chemical fertilizer and bio-organic fertilizer effectively improved soil fertility, microbial community structure, and lettuce yield and quality. These findings have valuable implications for vegetable safety and long-term environmental sustainability.

Keywords: bacterial community; bio-organic fertilizer; chemical fertilizer; fungal community; high-throughput sequencing.

FIGURE 1

Principal coordinate analysis (PCoA) plots of bacterial (A) and fungal (B) community composition at the OTU level. CK, chemical fertilizer with conventional fertilization rate; T1, chemical fertilizer reduction by 30% + 6,000 kg ha^–1 bio-organic fertilizer; T2, chemical fertilizer reduction by 30% + 9,000 kg ha^–1 bio-organic fertilizer; T3, chemical fertilizer reduction by 40% + 6,000 kg ha^–1 bio-organic fertilizer; T4, chemical fertilizer reduction by 40% + 9,000 kg ha^–1 bio-organic fertilizer.

FIGURE 2

Relative abundances of bacterial (A) and fungal (B) taxa at the phylum level. The relative abundances of bacteria (C) and fungi (D) were tested by the Kruskal–Wallis H-test. “Other” includes phyla with <1% average relative abundance. Values for individual treatments are the means of three replicate soil samples. The false discovery rate was used to control for multiple tests, and the Scheffe test was used to calculate the 95% confidence intervals. CK, T1, T2, T3, and T4 are as defined in the Figure 1 legend.

FIGURE 3

Redundancy analysis (RDA) of bacterial (A) and fungal (B) communities with soil chemical properties. EC, electrical conductivity; SOM, soil organic matter; TN, total nitrogen; TP, total phosphorus; TK, total potassium. CK, T1, T2, T3, and T4 are as defined in the Figure 1 legend.

FIGURE 4

Yield and quality parameters of lettuce grown in soils with different fertilization treatments. (A) Yield in 2019. (B) Yield in 2020. (C) Nitrate content of lettuce. (D) Vitamin C content of lettuce. (E) Soluble protein content of lettuce. (F) Soluble sugar content of lettuce. Values presented are the means ± SE (n ≥ 3). Vertical bars indicate the SE of the means. Different lowercase letters indicate significant differences among treatments at the same timepoint (Duncan’ s test, p < 0.05). CK, T1, T2, T3, and T4 are as defined in the Figure 1 legend.

FIGURE 5

Potential mechanism of action of reduced chemical fertilizer combined with bio-organic fertilizer in regulating soil microecology to improve the yield and quality of lettuce. The red arrow indicates an increase, and the blue arrow indicates a decrease. The red box indicates bacterial or fungal phyla with significant differences between treatments.