Tea plant-legume intercropping simultaneously improves soil fertility and tea quality by changing bacillus species composition

Abstract

Tea plant is an economically important crop in China, but long-term monoculture and substantial chemical nitrogen fertilizer input cause soil acidification, which in turn affects the nutrient supply and tea quality. Intercropping has drawn more attention in tea gardens because this pattern is expected to improve soil fertility and tea quality and change the soil microbial community composition. However, the roles of some key microorganisms in rhizosphere soils have not been well characterized. Hereby, a "soybean in summer and smooth vetch in winter" mode was selected to investigate the effects of intercropped legumes in a tea garden on soil fertility, tea quality, and the potential changes in beneficial bacteria such as Bacillus. Our data showed that when soybeans were turned into soil, intercropping system exhibited higher soil organic matter (SOM), total nitrogen (TN), tea quality indices and the expression of Camellia sinensis glutamine synthetase gene (CsGS). Notably, intercropping significantly affected the bacterial communities and decreased the relative abundance of Bacillus but increased its absolute abundance. Bacillus amyloliquefaciens BM1 was isolated from intercropped soil and showed outstanding plant growth-promoting (PGP) properties when coinoculated with rhizobia. In winter, intercropping with smooth vetch had a beneficial effect on soil properties and tea quality. Comparably, coinoculation with strain BM1 and Rhizobium leguminosarum Vic5 on smooth vetch (Vicia villosa) showed huge improvements in SOM, TN and quality of tea leaves, accompanied by the highest level of amino acids and lowest levels of polyphenol and caffeine (p < 0.05). According to these results, our findings demonstrate that intercropping with some legumes in the tea garden is a strategy that increases SOM, TN and tea quality, and some PGP Bacillus species are optional to obtain an amplification effect.

Figure 1

Conceptual model of the experimental design in this study. In summer, soybean was used as the intercropping plant, and Br. diazoefficiens USDA110 was inoculated as rhizobia (R) that formed nodules on soybean. Period I indicates that the soybeans were flowering and/or podding, and period II indicates that the soybeans were turned into the soil for approximately 40 days. Monoculture was used as the control (CK). In winter, smooth vetch and R. leguminosarum Vic5 were used as the intercropped plant and related rhizobia, respectively. In addition to monoculture and single inoculation, co-inoculation with rhizobia and B. amyloliquefaciens BM1 was also assessed under field conditions (RB). Period III indicates that smooth vetch was flowering and/or podding, and period IV indicates that smooth vetch was turned into the soil for approximately 30 days.

Figure 2

The effects of monoculture and intercropping (soybean–tea plant) under field conditions. a Soil pH. b Soil organic matter. c Total nitrogen. d Tea polyphenols in tea leaves. e Caffeine in tea leaves. f Amino acids in tea leaves. g Relative expression of CsGS. h Relative expression of CsGOGAT. I and II represent periods I and II, respectively. CK1 and CK2, tea plant monoculture in periods I and II; R1 and R2, intercropping with soybean in the tea garden in periods I and II. All data are shown as the mean ± SD (n = 3). Different letters represent significant differences (p < 0.05).

Figure 3

Intercropping changed the soil bacterial community composition in the tea garden. a Relative abundance of major bacterial communities (>1%). b Top 15 genera of each soil sample. c and d Differences in bacterial groups at the genus level between R1 and CK1 and R2 and CK2, respectively. The data were analyzed using STAMP with two-group comparisons. The effect size represents the difference in the relative abundance of some bacterial genera between two samples. The red and blue dots indicate samples with p values <0.05 by Welch’s t-test and an effect size ≥0.5 or ≤−0.5, whereas the gray dots indicate samples with p values >0.05 or effect size ranging from −0.5 to 0.5. The red, blue, and gray dots indicate increased, decreased, and unchanged relative abundance of bacterial groups, respectively. e and f, Relative and absolute abundance of Bacillus, respectively. I and II represent periods I and II. CK1 and CK2, tea plant monoculture in periods I and II; R1 and R2, intercropping with soybean in the tea garden in periods I and II. ^*, p < 0.05; ^**, p < 0.01.

Figure 4

The effects of B. amyloliquefaciens BM1 on the performance of smooth vetch and Astragulus in pots. a Height of legumes. b Dry weight of legumes. c Number of nodules. d Nitrogenase activity. Vic5, R. leguminosarum Vic5; Mh93, M. huakuii Mh93. ^*, p < 0.05; ^**, p < 0.01.

Figure 5

Effects of intercropping with smooth vetch on soil fertility and tea leaf secondary metabolites in field experiments. a Fresh weight of smooth vetch at 6 weeks post inoculation. b Activity of nitrogenase in single and dual inoculation. c Soil pH. d Soil organic matter. e Total nitrogen. f Tea polyphenols. g Caffeine. h Amino acids. Different letters represent significant differences (p < 0.05). III and IV represent periods III and IV in Fig. 1. CK3 and CK4, monoculture in periods III and IV; R3 and R4, intercropping with smooth vetch and inoculation with USDA110 in periods III and IV; RB3 and RB4, intercropping with smooth vetch and inoculation with USDA110 and BM1 in periods III and IV.