Effects of microbial agents on soil microenvironment and fruit quality of watermelon under continuous cropping

Abstract:【Objective】This experiment was conducted to explore the effects of microbial agents on rhi- zosphere microorganisms and growth conditions of continuously cropped watermelon, and to evaluate the application of microorganisms to overcome the obstacles in continuous cropping of watermelon.【Methods】Field experiments were conducted to study the effects of different microbial agent treatments on the soil microbial activity, change in floristic, functional diversity, carbon source metabolic characteristics, soil enzyme activity, and fruit quality of watermelon. Soils from continuous cropping was applied with Bacillus subtilis as treatment 1 (T1); applied with yeast as T2; applied with Bacillus subtilis + yeast as T3; and no application of microbial agents as the control (CK). The microbial agents were ap- plied into the soil before seedling transplantation【. Results】Compared with that of CK, the incidence of continuous cropping obstacle in T1, T2 and T3 decreased to 17%, 18% and 13%, respectively. After 192 hours of culturing, the result of AWCD showed that microbial activity in the microbial agent treatments was clearly higher than that of the control, and the mixed microbial agent T3 had the highest activity. According to the results of microbial count, the total number of microorganisms in the microbial treat- ments increased significantly compared with CK. The ratio of bacteria/fungi was highest in T1, and low- est in CK, which indicated the characteristics of microbial flora tended to change from fungal type to bacterial type. The microbial diversity index in T1, T2 and T3, Simpson and Shannon, were significant- ly higher than in CK, which meant that the species of microbial community were more abundant. The diversity, uniformity and stability of microbial community were higher than in CK. The Pielou index showed there was no significant difference among the treatments. The difference in McIntosh index was small among the microbial treatments (T1, T2 and T3) but the values were all higher than that of CK. Among T1, T2 and T3, the community uniformity and the overall utilization of carbon sources were similar. The richness index of CK was only 22, which was significantly lower than that of the other treatments. This indicated that microbial agents increased the diversity of soil microbial community. The microbial agents also improved the utilization intensity of carbon source. Carboxylic acids and polyphenol utilization intensity in the microbial treatments was significantly higher than in CK. The uti- lization characteristics of soil microbial carbon source and the utilization rate of different carbon sourc- es were changed. The dependence on carbon sources of carbohydrates and amino acids was reduced, and the utilization rate of polyphenols was increased. The results indicated the microbial community structure had been changed. PCA analysis of microbial functional diversity under different treatments showed that T2 was similar with T3 in microbial function. Compared with the other treatments, microbi- al agents increased soil enzyme activity. The soil of T3 had the highest activity in invertase, phospha- tase and catalase. T1 also significantly enhanced the activity of invertase. T2 had significant effect on in- vertase activity. The activity of catalase in all microbial treatments was higher than in CK, but there was no significant difference among the three treatments. In addition, there was no significant difference among all treatments in urease activity. After treatment with microbial agents, the fruit weight and the soluble solid content were higher than CK, and T3 was the highest. The peel thickness in T2 was low- est, while T1 and T3 increased the peel thickness compared with CK. The soluble sugar content in T3 treated with compound bacterium agent was 28.48%, 28.25% and 34.51% higher than that of T1, T2 and CK, respectively. The vitamin C content in T3 was 52.37% higher than that of CK. T1 and T2 were also significantly higher than CK (p < 0.05). The contents of soluble solids and vitamin C were positively correlated with the utilization intensity of carbon source of carboxylic acids (0.592, p < 0.05; 0.738, p < 0.01). And the utilization intensity of polyphenols also had a significant positive correlation with the contents of soluble solids (0.605, p < 0.01) and vitamin C (0.725, p < 0.01). Principal component analysis showed that T3 ranked the first with the highest score (2.578) while CK had the lowest score concerning the effect on soil on soil microbial diversity, soil enzyme activity and fruit quality.【Conclusion】Application of microbial agents is effective in increasing the microbial activity, bacterial/ fungal ratio, microbial community diversity and uniformity in continuous cropping soil. The treatment also changes soil microbial carbon source utilization characteristics, increases activities of some soil enzymes, reduces the incidence of continuous cropping obstacle, and thus promotes watermelon yield and fruit quality. Among the tested microbial treatments, T3 (Bacillus subtilis + yeast) showed the best ef- fect. The relationship between the utilization intensity of carbon source of microbes and fruit quality needs deeper study in respect to continuous cropping obstacle.