Effect of microbial inoculants on spinach growth characteristics and soil microbial diversity

The Huangpu River (which is the source of drinking water) has been highly contaminated in recent years by fertilizer-rich agricultural drainage from extensive fertilizer uses in vegetable farms for high yields in the suburbs of Shanghai. A fertilizer-use reduction experiment, with 20%~40% (T1~T5) reduction in chemical fertilizer use in combination with a newly composed mixture of two lad-developed microbial inoculant strains, was used to analyze spinach growth characteristics and soil microbial population in cultivated lands of Shanghai suburbs. The microbial DNA was extracted using the specific primer GC clamp to amplify PCR and ran a denaturing gradient gel electrophoresis (DGGE). The aim of the experiments was to investigate the characteristics of spinach and soil microbial environment under reduced fertilizer use, augmented with patent microbial agents. The results showed that reduced fertilization was conducive for improving the chlorophyll content and fluorescence parameters of spinach. Compared with T1 (100% chemical fertilizer) treatment, the combined treatments of reduced chemical fertilizer and inoculants presented higher chlorophyll content and fluorescence parameters of spinach at the later growth stage but lower at latter stages. SPAD under T5 (40% reduced fertilizer with 60% inoculant) treatment, and Fv/Fm under T3 (20% reduced fertilizer and 60% inoculant) treatment were 52.856 and 0.797 respectively. The results showed that the nitrate content of the edible parts of spinach was highest under CK (without fertilizer) treatment, while those of treatments with inoculants were obviously reduced. The absorption and use efficiency of N, P and K were highest under T2 (20% reduced fertilizer with 100% inoculant) treatment. Spinach yield was increased under combined treatments of reduced fertilizer and inoculant. Spinach yield under T4 (40% reduced fertilizer with 100% inoculant) treatment was the highest, which was approximately 227.73 g·pot^-1. Compared with the control (CK), yield under T4 treatment increased by 170%. Microbial richness index (from extracting total microbial DNA via PCR-DGGE and analyzing soil microbial diversity) was lowest in chemical fertilization treatment, while Shannon-Wierner index was 0.398, lower than that under CK treatment. Shannon-Wierner indexes under inoculant treatments were 0.547~0.983. The results suggested that microbial inoculants improved spinach yield, NPK uptake and use efficiency and soil microbial diversity. The combined application of 100% microbial inoculants with 40% reduction of chemical fertilizer (T4) significantly promoted spinach growth, while T3 treatment had the best effect on microbial diversity.