Effect of Fertilizer Application on Watermelon Growth, Structure of Dissolved Organic Matter and Microbial Functional Diversity in Organic Substrates

Dissolved organic matter (DOM) is the immediate energy, carbon, and other nutrient substrates for microbial growth and, therefore, plays an essential role in agroecosystem biochemical processes. The aim of this study was to elucidate the interrelationships among watermelon growth, DOM chemical characteristics, nutrient content, and microbial functional diversity in organic soilless growing substrates. We analyzed the effect of no fertilizer application (CK), PK fertilizer application (PK), and NPK fertilizer application (NPK) on fruit yield of watermelon, DOM chemical structure, nutrient status, and microbial functional diversity in organic substrates cultivated with watermelon. Compared with the CK, the NPK treatment resulted in a significantly lower fruit yield of watermelon, lower DOC concentration, significantly higher DON concentration, and higher NO3−-N concentration. The application of NPK chemical fertilizer in substrates decreased the degree of humification in DOM and microbial functional diversity compared with CK treatment. Our results demonstrate that the quality of DOM plays an important role in improving the fruit yield of watermelon and is tightly related to microbial functional diversity in organic substrates.