Characteristics and Succession of Rhizosphere Soil Microbial Communities in Continuous Cropping Watermelon

The aim of this study was to investigate the effects of continuous cropping on the construction and potential functions of bacterial and fungal communities in the rhizosphere soil of watermelon, and to clarify the adaptability of rhizosphere microorganisms to environmental changes, so as to provide a theoretical basis for ecological control of watermelon continuous cropping obstacles and healthy maintenance of farmland.

In this study, the rhizosphere soil of watermelon without continuous cropping (CK), continuous cropping for 2 times and continuous cropping for 6 times was used as the research object. 16S rRNA and ITS high-throughput sequencing were used to analyze the effects of continuous cropping on the bacterial and fungal communities of rhizosphere soil of watermelon.

With the increasing continuous cropping times, the bacterial diversity index in the rhizosphere soil of watermelon showed a trend of first decreasing and then increasing, while the fungal diversity index decreased significantly. At the bacterial genus level, multiple times of continuous cropping decreased the relative abundance of Sphingomonas and Lysobacter in watermelon rhizosphere soil; at the fungal genus level, the relative abundance of Fusarium increased with the continuous cropping times. Compared with CK, the network complexity of continuous cropping was higher, but the stability of network structure was lower. In addition, compared with CK, the relative abundance of biodegradation pathways of harmful substances and metabolic pathways of amino acids could be significantly reduced after continuous cropping for 6 times; the relative abundance of pathogenic fungi significantly increased in the fungal community after continuous cropping for 6 times. During community succession, the stochastic processes dominated rhizosphere bacterial community construction in watermelon under continuous cropping, while the deterministic processes dominated rhizosphere fungal community construction in watermelon.

Continuous cropping caused changes in community characteristics, functional composition and succession process of rhizosphere bacteria and fungi. The decrease of key functions of bacterial community, the increase of pathologic fungi and the decrease of stability of microbial community network might be the important factors leading to occurrence of watermelon continuous cropping obstacles.