Effect of fertilization regimes on continuous cropping growth constraints in watermelon is associated with abundance of key ecological clusters in the rhizosphere

The deterioration of the microbiome within the rhizosphere is the main reason behind continuous cropping obstacles and is affected by fertilization regimes. However, the potential associations among fertilization regimes, rhizosphere microbial communities, and the severity of continuous cropping obstacles are poorly understood. Here, we investigated how three-year fertilization regimes (including no fertilizer, chemical fertilizer, organic fertilizer, combination of chemical and organic fertilizer, and bio-organic fertilizer application) affect the rhizosphere microbiome and its associations with continuous cropping growth constraints during the whole growing period of watermelon, using a rhizobox system under greenhouse conditions. The results showed that different fertilization treatments induced drastic shifts in the richness, structure, and composition of rhizosphere microbial communities, whereas the growth stages induced few changes. Compared with no fertilizer treatment, the continuous application of chemical fertilizer inhibited the growth of watermelon. This was strongly associated with the reduced abundance of key ecological clusters within the bacterial co-occurrence network, which contains beneficial bacterial taxa, such as Lysobacter sp. and Haliangium sp. In contrast, continuous application of organic and bio-organic fertilizers promoted the growth of watermelon and was associated with the decreased abundance of key ecological clusters within the fungal co-occurrence network, which contains pathogenic fungal taxa, such as Fusarium sp., Cladosporium sp., and Acremonium sp. Overall, continuous chemical fertilizer application can induce severe continuous cropping obstacles by inhibiting the growth of beneficial bacteria. Continuous application of organic and bio-organic fertilizers can help alleviate such obstacles by suppressing pathogenic fungi. These findings reveal the microbial mechanisms underlying the effects of different fertilization regimes on continuous cropping growth constraints of watermelon.