Differential Effects of Rotatable Non-Host Crop Cultivation Duration on Physicochemical and Microbial Properties of Monocropping Soil

Crop rotation effectively mitigates soil continuous cropping obstacles. Exploring the impacts of different cropping systems (rotational cultivation vs. long-term monocropping of rotatable non-host crop) on the physicochemical (including pH, electrical conductivity, soil organic matter, available phosphorus, available potassium, ammonium nitrogen, and nitrate nitrogen) and microbial properties (including the abundances of Fusarium oxysporum f. sp. cubense (FOC) total bacteria, fungi, and actinomycetes, as well as catalase, urease, and chitinase activities) of monocropped soil is essential for targeted obstacle management. In a long-term fixed-site experiment, three treatments were established: continuous banana monocropping (BB), single-season pineapple rotation (BP), and long-term pineapple monocropping after continuous banana monocropping (PP). Soil physicochemical and microbial properties were determined, and their interrelationships were analyzed. Compared with BB, both BP and PP significantly reduced the target gene copies of FOC with the FOC target gene copies in BP being significantly (p < 0.05) higher than in PP. BP significantly (p < 0.05) increased the target gene copies of fungi (F), bacteria (B) and actinomycetes (A) as well as the activities of catalase (Cat), urease (Ure) and chitinase (Chi). In contrast, PP caused no significant changes in actinomycetes’ target gene copies or any tested enzyme activity. All of the microbial target gene copies and enzyme activities in BP were significantly (p < 0.05) higher than those in PP. Relative to BB, BP significantly (p < 0.05) decreased the soil available potassium (SK), available phosphorus (SP) and soil organic matter (SOM). Soil pH and SOM were significantly (p < 0.05) higher in BP than in PP, while electrical conductivity (EC) and nitrate nitrogen (NO3−) were significantly (p < 0.05) higher in PP than in BB. Model analysis indicated that SOM, SK, SP and NH4+-N were the core physicochemical factors explaining the variation in FOC target gene copies, and the target gene copies of actinomycetes and fungi, together with catalase, urease and chitinase activities, were the key biological drivers. Verification tests showed that the proportions of pathogen-antagonistic fungi and actinomycetes in BP reached 23.6% and 10%, respectively, which were significantly (p < 0.05) higher than those in PP and BB. Banana seedlings grown in BP soil had significantly (p < 0.05) greater plant height, stem diameter and leaf area than those in BB. PP showed a mild upward trend in growth indicators, but the difference from BB was not significant. In conclusion, single-season pineapple rotation and the long-term monocropping of rotatable non-host crop both reduce FOC target gene copies in continuously cropped soil, yet the two patterns operate through distinct suppression mechanisms for continuous cropping obstacles. Single-season rotation achieves better improvement in soil physicochemical and microbial properties, while the long-term monocropping of a rotatable non-host crop showed lower soil FOC target gene copies.