Synergistic Effects of Earthworm and Straw Application on Soil Enzyme Activities and Nutrient Cycling in Continuous Greenhouse Watermelon Systems

Continuous greenhouse watermelon cultivation is widely constrained by declining soil function, impaired nutrient cycling, and increasing soil-borne disease pressure. Developing biologically driven strategies to restore soil–crop coupling is therefore critical for sustainable protected horticulture. Here, we conducted a two-year field experiment (2024–2025) using a randomized block design with three treatments (CK, ST, and STE), three replicates per treatment, and a plot area of 22.5 m2 to evaluate how straw application alone and in combination with earthworms regulate soil processes and crop performance in a continuous greenhouse watermelon system. Compared with CK and ST, earthworm–straw co-application (STE) exerted stronger effects, particularly during the mid-to-late growth stages. In 2024, STE increased soil organic matter by 25.34% and 30.28% relative to CK at the fruiting and harvest stages, respectively; in 2025, the corresponding increases were 25.22% and 27.62%. STE also significantly increased total nitrogen at nearly all growth stages, with the maximum increase reaching 67.23% relative to CK at harvest. In 2025, total phosphorus under STE was significantly higher than under CK and ST across all growth stages, with increases of 75.82% and 79.63%, respectively, at the fruiting stage. Neutral phosphatase activity was markedly enhanced, increasing by 292.24% at the fruiting stage in 2025. These improvements were accompanied by higher plot yield and lower wilt disease incidence, with yield increasing by 34.00% in 2024 and 21.29% in 2025 relative to CK, while disease incidence decreased by 41.46% and 56.06%, respectively. Integrative Mantel tests showed that total nitrogen was the factor most strongly associated with watermelon yield, with the correlation coefficient increasing from r = 0.490 (p = 0.001) in 2024 to r = 0.662 (p = 0.001) in 2025. Co-occurrence network analysis further revealed a strong positive correlation between yield and total nitrogen (r = 0.848 in 2024; r = 0.673 in 2025) and a negative correlation between disease incidence and total nitrogen (r = −0.661 in 2024; r = −0.822 in 2025), indicating progressively strengthened soil–plant functional coupling over time. Our findings demonstrate that earthworm–straw co-application strengthened soil nutrient transformation capacity and enhanced soil suppressiveness against wilt disease, thereby providing an effective ecology-based strategy for alleviating continuous-cropping constraints in greenhouse watermelon systems.