Objectives The objective of this study was to explore the effects and mechanisms of bio-organic fertilizers on alleviating soil related problems in continuous cropping, balancing soil nutrients, and controlling soil enzyme activities under cucumber continuous cropping conditions. The results will lay a foundation for bio-organic fertilizers on improving soil fertility, improving continuous cropping obstacles, and promoting sustainable agricultural development.

Methods A three-season field trial was conducted at Rugao Agricultural Science Research Institute using high-intensity continuous planting of cucumber. Applications of bio-organic fertilizers of 0, 10 and 20 t/ha were the treatments. The bio-fertilizer was once applied at the first cucumber season and cucumber was planted continuously for three seasons. Soil samples of 0−20 cm deep were collected at maturate stages of each season, the soil physical and chemical properties, enzyme activities, microbial biomass carbon and nitrogen content as well as cucumber yield were measured.

Results The soil bulk density and electrical conductivity increased with cucumber planting season, while soil fertility indexes, enzyme activity, microbial biomass carbon and nitrogen content and cucumber yield decreased in CK treatment; the application of biological organic fertilizer reduced soil bulk density and electrical conductivity, increased soil total porosity, pH, organic matter, total nitrogen, nitrate nitrogen and available phosphorus content in the same cucumber mature period. Over the continuous planting time, the contents of soil organic matter, available phosphorus and nitrate nitrogen increased by 39.3%–83.9%, 5.98%–14.2% and 10.0%–22.0% respectively, compared to CK, while the soil electrical conductivity decreased by 8.57%–12.73%.At cucumber ripening stage, the activities of soil urease, catalase, sucrose and phosphatase in different treatments followed the order: B20 > B10 > CK, and the soil four enzymatic activities in B10 and B20 treatments after 3 seasons planting were higher than those in CK in the first season; Over time, the soil microbial biomass carbon and nitrogen contents in the bio-organic fertilizer treatment became significantly higher than that in CK, the content of soil microorganism biomass carbon and nitrogen inB10 and B20 treatments increased by 4.26%–23.17% and 21.81%–56.23% respectively, compared with CK. The cucumber yield in the first, second and third season of B10 treatment and B20 treatment increased by 3.93%, 9.73%, 10.65% and 12.92%, 18.83%, 20.21% respectively, compared to CK. The correlation between cucumber yield and soil bulk density, organic matter, nitrate nitrogen, available phosphorus and biological index was significant during the continuous cropping of cucumber, and soil pH and electrical conductivity were significantly correlated with soil catalase activity, invertase activity, microbial biomass nitrogen content and cucumber yield.

Conclusions Bio-organic fertilizer can improve the soil physical and chemical properties in the continuous cucumber cropping system, alleviate the secondary salinization of cucumber continuous cropping, increase soil enzyme activity, microbial biomass carbon and nitrogen content as well as cucumber yield, but without significant change in soil physical and chemical properties and biological characteristics during continuous cropping, thereby alleviate soil related problems in continuous cucumber cropping.