Combining organic fertilizer with soil conditioner alleviates continuous cropping obstacles in protected cowpea by improving soil properties and microbiome

Cowpea (Vigna unguiculata) is a nutritious vegetable and a key crop in China, encompassing a sizeable cultivation area. However, continuous cropping has led to declines in yield and quality, limiting sustainable development in the protected cowpea industry. This study investigated the effects of combining organic fertilizer with soil conditioner (an amendment containing beneficial microbes and organic components) on protected cowpea production, soil physicochemical properties, and microbial community structure in soils subjected to five consecutive cowpea cropping cycles. The results demonstrated that combined application significantly improved cowpea yield and quality. Specifically, soil pH decreased by 5.4%, while soil organic matter (OM) content increased markedly, especially during the vegetative growth stage, displaying a 70.4% increase. The activities of key soil enzymes, including sucrase (SC), protease (PT), nitrate reductase (NR), dehydrogenase (DHO), and phosphatase, were enhanced, while the urease (UE) activity decreased. Additionally, the combined application improved bacterial and fungal abundance and diversity. Operational taxonomic unit (OTU) richness exhibited positive correlations with various soil nutrient indicators, enzyme activities, and physicochemical properties. Moreover, cowpea protein, carbohydrate, and energy contents were positively correlated with specific soil enzyme activities and nutrient levels. The combined treatment increased the cowpea yield by nearly 32,849 kg per ha and protein content by 32.3% compared with applying only organic fertilizer while optimizing the soil microbial community, improving soil structure and fertility, and effectively mitigating the issues of continuous cropping in protected cowpea production.