Intercropping ajowan (Trachyspermum ammi L.) and Securigera (Securigera securidaca L.) enhances soil quality, biodiversity, and land use efficiency under integrated phosphorus management

Intercropping is a sustainable agricultural system that provides numerous ecological benefits, including enhanced biodiversity, improved soil quality, and increased productivity. This study assessed the ecological advantages of intercropping Ajowan (Trachyspermum ammi L.) and Securigera ( Securigera securidaca L.) under various phosphorus fertilizer treatments and planting ratios. A two-year field experiment (2021–2022) was conducted in Mamasani County, Iran, using a randomized complete block design with three replications. The treatments included five types of fertilizers: 100 kg ha− 1 of triple superphosphate (TSP), 50 kg ha− 1 of TSP with phosphate solubilizing bacteria (PSB), 50 kg ha− 1 of TSP with arbuscular mycorrhizal fungi (AMF), 50 kg ha− 1 of TSP with PSB + AMF, and a control. The intercropping ratios tested were 1:1 (A₁S₁), 1:2 (A₁S₂), 2:1 (A₂S₁), as well as sole cropping for each species. Measurements included root colonization, microbial respiration, phosphatase activity, seed yield, land equivalent ratio (LER) and phosphorus use efficiency (PUE). The results indicated that the combination of AMF + PSB with the 1:2 intercropping ratio resulted in the highest root colonization (70.7%), basal respiration (95.6 mg CO₂ kg− 1 day− 1), and phosphatase activity (562.8 µg PNP g− 1 h− 1). Although sole cropping with chemical fertilizer produced the highest seed yield for both species, the land equivalent ratio (LER) for the 1:1 and 1:2 intercropping systems was greater than 1 (1.13 and 1.17, respectively), indicating higher land use efficiency. These findings underscore the effectiveness of integrated AMF + PSB applications in enhancing soil health, land productivity, and biodiversity in intercropping systems, presenting a sustainable strategy for the cultivation of medicinal plants. Combined AMF + PSB increased root colonization by 70% compared to control. Intercropping patterns achieved LER > 1, showing 13–17% land use efficiency gain. Soil microbial respiration increased by 150% in optimal intercropping with biofertilizers. Phosphatase enzyme activity enhanced by 86% with integrated nutrient management. Insect diversity was significantly higher in intercropping compared to monoculture.