Long-Term Intercropping With Nitrogen Management to Improve Soil Quality and Control Crop Diseases

Long-term positioning experiments have demonstrated significant benefits in agricultural production and environmental protection. Faba bean-wheat intercropping with nitrogen fertiliser can effectively mitigate the occurrence of faba bean wilt disease. Identifying the optimal nitrogen application rate is essential for enhancing the disease control efficacy of intercropping. This study aimed to investigate the long-term effects of varying nitrogen application levels on the physical, chemical, and biological changes in the rhizosphere soil of faba bean under intercropping conditions and to examine their relationship with the incidence of faba bean wilt disease. In a 9-year field experiment, two treatments of faba bean-wheat intercropping for 1 year (IF-1) and 9 years (IF-9) were established to investigate the incidence of faba bean wilt under four nitrogen levels (N0: 0 kg ha^-1; N1: 45 kg ha^-1; N2: 90 kg ha^-1; N3: 135 kg ha^-1). Rhizosphere soil from faba bean plants was collected to assess the corresponding physical, chemical, and biological indicators. Long-term intercropping promoted the growth of faba bean plants and effectively controlled faba bean wilt disease by improving soil structure and fertility and soil quality (SQI). Under different nitrogen application levels, certain soil physical properties (moisture content, macroaggregate proportion, MWD, and GMD) and chemical properties (SOM, total carbon, SOC, total nutrients, and available nutrients) peaked under N2 (90 kg ha^-1), with SQI showing a similar trend. Additionally, long-term intercropping enhanced enzyme activity in the faba bean rhizosphere, reshaped microbial community composition, maximised the benefits of beneficial microbes, reduced the abundance of the pathogenic fungus Fusarium, and achieved optimal disease control under N2. Under long-term intercropping with nitrogen fertiliser application at N2 (90 kg ha^-1), the physical structure of the faba bean rhizosphere soil was significantly improved, soil quality and fertility were enhanced, and the abundance of plant pathogens was reduced by modifying the microbial community composition. This effectively alleviated faba bean disease, promoted healthy plant growth, and maintained soil function.