Nitrogen Fertilizer Management Effectively Improves Soil Quality in Continuous Monocropping and Alleviates Fusarium Wilt in Faba Bean

Long-term continuous monocropping of faba bean destroys the soil ecological balance and increases the occurrence of Fusarium wilt. This study investigated the effects of nitrogen management on soil quality and Fusarium wilt under 1 and 9 years of monocropping at four nitrogen levels (N0-N3; 0, 45, 90 and 135 kg·hm^-2 nitrogen fertilizer, respectively). Compared with 1 year of monocropping, 9 years of monocropping significantly reduced plant growth and promoted the occurrence of Fusarium wilt. Under 9 years of monocropping, N1-N3 significantly increased plant height by 13.53%-34.84% and reduced the disease incidence by 7.21%-12.26% compared with N0, with N2 (90 kg·hm^-2) exhibiting the best effect. Compared with N0, N2 reduced soil bulk density by 9.40% and simultaneously enhanced the percentage of soil aggregates with R > 0.25 mm by 17.54%. N2 altered the community structure and composition of soil bacteria and fungi; specifically, it increased the abundance of Arthrobacter while decreasing that of Fusarium and Gibberella. Moreover, N2 contributed to increasing soil urease activity by 40.41%, soil neutral phosphatase activity by 22.59% and soil organic matter by 20.08%. Thus, applying 90 kg·hm^-2 nitrogen fertilizer effectively enhances soil quality and mitigates Fusarium wilt under continuous monocropping of faba bean.