Organic fertilizer levels regulate Fusarium graminearum colonization by reshaping soil microbial community

Soil immunity is defined as the capacity of soil to defend against diseases and sustain plant health, which is fundamentally driven by soil microbial communities. Wheat Fusarium head blight, which is caused mainly by Fusarium graminearum , threatens agricultural sustainability. Topsoil is a major pathogen habitat because of straw return, thus reducing soil pathogen abundance is critical for the environmentally sustainable management of soil-borne diseases. Organic management is regarded as a potential way to inhibit pathogen proliferation by shaping complex microbial interactions in soil to prevent plant diseases. Therefore, determining the optimal amount of organic fertilizer for improving soil immunity to cultivate disease-suppressing soils is imperative. A six-year field trial with a 28-d experiment was conducted to determine the effects of organic fertilizer rates (CK, 0 t/ha; 1TSM, 15 t/ha; 2TSM, 30 t/ha; 4TSM, 60 t/ha) on crop yields, soil fertility, and microbial traits. The results revealed that compared with the CK, the 2TSM and 4TSM increased crop yields, soil fertility, and network complexity and enhanced the inhibitory effect on Fusarium graminearum abundance. However, the soil microbial community remained highly stable only under the 2TSM after Fusarium graminearum inoculation. Microbial co-occurrence network and topological role analyses both suggested that Mortierella may be the keystone taxon involved in soil immunity. Furthermore, plant coleoptile infection and another 28-d incubation experiments revealed that Mortierella alpina inoculation not only increased network complexity in soil but also reduced plant diseases. Overall, 30 t/ha organic fertilizer application is a reasonable strategy for increasing soil productivity. Changes in soil C:N ratio induced by fertilization may account for the alterations in soil immunity. Incubation combined with plant coleoptile infection assays revealed a potential mechanism: 2TSM-stimulated Mortierella alpina -driven Fusarium graminearum suppression and network complexity improvement represent feasible pathways to achieve plant and soil health.