Fungal Community Responses to Natural Humus Amendment Potentially Facilitate the Enhancement of Saline–Alkali Soil Multifunctionality

Natural humus, characterized by its high organic carbon content and high degree of humification, is widely used in soil improvement. However, the impact of natural humus on the multifunctionality of saline-alkali soils and its relationship with soil microbial diversity remain poorly understood. This study conducted experiments with varying concentrations of natural humus to explore changes in soil multifunctionality and its driving factors. The results indicate that the addition of natural humus increases soil organic matter (by 23.5-45.73%) and alkali-hydrolyzable nitrogen (by 40-81.57%), while reducing electrical conductivity (by 1.8-35.9%). These changes enhance soil microbial diversity and improve soil multifunctionality. As natural humus is a high C/N material, nitrogen limitation in soil microorganisms may occur with increasing humus addition. However, the increase in K-strategy fungi (which are more efficient in resource utilization) helps maintain a relatively high level of soil multifunctionality. At the maximum application rate (30 t/ha), soil multifunctionality reached its peak value of 0.41. These findings highlight the significant role of natural humus in improving saline-alkali soils and enhancing soil functions, particularly through its effects on microbial communities, especially K-strategy fungi.