The Effects of the Long-Term Application of Different Nitrogen Fertilizers on Brown Earth Fertility Indices and Fungal Communities

Soil fungi play a crucial role in soil microbes, the composition and variety of whose communities can be altered due to nitrogen constraints, thereby affecting the plant’s development. This study aimed to investigate the relationship between the composition of soil fungi communities, fertility index, and the structure of soil fungal communities under varying nitrogen fertilizer conditions, using a long-term positioning test on the brown earth of Northeast China. It examined the impact of 31 years of applying of no fertilizer (CK, 0 kg N hm−2 a−1), the single application of inorganic fertilizer (N2, urea 135 kg N hm−2 a−1; N4, urea 270 kg N hm−2·a−1), the single application of organic fertilizer (M4, pig housing fertilizer 270 kg N hm−2 a−1), and mixed nitrogen fertilizer (M2N2, urea 135 N hm−2 a−1 + pig housing fertilizer 135 kg N hm−2 a−1) on the fertility index and fungal community structure of brown earth. The findings indicated the following: Long-term non-fertilization and the single application of chemical nitrogen fertilizer reduced the soil pH value and increased the soil bulk density. The application of organic fertilizer reduced soil bulk density and slowed down the reduction of soil fungal richness caused by nitrogen fertilizer application. The long-term application of different nitrogen fertilizers did not alter the dominant fungal phylum, showing that the dominant phylum in all treatments was Ascomycota. The pH, organic matter, total phosphorus, available phosphorus, total nitrogen, alkaline nitrogen, and available potassium were the main soil factors affecting the structural diversity of soil fungal communities. Total phosphorus explained the greatest differences in soil fungal communities.