Poplar–Soybean Intercropping and Fertilizer Application Increased Soil Fungal Diversity and Nutrient Content

Abstract

Intercropping significantly affects soil microbial communities and nutrient content; however, the influence of intercropping with fertilizer application on them has yet to be elucidated. A pot experiment was performed with three planting patterns: P (poplar monoculture; Populus deltoides), S (soybean monoculture; Glycine max) and PS (poplar–soybean intercropping), with three fertilizer application rates of 0, 5, and 10 g (10 kg of soil per pot). The soil properties and fungal community under different treatments were analyzed. Planting patterns and fertilizer application rates significantly influenced soil properties and fungal communities. The PS pattern exhibited significantly higher soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), and ammonium nitrogen (NH₄⁺-N) contents; greater urease (UR) and sucrase (SUR) activities; and greater soil fungal diversity than did the S and P patterns. Soil nitrate nitrogen (NO₃⁻-N), available phosphorus (NH₄⁺-N, AP), available potassium (AK) contents, UR, SUR activities; and fungal diversity increased with a higher fertilizer application rate, with the highest values occurring in the 10 g fertilizer treatment. Structural equation modeling revealed that intercropping and fertilizer application affected soil nutrients by altering fungal community and enzyme activity. Soil water content and NO₃⁻-N content were the dominant factors affecting the soil fungal community (p < 0.05). The PS pattern significantly decreases the relative abundance of soil pathogenic fungi and increases the relative abundance of soil fungi related to SOM transformation and nutrient utilization efficiency. In conclusion, these findings emphasize the importance of poplar intercropping and fertilizer application on soil quality, providing guidance for the selection and management of planting patterns in agriculture to improve soil quality.