Synergistic changes in soil physicochemical properties and microbial communities after earthworm breeding at poplar plantations

Earthworm breeding has emerged as a sustainable agroforestry practice with high potential to increase nutrient cycling, increase soil fertility, and regulate microbial ecology. However, the effects of earthworm breeding on soil microbial communities at poplar plantations, particularly the spatial variations across different plantation niches, remain largely unexplored. Therefore, we selected a poplar plantation with two years of earthworm breeding (PE) as an experimental system. Three sampling sites—tree rows (PE1), earthworm-breeding rows (PE2), and nonbreeding rows (PE3)—were established for analysing soil properties and microbial communities. A poplar plantation without earthworm breeding served as the control (CK) and was subsequently partitioned into three spatially matched sites (CK1, CK2, and CK3) for comparison with the corresponding PE sites (PE1, PE2, and PE3, respectively). The results indicated that compared with those at the corresponding CK sites, the soil water content (SWC), organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), ammonium nitrogen (NH 4 + –N), nitrate nitrogen (NO 3 – –N), available phosphorus (AP), and available potassium (AK) at the PE sites were significantly higher. Compared with that at CK2, the soil fertility at PE2 was significantly enhanced, as reflected by marked increases in all major soil nutrients and their available forms. In particular, the contents of NH 4 + –N and AK showed the most pronounced growth, indicating that earthworm breeding effectively promotes the transformation and release of key nutrients. Bacterial and fungal richness increased at PE2, whereas fungal diversity decreased. The bacterial community structure was influenced mainly by SOC, TN, AP, and NH 4 + –N, whereas the fungal structure was correlated with NO 3 – –N and pH. Earthworm activity promoted the abundance of saprotrophic fungi and functional microbes involved in organic matter degradation while suppressing the abundance of pathogens. However, it also reduced arbuscular mycorrhizal fungi (AMF), revealing a trade-off between short-term nutrient mineralization and long-term soil health. By elucidating the synergistic changes between soil properties and microbial communities, this study provides a practical basis for optimizing poplar–earthworm systems within sustainable agroforestry management. • Spatial heterogeneity within the plantation drives differential soil and microbial responses. • Earthworm breeding rows function as hotspots for nutrient cycling and fertility improvement. • A critical trade-off emerges: enhanced nutrient mineralization and pathogen suppression versus reduced AMF abundance.