Excessive pesticide input drives deterministic reassembly of soil fungal communities through niche broadening and reduced phylogenetic clustering

Excessive fertilizer and pesticide inputs are widespread in intensive agriculture, yet their combined effects on soil fungal communities and the mechanisms underlying community reassembly remain poorly understood. We conducted a six-year field experiment in the North China Plain with four treatments: conventional fertilizer and pesticide inputs, excessive fertilizer, excessive pesticide, and combined excessive inputs. We assessed fungal diversity, composition, co-occurrence networks, habitat niche breadth, phylogenetic structure, and assembly processes. Excessive fertilization increased soil carbon and nutrient availability, crop yield, and fungal diversity, but did not broaden mean habitat niche breadth, indicating that resource enrichment mainly relaxed substrate limitation and promoted taxon coexistence. In contrast, excessive pesticide input reduced fungal diversity, crop biomass, yield, and biological indicators, while increasing habitat niche breadth and weakening phylogenetic clustering. Mean niche breadth was negatively related to the nearest taxon index, suggesting that pesticide filtering favored broad-niche taxa from relatively distant fungal lineages. Null-model analysis showed that stochastic processes dominated under conventional management, whereas heterogeneous selection increased under excessive pesticide input and was strongest under combined excessive inputs. These findings show that pesticide pressure reduced fungal taxonomic diversity while increasing the average habitat breadth and phylogenetic separation of the remaining taxa. Nutrient enrichment did not offset pesticide-associated ecological stress under combined excessive inputs. Overall, excessive pesticide input was the primary driver of deterministic fungal community reassembly, and the coordinated shifts in niche breadth and phylogenetic structure highlight their potential as sensitive indicators of agrochemical disturbance in intensively managed croplands.