Effects of grazing intensity on forage fiber composition and rumen fungal community structure in sheep grazing a desert steppe

Rumen fungal communities play a central role in forage fiber degradation in grazing ruminants, yet their responses to grazing intensity under natural field conditions remain poorly understood. This study investigated the effects of light (LG), moderate (MG), and heavy (HG) grazing intensity on forage fiber composition and rumen fungal community structure in sheep grazing a desert steppe ecosystem, and examined the associations between forage fiber traits and dominant fungal genera. Compared with LG, both MG and HG were associated with significantly higher neutral detergent fiber (NDF), hemicellulose content, and hemicellulose proportion, together with a lower acid detergent fiber-to-NDF ratio (ADF/NDF), whereas acid detergent fiber (ADF) remained stable across treatments. Grazing intensity did not significantly affect fungal alpha diversity, but ordination analysis revealed treatment-related shifts in community composition. Chytridiomycota and Ascomycota were the dominant phyla across all treatments. At the genus level, Neocallimastix and Piromyces showed higher relative abundances under HG, whereas Aspergillus showed the opposite pattern. Linear discriminant analysis effect size (LEfSe) identified distinct fungal biomarkers among grazing treatments: HG was mainly enriched in Chytridiomycota and anaerobic fungal lineages, whereas LG and MG were more strongly associated with Ascomycota -affiliated taxa. Redundancy analysis (RDA) indicated that forage fiber traits were significantly associated with fungal community variation ( P = 0.042), jointly explaining 17.0% of the total variation. Pearson correlation analysis further showed that Aspergillus abundance was negatively correlated with NDF, hemicellulose content, and hemicellulose proportion; Ambispora was positively correlated with hemicellulose proportion; and Colletotrichum was negatively correlated with both ADF and NDF. Grazing intensity shaped rumen fungal community composition through dietary substrate modification and alteration of the soil-plant interface, without altering alpha diversity. The ecological heterogeneity revealed among dominant fungal taxa highlights the importance of distinguishing functional ecological groups when interpreting rumen fungal community data in grazing systems.