Seasonal variation and drought effects on colonization and community structure of arbuscular mycorrhizal fungi and dark septate endophytes in desert plants.

Arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) are common root-associated fungal groups in natural ecosystems. However, how their colonization and community composition vary among seasons under natural drought conditions remains unclear. We investigated Populus euphratica and Haloxylon ammodendron in the Ebinur Lake Wetland National Nature Reserve, China. Microscopic observation and high-throughput sequencing were combined to characterize AMF and DSE colonization and community composition across spring, summer, and autumn under low, moderate, and high drought conditions. The results showed that drought strongly affected fungal colonization, with responses varying between host species. AMF colonization in P. euphratica increased to 90% under high drought conditions, and its response to drought differed among seasons, whereas DSE colonization responses were less season-dependent. AMF communities changed markedly across drought conditions, whereas DSE communities remained comparatively stable. Soil environmental variation was driven mainly by seasonality, and the colonization rates of both fungal groups were positively associated with soil ammonium nitrogen. Co-occurrence structure also varied seasonally, with the autumn network showing the greatest connectivity and complexity and a predominance of positive correlations. Overall, drought, seasonality, and host identity jointly shaped the colonization, community composition, and association patterns of root endophytic fungi in desert plants. These findings reveal contrasting responses of AMF and DSE to environmental stress and provide ecological insights relevant to dryland restoration.