Abstract: The spatial pattern and community assembly processes of soil microbial taxa are critical for understanding biodiversity formation and maintenance mechanisms. While rare fungal taxa likely exhibit distinct biogeographic patterns and assembly processes compared to abundant taxa, such differentiations remain poorly characterized, particularly at continental scales. Here, we investigated distance-decay patterns and underlying assembly mechanisms for abundant and rare fungal taxa in 129 soil samples collected across 4000 km in Chinese northern grasslands, based on high-throughput sequencing data. A total of 208 abundant operational taxonomic units (OTUs, relative abundance >0.1%) and 5779 rare OTUs (relative abundance <0.01%) were identified. Both abundant and rare fungal taxa showed significant distance–decay relationships (P < 0.001), but the turnover rate for rare taxa (0.0024 per 100 km) was nearly half that of abundant taxa (0.0054 per 100 km) based on the binary Bray–Curtis distance. The lower turnover of rare fungal taxa was likely due to their community assembly mechanism dominated by stochastic processes, which were less influenced by environmental gradients. In contrast, abundant taxa assembly was dominated by deterministic factors like soil variables and plant traits, which varied significantly along the geographic distance. Consistently, rare fungal taxa were also less sensitive to environmental changes, with a lower turnover rate by environmental distance (0.0027 vs. 0.0099) than abundant taxa. Our findings revealed that rare fungal taxa—shaped mainly by stochastic processes—had lower spatial turnover compared to abundant taxa, which are dominated by deterministic processes. This deepens our understanding of rare microbial biogeography.

Key words: abundant fungal taxa, rare fungal taxa, distance decay, spatial turnover, community assembly