Spatial distribution and environmental drivers of comammox Nitrospira versus canonical ammonia oxidizers in subtropical forests

Ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) have been recognized as important nitrifiers in various ecosystems; however, the complete ammonia oxidation (comammox) bacteria affiliated with the genus Nitrospira are also an important contributor to soil nitrification, which has been demonstrated in agricultural and wetland ecosystems. Despite these findings, comammox Nitrospira in subtropical forest soils remain poorly understood. Here, we investigated the diversity, community composition, and abundance of comammox Nitrospira along a subtropical forest transect in Southeast China spanning ∼900 km, including coniferous and broadleaf forests at three representative sites. Most detected OTUs belonged to Clade B, with only a few assigned to Clade A2 and none to Clade A1. Comammox Nitrospira were present across all samples, with amoA gene abundances ranging from 3.14 × 10 8 to 14.9 × 10 8 copies g −1 dry soil, accounting for 21.9%-56.6% of the total amoA gene copies. Their abundance decreased with increasing latitude and was higher in broadleaf than in coniferous soils. Comammox Nitrospira and AOA were numerically dominant over AOB in all soil samples. The community composition of comammox Nitrospira exhibited significant site-specific characteristics with the northernmost Zhenjiang site distinguished from the two southern sites, and was significantly related to soil p H and mean annual temperature ( p < 0.01), while forest type had a weak and non-significant effect. Alpha diversity was positively associated with soil water content ( p < 0.05). Comammox Nitrospira abundance was negatively correlated with SOC but positively correlated with mean annual precipitation and temperature ( p < 0.01). This study addresses the knowledge gap of comammox Nitrospira in subtropical forests and improves our understanding of niche differentiation among nitrifying microbial communities in forest ecosystems.