Seasonal and spatial dynamics of soil nitrogen cycling microbial communities in an agricultural riparian ecosystem

The riparian zone, located between agricultural region and aquatic environments, is a hotspot for microbe-driven nitrogen (N) reduction. However, the impact of seasonal and spatial variations on the N-cycling microbial community and the underlying mechanisms in riparian soils remain poorly understood. This study took the riparian zone of the North Canal in Hebei Province, China, as an example. Four sampling profiles (A, B, C, and D) were established at 1, 20, 40, and 50 m from the water body, respectively. Each profile was divided into 0-20, 20-40, 40-60, and 60-80 cm layers, and soil properties and microorganisms were investigated to reveal the seasonal and spatial dynamics of microbial N-cycling communities and denitrification potential (DP). The most pronounced disparities were observed across soil profiles, with significant variations in the abundance of most N-cycling genes and DP. Both rivers and agricultural fields affect neighbouring riparian zones, leading to a decline in soil DP. Moreover, the waterside profile A displayed the most distinct N-cycling microbial composition, along with significantly higher network complexity and stronger deterministic assembly processes. Vertical stratification analysis revealed that N-cycling microbial diversity and DP decreased significantly with soil depth, with topsoil layers exhibiting greater network complexity and stronger deterministic processes. Seasonal comparisons showed that microbial diversity, network complexity, and deterministic processes were markedly enhanced during the wet season. Multiple physicochemical factors collectively regulated N-cycling community and DP, with soil nitrate and moisture content emerging as the most influential drivers. These findings enhance our mechanistic understanding of microbial N removal processes in riparian ecosystems.