Agricultural practices and ditch size drive microbial community assembly and mediate N- and P-transformation in multistage drainage networks of paddy fields: Insights from a large-scale irrigation district in eastern China

Agricultural drainage ditches (ADDs) are ubiquitous and regarded as active zones for biogeochemical reactions and microbe-mediated pollutant removal. However, little is known about the microbial distribution and community assembly in ADDs. Here, a typical large-scale irrigation district, including five orders of farmland drainage systems (namely field, sublateral, head, branch, and trunk ditches that could efficiently remove excess water from paddy fields to downstream water bodies), was selected to investigate the ecological processes of microbial communities and N- and P-transformation processes in multistage ditches. We found that scale effects drove distinct environmental gradients and microbial community dissimilarities and that the five ordered ditches were grouped into three clusters (field vs. sublateral vs. head, branch, and trunk ditches). Specifically, the microbial communities in the field ditches located adjacent to the paddy fields were strongly selected by agricultural fertilization and irrigation drainage, enriching salt-tolerant microbes with high nitrification and inorganic P solubilization capabilities. In comparison, the sublateral ditches showed the highest removal performance for total nitrogen (13.28-55.80%) and total phosphorus (9.06-65.07%) during the growth of rice, which was mainly attributed to the enrichment of versatile microbiota (e.g., C39, Nitrospira, and Novosphingobium) as a result of the increased stochastic processes driven by the low redox potential. Notably, the specific gene (i.e., hzsB) for anaerobic ammonium oxidation in sublateral ditches was 1-2 orders of magnitude higher than in adjacent ditches, further contributing to N loss. As field water was discharged into the large-sized head, branch, and trunk ditches, the nutrient levels decreased sharply. At the same time, deterministic processes gained more importance (∼82%), leading to the flourishing of Synechococcus and increasing the potential risk of eutrophication. Overall, the microbial communities in multistage ADDs were co-shaped by agricultural practices and ditch size, which further governed the N and P removal performance. These results provide unique insights into microbiota assembly patterns and dynamics in multistage ADDs and important ecological knowledge for controlling agricultural non-point source pollutants by managing of small-sized ditches.