Preferential Flow Drives Coupled Nitrogen Migration and Transformation in the Vadose Zone: Microbial Hotspots and...

Preferential flow (Pf) in agricultural vadose zones poses environmental risks by accelerating transport of contaminants to groundwater while simultaneously functioning as biogeochemical hotspots. However, current research often simplifies vadose zone processes, overlooking the intricate coupling among pore structure, hydrological flow, and microbial communities in shaping nitrogen cycling. Field investigations in Northeast China identified significant differences between Pf zones and the soil matrix zones in terms of physical-chemical properties and molecular biological characteristics: Pf zones exhibited 26.4% higher organic/total nitrogen but 24.4-38.0% lower inorganic nitrogen species compared to matrix zones, along with an average 5.7-fold increase in nitrification gene abundance (amoABC and hao). High hydraulic gradients induced by Pf enhanced oxygen/nutrient transport, creating spatially heterogeneous redox gradients that restructured microbial communities. This interplay between hydrologic forcing and microbial activity established Pf zones as critical nitrogen transformation hotspots characterized by diverse functional potentials shaped by spatially variable environments. Our findings provide a mechanistic framework to refine nitrogen cycle models in the vadose zone and inform strategies to mitigate agricultural nitrogen loss through microbial metabolic control.