Research Progress on the Effects of Alfalfa on Farmland Runoff Nitrogen in the Arid Northwest Region of China

Water scarcity in the arid northwest region of China, coupled with agricultural non-point source pollution, particularly nitrogen loss via farmland runoff, has become a critical issue threatening regional water environment security. Compared to traditional engineering measures, ecological interception technologies utilizing plant-microbe combined systems show great potential. Alfalfa (Medicago sativa L.), as an important leguminous forage, possesses unique advantages in controlling nitrogen loss due to its strong nitrogen fixation capacity and ecological adaptability. This review aims to systematically elaborate the core mediating role of soil microorganisms in alfalfa's regulation of the farmland nitrogen cycle, with a focus on analyzing how alfalfa achieves dual control effects-"increasing source" and "reducing flow"-on farmland runoff nitrogen in the arid northwest region through biological nitrogen fixation, stimulation of rhizosphere nitrogen transformation, and construction of microbial ecological networks. Finally, the review summarizes the limitations of current research and provides prospects for future research directions, intending to offer a theoretical basis for managing agricultural non-point source pollution and achieving green sustainable development in the arid northwest region.