Productivity, soil fertility and enzyme activity of mixed forage grasslands improved by alfalfa and nitrogen addition in Horqin Sandy Land, China

Legume forage addition helps achieve nitrogen reduction, efficiency improvement, and sustainable development of artificial grasslands. This study employed the multi-criteria decision-making model TOPSIS to determine the optimal alfalfa addition amount and nitrogen application level in Horqin Sandy Land, China, by intercropping alfalfa with 2-month-old Bromus inermis, and using alfalfa addition single-sowing amounts of 0% (B0), 10% (B1), 20% (B2), 30% (B3), and 40% (B4), and nitrogen application levels of 0 (N0), 105 (N1), 210 (N2), and 315 kg hm^-2 (N3). The results indicate that the maximum enzyme activity was primarily observed in the treatments with 30% alfalfa amendment. Compared to the control (N0B0), the activities of urease, protease, alkaline phosphatase, and catalase increased by 66.7%, 74.1%, 22.8%, and 5.41%, respectively. The interaction between nitrogen application and alfalfa amendment significantly improve soil enzyme activities. Microbial biomass carbon and nitrogen contents were highest in the N3B3 treatment. Forage yield was highest in N3B2 (13,767.07 kg hm^-2). Comprehensive evaluation using the TOPSIS model revealed that the optimal mixture ratio and nitrogen application rate combination for artificial grassland establishment in Horqin Sandy Land was 30% alfalfa addition to B. inermis with a 210 kg hm^-2 nitrogen application rate.