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.

Sowing mixed artificial grassland can aid grassland husbandry and restoration. However, the optimal mix and sowing mode for forage grass and legume grass in Horqin Sandy Land, China, remains unidentified. In this study, grassland production performance and stability were investigated using three mixed-sowing modes (peer-mixed, cross-mixed, and intercropping) and five mixed sowing ratios (awnless brome: alfalfa at 1:0, 1:1, 3:2, 2:1, 5:2, 3:1, 0:1). The cross-mixed sowing mode with a 2:1 ratio had the highest yields and greatest total nitrogen accumulation. All treatments had a land equivalent ratio ≥ 1, and C3 (cross-mixed at a 2:1 ratio, 2021 and 2022) had the largest value. The C3 treatment had the highest LERN, which was not significantly different from those of C1 (cross-mixed at a 1:1 ratio), C2 (cross-mixed at a 3:2 ratio), I2 (intercropping at a 3:2 ratio), and I3 (intercropping at a 2:1 ratio) treatments in 2021 and 2022. The sustainable yield index of C3 was significantly higher than those of the other treatments, excluding C2 (P < 0.05). TOPSIS evaluation indicated that cross-mixing at a 2:1 ratio is ideal and represents a crucial intercropping technique for establishing efficient and sustainable grassland ecosystems, which could further facilitate animal husbandry development.

Transforming land use patterns prevents and controls desertification. In the Horqin Sandy Land, we evaluated the soil restoration effects of planting corn (from 2014 to 2018) on previously uncultivated land (in 2013), followed by the transition to alfalfa cultivation under five nitrogen application levels (from 2019 to 2023). After corn cultivation, the soil available nitrogen (AN), C/N ratio, C/P ratio, and N/P ratio decreased by 39.02%, 7.14%, 21.35%, and 12.83%, respectively, compared to those of uncultivated land. However, following the planting of alfalfa, especially in 2023, the bulk density values were the lowest, while the AN, soil organic carbon, total nitrogen, and total phosphorus values were the highest. An AHP-fuzzy comprehensive evaluation showed that the available phosphorus (AP), SOC, C/N, C/P, and N/P had significant weights of 0.12, 0.128, 0.133, and 0.128, respectively, and thus were key soil quality determinants. The soil quality assessment values for the N1 and N2 treatments were the highest at 0.208 and 0.202, respectively. Conclusively, the intensive cultivation of alfalfa under 51.75 and 103.5 kg/ha N improves soil fertility. This study provides theoretical support for the restoration of desertified soils in the Horqin Sandy Land.

The incorporation of alfalfa into grass systems reduces reliance on nitrogen fertilizer application. Over two consecutive years, we investigated the regulation of carbon and nitrogen metabolism in grasses and productivity enhancement under four nitrogen application rates (0, 105, 210, and 315 kg·ha⁻¹) and five alfalfa incorporation levels (0%, 10%, 20%, 30%, and 40%); incorporation (%) refers strictly to seeding proportion (% of the monoculture seeding rate). Within the range of 20–30% alfalfa incorporation and 105–210 kg·ha⁻¹ nitrogen application, key physiological and biochemical parameters, except the net photosynthetic rate (P_n), reached their peak values compared to the N0A0 (no nitrogen and no alfalfa) treatment. Transpiration rate (T_r), intercellular CO₂ concentration (C_i), and stomatal conductance (G_s) increased by 43.64%, 40%, and 48.09%, respectively. P_n peaked under the N2A0 treatment (210 kg·ha⁻¹ nitrogen application and no alfalfa), increased by 65.63%. Nitrate reductase (NR), glutamine synthetase (GS), and ribulose-1,5-bisphosphate carboxylase (RuBisCO) activity increased by 154.60%, 112.39%, and 199.19%, respectively. Total sugar (TS) and protein production (YCP) increased by 122.22% and 145.17%, respectively. The entropy-weighted TOPSIS evaluation based on multi-objective assessment showed that the combination of 20% alfalfa incorporation with 105 kg N·ha⁻¹ application is an efficient model for enhancing forage productivity in the Horqin Sandy Land.

The effect of nitrogen (N) (0, 75, 150, 225 and 300 kg N ha⁻¹, N0-N4) on the yield, photosynthetic properties and nitrogen-use efficiency of forage oat (Avena sativa L.) was studied in the Horqin Sandy Land in northern China. Mantel tests and chord diagrams were used to investigate the correlation of traits. Plant height, stem–leaf ratio, hay production, chlorophyll content and heading stage photosynthesis indices showed a unimodal response with highest values at N3 (225 kg N ha⁻¹; p < 0.05). The dry matter yield at filling stage, net photosynthetic rate (Pn) at heading stage and total chlorophyll content (Total Chl) of N3 were 74.52%, 101.65%, and 62.18% higher than that of N0. The nitrogen contribution ratio (NFCR) of N3 reached 42.70%, which was 60.59% greater than that of N1. Nitrogen uptake efficiency (NUpE) was achieved at N3 that was 21.50% relative to N1.The N3 treatment showed the best comprehensive performance across agronomic, photosynthetic and nitrogen-use efficiency traits. In contrast, N3 achieved a favorable balance between yield and nitrogen use efficiency and can be regarded as a suitable nitrogen application rate for high-yield forage oat production in this region.