Soil organic carbon (SOC) is a key indicator of soil quality and an important component of the global carbon cycle. Enhancing SOC through crop rotation is a promising strategy; yet, the underlying mechanisms for SOC accumulation remain unclear. This study aimed to evaluate the effects of different pasture age, pasture species, irrigation, and nitrogen (N) fertilization treatments on SOC content and storage in pastureland, analyzing the SOC content and below-ground biomass (BGB) data of different soil layers (0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm) of each treatment under three factors (pasture species (Bromus inermis, Medicago varia, the 1:1 mixture), irrigation (CK, dry-season supplementation), and N fertilization (0 kg N hm⁻² y⁻¹, 75 kg N hm⁻² y⁻¹, and 150 kg N hm⁻² y⁻¹)), as well as the interaction effects of these factors. Pasture species, water and N addition levels, and pasture age all had significant (p < 0.05) effects on BGB. At the age of 1–3, the SOC content of monocultured Bromus inermis was slightly higher than the monocultured Medicago varia and the mixture, and at the age of 4–5, monocultured Medicago varia and the mixture were slightly higher than the monocultured Bromus inermis. Among them, the mixture was the highest. At the age of 2–5, the BGB of pastureland was significantly influenced by pasture species, N and water addition, and pasture age. Over a 5-year period, SOCs in the surface layer of the fallowed cropland accumulated 32.35 Mg ha⁻¹, showing a very good carbon sequestration effect; especially the planting of a mixed pasture had a more significant positive effect on the accumulation of SOC. Therefore, for the low and medium yielding fields in China, according to the crop utilization target and production cycle, the purpose of improving soil quality can be effectively achieved through crop and grass rotation.

The degradation of black soil cropland has occurred to varying degrees in the northern agropastoral ecotone. Crop–forage rotation is an effective way to improve soil quality, with Medicago being the preferred perennial legume. The C, N, and P stoichiometric ratios are key indicators of soil quality and organic matter composition, reflecting the status of the internal C, N, and P cycles in soil. This study aims to investigate the ecological stoichiometric ratios of Medicago grassland soils with different planting durations, explore the regulatory effects of nitrogen fertilizer on soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) content, and assess the impacts of these changes on the Medicago grassland ecosystem. This study was conducted on the long-term cultivated grassland core experimental platform of the Hulunber National Field Station. Based on forage yield and soil nutrient measurements, field-based observations and laboratory analyses were carried out. Medicago × varia was the study subject, with different nitrogen fertilizer treatments: CK (0 kg N ha⁻¹), N₇₅ (75 kg N ha⁻¹), and N₁₅₀ (150 kg N ha⁻¹). A randomized block design was adopted. Variance analysis, boxplot statistics, and scatterplot fitting methods were used to examine soil properties and assess the effects of nitrogen application on the C, N, and P stoichiometry of soils in established perennial Medicago grasslands. The results indicate that, based on the growth characteristics of alfalfa, soil nutrient dynamics, and its effectiveness in improving soil quality, the optimal rotation period for alfalfa in the northern agropastoral ecotone is 4–5 years, but it can also be shortened to 3 years. Soil carbon, nitrogen, and phosphorus contents are significantly influenced by the planting duration. As the planting years increase, soil carbon and nitrogen contents first increase and then decrease, while soil phosphorus content initially decreases followed by a slight increase. Soil pH gradually rises with both planting years and soil depth. Both low and high levels of nitrogen fertilizer application reduce soil organic carbon concentration (by 0.40% and 10.14%, respectively). Low nitrogen fertilizer application increases soil nitrogen concentration (by 1.50%), whereas high nitrogen fertilizer application decreases it (by 7.6%). Both nitrogen levels increase soil phosphorus concentration (by 36.67% and 35.26%, respectively). For soil from an alfalfa grassland planted for 8 years, the carbon-to-nitrogen ratio ranges from 9.08 to 9.76, the carbon-to-phosphorus ratio from 13.00 to 151.32, and the nitrogen-to-phosphorus ratio from 1.65 to 17.14. In summary, alfalfa yield is primarily influenced by the nitrogen fertilizer application rate, planting duration, stoichiometric ratios, and pH. Nitrogen fertilizer application has a positive regulatory effect on soil stoichiometric ratios. The annual yield can reach 8.94 to 10.07 tons per hectare., but phosphorus remains a limiting factor. These findings provide crucial data for understanding the impact of ecological stoichiometry on crop–forage rotation cycles, as well as optimal land use and quality improvement.