Dynamic Changes in Carbon and Nitrogen Storage and Sequestration of Alfalfa Pastureland in Different Planting Years Under Temperate Continental Arid Climate Conditions

Alfalfa (<i>Medicago sativa</i> L.), a drought-tolerant legume, significantly influences carbon and nitrogen cycling in arid and semi-arid regions. This study investigated carbon and nitrogen storage and sequestration dynamics in alfalfa pastureland cultivated for 2-7 years under temperate continental arid climate conditions (110-190 mm annual precipitation). Overall, the biomass, carbon and nitrogen sequestration in alfalfa pasture, and carbon and nitrogen storage and sequestration in soil exhibited a quadratic pattern with planting years. The above-ground biomass peaked at 19.28 t·hm^-2, with carbon and nitrogen sequestration reaching the highest level at 10.18 t·hm^-2 and 0.511 t·hm^-2, respectively, in year 5. Both annual carbon and nitrogen sequestration of the below-ground vegetation exhibited an increase, reaching a peak before decreasing with planting year, and from Y3 to Y7, the sequestration values were consistently higher than those in Y2. Soil carbon and nitrogen sequestration peaked in year 3. Compared to the adjacent fallow lands, alfalfa pasturelands maintained positive soil carbon sequestration until year 6 but became negative (-8.03 t·hm^-2) by year 7. From years 2-6, alfalfa pasture fixed carbon and nitrogen at comparable rates but returned disproportionately less carbon than nitrogen to the soil. To optimize sustainability, we recommend (1) rotating alfalfa after 6 years to prevent soil nutrient depletion and (2) applying carbon-rich fertilizers post-year 3 to balance nutrients and prolong productivity in arid climates.