Background: Alfalfa (Medicago sativa L.) is a vital forage crop, but optimizing nitrogen (N) application remains crucial for maximizing yield and minimizing environmental impact, especially under variable climatic conditions. Precipitation regimes significantly influence N dynamics and plant growth, necessitating tailored N management strategies. Agricultural systems models, such as APSIM (Agricultural Production Systems sIMulator), offer powerful tools for simulating complex crop-soil-weather interactions to inform management decisions.

Objective: This study aimed to utilize the APSIM model to determine optimal nitrogen application rates for alfalfa yield across varying precipitation regimes (wet, normal, and dry years).

Methods: The APSIM model was calibrated and validated using observed alfalfa growth, yield, and soil N data from a representative agricultural region. Historical weather data were analyzed to define distinct wet, normal, and dry year precipitation scenarios. Subsequently, a range of N application rates (0 to 250 kg N ha$^{-1}$) were simulated for alfalfa under each precipitation regime. Key output variables included alfalfa hay yield, water use efficiency (WUE), and nitrogen use efficiency (NUE). Statistical analyses were performed to identify optimal N rates for each scenario.

Results: The APSIM model demonstrated robust performance in simulating alfalfa yield and N uptake (R2 > 0.85). Simulation results indicated that optimal N application rates varied significantly with precipitation. In wet years, higher N rates (e.g., 150 kg N ha$^{-1})maximizedyield,whilenormalyearsrequiredmoderaterates(e.g.,100kgNha^{-1}).DryyearsshoweddiminishingreturnsorevennegativeimpactswithincreasingN,suggestingloweroptimalrates(e.g.,50kgNha^{-1}$) or even reliance on biological N fixation. These varying optimal rates also influenced WUE and NUE, with more efficient resource use observed when N application aligned with water availability.

Conclusion: The APSIM model provides a valuable framework for optimizing N application in alfalfa production. Tailoring N management based on anticipated precipitation regimes can significantly enhance alfalfa yield, improve resource use efficiency, and promote more sustainable agricultural practices. These findings underscore the importance of climate-adaptive nutrient management for future forage production.  

APSIM model, Alfalfa, Nitrogen optimization

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