Optimizing Management of Alfalfa (Medicago sativa L.) Nitrogen Fertilizer Based on Critical Nitrogen Concentration Dilution Curve Model

Abstract

The critical nitrogen dilution curve (CNDC) model enables precise nitrogen management by quantifying the threshold of nitrogen deficiency in crops, thereby enhancing both crop productivity and nitrogen use efficiency. However, its applicability to perennial crops remains unclear. In this study, alfalfa (Medicago sativa L.), a perennial leguminous forage, was used as the model crop. Based on two years of field experiments, CNDC models of aboveground biomass were constructed under two nitrogen fertilizer regimes: urea (0, 80, 160, and 240 kg·ha^-1, applied in a 6:2:2 basal-to-topdressing ratio) and controlled-release urea (CRU; 0, 80, 160, and 240 kg·ha^-1, applied as a single basal dose). Using these models, the nitrogen nutrition index (NNI) and cumulative nitrogen deficit (N_and) models were developed to diagnose alfalfa nitrogen status, and the optimal nitrogen application rates were determined via regression analysis. The results showed that critical nitrogen concentration and aboveground biomass followed a power function relationship under both fertilizer types. For CRU treatments, parameters a and b were 3.41 and 0.20 (first cut), 3.15 and 0.12 (second cut), and 2.24 and 0.40 (third cut), respectively. For urea treatments, a and b were 3.13 and 0.35 (first cut), 2.21 and 0.16 (second cut), and 1.75 and 0.73 (third cut). The normalized root mean square error (n-RMSE) of the models ranged from 3.1% to 13%, indicating high model reliability. Based on the NNI, N_and, and yield response models, the optimal nitrogen application rates were 175.44~181.71 kg·ha^-1 for urea and 145.63~153.46 kg·ha^-1 for CRU, corresponding to theoretical maximum yields of 14.76~17.40 t·ha^-1 and 16.76~20.66 t·ha^-1, respectively. Compared to urea, CRU reduced nitrogen input by 18.41~20.47% while achieving equivalent or higher theoretical yields. This study provides a scientific basis for nitrogen status diagnosis and precision nitrogen application in alfalfa cultivation.

Keywords: alfalfa; critical nitrogen concentration; nitrogen application rates; nitrogen nutrition diagnostics.

Figure 1

The dynamic accumulation of aboveground biomass of alfalfa. (a–c) show changes in aboveground biomass of alfalfa plants with urea application; (d–f) show changes in the aboveground biomass of alfalfa plants with CRU application.

Figure 2

Changes in nitrogen concentration in alfalfa plants at different stages of growth. (a–c) show the changes in nitrogen concentration of alfalfa plants with urea application; (d–f) show the changes in nitrogen concentration of alfalfa plants with CRU application.

Figure 2

Changes in nitrogen concentration in alfalfa plants at different stages of growth. (a–c) show the changes in nitrogen concentration of alfalfa plants with urea application; (d–f) show the changes in nitrogen concentration of alfalfa plants with CRU application.

Figure 3

Nitrogen concentration dilution curves in aboveground biomass of alfalfa.

Figure 4

The 1:1 relationship diagram of simulated and observed values of critical N concentration.

Figure 5

Dynamic changes in the NNI of alfalfa plants under different nitrogen fertilizer management. (a–c) show the changes in NNI of alfalfa plants with urea application; (d–f) show the changes in NNI of alfalfa plants with CRU application.

Figure 6

The relationship between NNI and RY of alfalfa.

Figure 7

The dynamic changes in N_and of alfalfa plants. (a–c) show the changes in N_and of alfalfa plants with urea application; (d–f) show the changes in N_and of alfalfa plants with CRU application.

Figure 8

Relationship between N_and and RY of alfalfa.

Figure 9

Daily precipitation and air temperature during the alfalfa growing period.