Interaction of Nitrogen, Phosphorus, and Potassium Fertilisation and Precipitation on the Nitrogen Use Efficiency of Rainfed Grass

Nitrogen (N) fertilisers should be utilised as efficiently as possible. In addition to N fertiliser doses, nitrogen use efficiency (NUE) is influenced by other factors. The effects of four different N, phosphorus (P), and potassium (K) supply levels (0–3) and rainfall periods (dry, normal, wet) were investigated on NUE indices in six selected years of a field experiment. Generally, rainfall and N had the strongest effects. N × rainfall supply interaction increased biomass production from 0.915 t ha−1 (N0, dry) to 10.4 t ha−1 (N3, wet). The N balance varied between −47.3 kg ha−1 (N0, wet) and 218 kg ha−1 (N3, dry). The N uptake per unit N of fertiliser (FNRE) was mainly determined by the P × rainfall interaction, varying between 26.13% (P0, dry) and 83.73% (P3, wet). Both the biomass increment per unit N of fertiliser (agronomic efficiency—AE) and the biomass production per unit N uptake (internal efficiency—IE) were mainly influenced by the N × rainfall interaction, with AE decreasing from 43.7 kg kg−1 (N1, wet) to 10.6 kg kg−1 (N3, dry) and IE from 114 kg kg−1 (N0, normal) to 45.9 kg kg−1 (N3, wet). Both P and, to a lesser extent, K had a significant positive effect on these indices. The N dose of 200 kg ha−1 year−1, the P2O5 supply of 153 mg kg−1, and the K2O supply of 279 mg kg−1 proved to be optimal in terms of NUE indices.