Objectives The Nutrient Expert (NE) system is thought feasible and practical in fertilizer recommendation as it does not need soil or plant test. We verified the effects of the NE system in melon production through yield and quality effect and soil N behavior in the ecologically fragile area.

Methods Melon field experiments were carried out in Guazhou County, Gansu Province in 2017 and 2018, using melon cultivar Loulan17 as materials. On the base of NE recommended N rate of 300 kg/hm² (N300), the N application rate treatments were set up by ± 25% (N225, N375) and ± 50% (N150 and N450). At harvest, melon yield, quality, above-ground N uptake and fertilizer N use efficiencies were measured. Soil samples were collected at 0–100 and 100–200 cm depth, and NO₃^–-N content were determined, at last the total nitrogen loss were calculated.

Results In the both year 2017 and 2018, the melon yield was highest in N300 treatment recommended by NE, with an average increase of 24.7% compared with N0 treatment for two years. When N rate was <300 kg/hm², the melon yield increased with the N rate increased; when N rate was > 300 kg/hm², melon yield didn’t continue increasing with N rate. The melon quality and commodity rate was the best at N application rate of 300 kg/hm², and insufficient or excessive N application was not conducive to the formation of quality. N use efficiency and internal efficiency of N decreased with the N application, but there was no significant difference between N225 and N300. N harvest index was highest in N300. The accumulation of nitrate N in 0–200 cm soil layer increased with the N application, the highest soil NO₃^--N content were observed in 0–100 cm soil layers, account for 43.9%–55.3% of 0–200 cm layer in 2017, while in 2018, the highest soil NO₃^--N content was observed at 100–200 cm soil layers, account for 44.8%–69.9% of 0–200 cm layer. Soil NO₃^--N accumulation and N apparent loss were increased with the N application rates , and melon N uptakes, NO₃^--N accumulation and N apparent loss accounted for 33.2%, 33.1% and 42.1% respectively. Multi-curve analysis of melon yield, N uptake and nitrate N residue showed that the highest N uptake rate of N application was 323 kg/hm² and maximum yield of N application was 293 kg/hm². The yield increased by 886.5 kg/hm² or 2.1% and nitrate N increased by 8.5 kg/hm² or 37.6% with the increase of N application rate per 30 kg/hm².

Conclusions The NE recommended N rate of 300 kg/hm² has achieved the highest yield and quality of melon and the highest nitrogen harvest index. With N rate of 300 kg/hm², the NO₃^--N accumulation in 0–100 cm and 100–200 cm soil layers are significantly lower the higher N rates, and not significantly higher than the lower N rates. Higher than N rate of 300 kg/hm² will only increase the biomass of veins and leaves, and decrease the yield and quality of melon, and increase the N loss significantly. The satisfactory yield, quality and environmental outcomes indicate the feasible and practical effect of the NE system in fertilizer recommendation for melon production in Gansu Oasis.