Objectives We conducted a high-abundance ¹⁵N isotope microplot experiment to quantitatively investigate the fate of fertilizer N through crop uptake, soil residue, and losses under varying N application rates in a plastic-mulched and drip-irrigated watermelon production system in the arid region of Northwest China. Soil apparent N balance was calculated to provide a scientific basis for optimizing N management and reducing environmental emissions in regional watermelon production.

Methods A field experiment was carried out at the Zhangye Water-Saving Experiment Station of the Gansu Academy of Agricultural Sciences. Four N application rate treatments were established: N0 (0 kg/hm²), N100 (100 kg/hm²), N200 (200 kg/hm²), and N300 (300 kg/hm²). Within each plot, a 0.8 m×2 m=1.6 m² microplot was designated and applied with urea enriched to 10% ¹⁵N abundance (46% N). During the watermelon growing season, water samples from rainfall and irrigation events were collected to determine N content. At harvest, all eight watermelon plants within each microplot were sampled, separated into stem, leaf, and fruit samples for analysis of dry biomass, N content, ¹⁵N abundance, and fruit soluble solids content. Concurrently, soil samples were collected from the 0–100 cm profile at 20 cm intervals to measure ¹⁵N abundance and calculate residual fertilizer-N in each layer. N loss was estimated as the difference between the applied N rate and the sum of plant N uptake and soil N residue.

Results N application rate significantly affected watermelon yield, quality, and N use efficiency (NUE). Both yield and soluble solids content initially increased and then decreased with rising N rates, peaking at the N200 treatment; this response was particularly pronounced in marginal fruit sweetness. Plant dry matter and N accumulation increased continuously with higher N inputs. However, NUE and the N harvest index declined with increasing N rates, indicating that excessive N application hindered N translocation to the fruits. Of the total N absorbed by watermelon, 60.64%–74.30% originated from the soil. Regarding the applied fertilizer-N, 11.63%–22.79% was taken up by the crop in the current season, 41.99%–59.06% remained in the soil, and 28.84%–35.22% was lost. Residual N was primarily concentrated in the 0–40 cm soil layer, and both residual and loss amounts increased with higher N application rates. Soil N balance analysis revealed that the system shifted from N deficit to surplus when the application rate exceeded 103 kg·hm², thereby elevating the risk of environmental pollution.

Conclusion Over 60% of the N absorbed by watermelon is derived from soil reserves. As N rates increased from 100 to 300 kg/hm², soil-derived N uptake remained relatively stable, while fertilizer-N uptake increased by less than 10%. The additional fertilizer N was primarily allocated to increasing plant biomass and vegetative N accumulation, consequently lowering the NHI. The fate of applied N is distributed as follows: 11.63%–22.79% is taken up by watermelon in the current season, 41.99%–59.06% reside in the soil, and 28.84%–35.22% is lost. Soil-residual N was mainly retained in the 0–40 cm layer and remains potentially available for subsequent crops. While the soil N balance threshold for the current season was 103 kg/hm², considering both the agronomic benefits and the role of fertilization in replenishing soil N pools, the recommended optimal N rate for mulched drip-irrigated watermelon in the Hexi Corridor arid region should not exceed 200 kg/hm².