Integrated water–energy–nitrogen performance under nitrogen and biochar treatments in shallow groundwater tomato production

Abstract

Efficient management of water, energy, and nitrogen is essential for sustaining irrigated crop production in arid regions. However, under hydrologically buffered conditions, similar crop yields may conceal substantial differences in input demand and resource productivity. This study evaluated the integrated water-energy-nitrogen performance of tomato cultivated in clay loam soil under shallow groundwater influence and drip irrigation managed at a uniform soil moisture level. A two-season field experiment compared three N-rates: 431, 345, and 215 kg N ha -1 , corresponding to 100%, 80%, and 50% of the full N-rate, with and without biochar application. Fresh yield did not differ significantly among treatments (p>0.05) and ranged from 81.4 to 108.6 t ha -1 in pooled treatment means. Water productivity showed a significant Season × N-rate interaction (F 2,8.52 = 5.50, p=0.029), although no Tukey-adjusted pairwise comparison among N-rates was significant within either season. Irrigation energy ranged from 1.27 to 1.83 GJ ha -1 and was not significantly affected by season, biochar, N-rate, or their interactions (p>0.146). Fertilizer-associated N energy was 26.14, 20.92, and 13.08 GJ ha -1 under the 100%, 80%, and 50% N-rates, respectively. This corresponds to reductions of ~20% and 50% relative to the full N-rate. Fruit quality responses were governed mainly by season, with management effects restricted to specific variables and treatment contexts. The 80% N-rate included the highest pooled-yield treatment, W8 at 108.6 t ha -1 , while reducing fertilizer-associated energy by approximately 20%. The 50% N-rate produced the lowest total accounted energy, ranging from 14.43 to 14.91 GJ ha -1 , and the highest numerical input-productivity values; B5 achieved a water productivity of 39.59 kg m -3 , an energy productivity of 6.87 kg MJ -1 , and partial factor productivity of N of 459.09 kg yield kg -1 N. These findings show that limited yield differentiation did not imply equivalent management performance. Under the shallow-groundwater conditions evaluated, the 80% N-rate represented a candidate production-input compromise, whereas the 50% N-rate prioritized lower energy demand and higher numerical input productivity. Integrated indicators therefore provided management information that was not evident from yield alone, without identifying a statistically established universal optimum.