Cutting Nitrogen Fertilizer by Half Barely Dented Tomato Yields, but Slashed Energy Use Dramatically

In the sun-scorched farmland of Egypt, where every drop of water and every kilogram of fertilizer carries both economic and environmental weight, a new two-season field experiment has revealed something quietly radical: tomato plants produced nearly identical yields whether farmers applied their full nitrogen ration or cut it in half. Yet the energy cost of producing those tomatoes differed enormously depending on how much nitrogen went into the soil. The findings, published in Frontiers in Agronomy, challenge a core assumption of modern irrigation agriculture—that yield alone is the best measure of how well a farming system performs.

The research team, led by Maha Elbana of Beni-Suef University in Egypt, alongside collaborators from the China Meteorological Administration, the National Authority for Remote Sensing and Space Sciences, and the International Center for Agricultural Research in the Dry Areas, conducted their experiment at the Arid Lands Agricultural Studies and Research Institute of Ain Shams University. Their goal was ambitious: to simultaneously track water use, energy demand, nitrogen efficiency, fruit quality, and crop yield under conditions where a shallow water table sits close enough to the root zone to buffer the soil’s moisture supply.

The experimental setup was carefully controlled. Tomato plants of the variety Rich Zoan K186 were grown in clay loam soil—composed of 34.4 percent clay, 35.9 percent silt, and 29.7 percent sand—across two growing seasons: Spring 2023 and Autumn 2023-2024. A pressurized drip irrigation system, governed by daily soil-moisture readings from time-domain reflectometry probes, ensured that every plot received water only when root-zone moisture dropped to 50 percent of available water. This uniform irrigation protocol meant that differences among treatments could not be attributed to water stress; instead, the researchers manipulated two factors: nitrogen rate and biochar application.

Six treatment combinations were tested. Three nitrogen rates—431, 345, and 215 kilograms of nitrogen per hectare, corresponding to 100, 80, and 50 percent of the local farmer practice—were crossed with two biochar levels: zero or 35 tonnes per hectare. The biochar, derived from corn cobs, was incorporated into the top 30 centimeters of soil one week before transplanting. Each plot was hydraulically isolated with trenches to minimize lateral water movement, and the experiment was arranged in a randomized complete block design with three replicates.

The most striking result was what did not happen. Marketable fresh yield showed no statistically significant differences among any of the six treatment combinations, ranging from 81.4 to 108.6 tonnes per hectare in pooled treatment means. In a hydrologically buffered system—where shallow groundwater and high soil-water storage in fine-textured soil maintain root-zone moisture near a defined threshold—the crop simply did not respond to changes in nitrogen input with the yield differences that conventional agronomy might predict. The researchers’ null hypothesis for yield was not rejected.

But water productivity told a more nuanced story. A significant Season × Nitrogen-rate interaction emerged (F2,8.52 = 5.50, p = 0.029), indicating that the response of water productivity to nitrogen input varied between the two growing seasons. However, when the researchers applied Tukey-adjusted pairwise comparisons within each season, no individual nitrogen rate proved statistically superior to another. The pattern suggests that seasonal environmental conditions—temperature, radiation, evaporative demand—modulated how efficiently the crop converted water into yield under different nitrogen regimes, but not in a way that consistently favored one rate over another.

The energy accounting revealed the experiment’s most consequential finding. Fertilizer-associated nitrogen energy—calculated using an embodied energy coefficient of 60.6 megajoules per kilogram of nitrogen, reflecting the intensive industrial process of mineral nitrogen production—dominated the total accounted energy budget. At the full nitrogen rate, fertilizer-associated energy reached 26,143 megajoules per hectare, compared with just 1,270 to 1,831 megajoules per hectare for irrigation pumping. In other words, the energy embedded in nitrogen fertilizer was roughly 7 to 20 times greater than the energy required to pump irrigation water. Cutting nitrogen to 80 percent reduced fertilizer-associated energy by approximately 20 percent; cutting it to 50 percent reduced it by half.

Total accounted energy—combining irrigation pumping and fertilizer-associated nitrogen energy—ranged from approximately 14,430 megajoules per hectare under the 50 percent nitrogen treatments to over 27,500 megajoules per hectare under the full rate. Because irrigation energy remained statistically unchanged across treatments (p > 0.146), the nitrogen rate was the primary determinant of the overall energy footprint. This finding carries significant implications for arid-region agriculture, where mineral nitrogen production represents a substantial share of greenhouse gas emissions and where reducing unnecessary fertilizer inputs simultaneously addresses soil acidification, nitrate leaching, and climate burdens.

The integrated performance profiles, visualized through directionally normalized radar plots and standardized heatmaps, exposed trade-offs that yield alone could never reveal. Treatment W8—no biochar, 80 percent nitrogen—produced the highest pooled numerical yield at 108,583 kilograms per hectare while reducing fertilizer-associated energy by roughly 20 percent compared with the full rate. Meanwhile, treatment B5—biochar, 50 percent nitrogen—achieved the highest pooled water productivity at 39.59 kilograms per cubic meter, the highest energy productivity at 6.87 kilograms per megajoule, and a partial factor productivity of nitrogen of 459.09 kilograms of yield per kilogram of nitrogen applied. These ratio-based indicators, the researchers caution, increase partly because of smaller denominators and should not be interpreted as proof of agronomic superiority on their own.

Fruit quality responses were governed primarily by season rather than by management. Fruit acidity, nitrogen concentration, hardness, and nitrate-nitrogen levels all showed significant seasonal effects, while biochar and nitrogen-rate effects were restricted to specific variables and treatment contexts. The only statistically significant adjusted pairwise difference among nitrogen rates occurred for fruit acidity in spring under biochar, where the full nitrogen rate produced higher acidity than the 80 percent rate. Reducing nitrogen input did not produce a generalized deterioration in fruit quality, supporting the notion that moderate nitrogen reduction can maintain both yield and fruit composition under well-managed drip irrigation.

The broader implications extend beyond a single field in Egypt. The study demonstrates that in hydrologically buffered systems—where shallow groundwater, fine soil texture, and sensor-based irrigation conspire to equalize water availability across treatments—yield becomes a blunt instrument for evaluating management performance. Integrated indicators of water productivity, energy productivity, and nitrogen-use efficiency reveal contrasts that yield conceals. The researchers emphasize that their energy accounting represents a partial operational boundary, excluding biochar production, irrigation infrastructure, and post-harvest operations, and that future work should quantify nitrogen uptake, expand the life-cycle boundary, and test whether these trade-offs persist across different soil textures and longer timeframes. For now, the message is clear: two fields that look identical by the harvest scale may be worlds apart in the energy they consume and the resources they demand.

Subject of Research: Integrated water, energy, and nitrogen performance of drip-irrigated tomato production under varying nitrogen rates and biochar application in shallow groundwater conditions

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

Article References: Elbana, M., Wang, L., El-Shirbeny, M. A., Gamal, R., Nangia, V., & Abou Hadid, A. F. (2026). Integrated water–energy–nitrogen performance under nitrogen and biochar treatments in shallow groundwater tomato production. Frontiers in Agronomy, 8, Article 1922454. https://doi.org/10.3389/fagro.2026.1922454

Image Credits: AI Generated

DOI: 10.3389/fagro.2026.1922454

Keywords: tomato, nitrogen fertilizer, biochar, drip irrigation, water productivity, energy productivity, shallow groundwater, arid agriculture, soil moisture, fertilizer energy, Egypt, sustainable farming