Barcelona rooftop tomato study tracks how light and nutrients affect yields

A newly reported study of a rooftop greenhouse at the Universitat Autònoma de Barcelona found that maintaining sunlight through its covering and adjusting nutrients helped improve tomato yields across eight years of cultivation. The university-provided report was published on October 7, while the underlying research appeared in Agronomy for Sustainable Development in June. Its findings offer practical evidence for urban growers, with important limits: the researchers studied one evolving production system, chiefly at one Barcelona site.

The researchers examined 11 hydroponic tomato crop cycles from 2015 to 2023, mostly in a 125-square-metre rooftop greenhouse thermally integrated with a university building. One cycle grew tomatoes indoors under artificial light. Crop varieties, irrigation, nutrient recipes and equipment changed during the study, so comparisons between years reflect a sequence of operating decisions rather than controlled tests that isolate every individual change.

How greenhouse light affected Barcelona tomato yields

The study reports an overall 31.2% decline in yield over eight years as the greenhouse covering let through less sunlight. Some polycarbonate sheets were replaced in 2023 after hail damage. Measured light transmission then rose to 46.4%, and yield in the researchers’ comparison of Arawak tomato cycles increased 56.5% from 2022 to 2023. Those changes occurred within a system whose other operating conditions also evolved.

A sensitivity analysis considered replacing the covering every four to six years instead of following a manufacturer’s 10-year recommendation. Against the 10-year scenario, the researchers estimated productivity gains of 19.4% to 31.8% and a 6.7% to 7.7% reduction in modeled global-warming impact. More frequent replacement also raised modeled ecotoxicity impact because the covering itself contributes to that measure. The proposed interval is a scenario from this greenhouse, not a replacement schedule tested across multiple sites.

What recovered fertilizer changed in the study

Dose mattered. An earlier 2022 cycle using 140 grams of struvite per plant produced low yields; the authors identified excessive application and nutrient imbalance as likely contributors. In their comparison of 2023 Arawak cycles, the modeled marine-eutrophication impact for the struvite cycle was 16.4 times lower than for the mineral-fertilizer cycle. The authors found that leachate emissions accounted for 4.5% of that impact in the struvite cycle. These figures describe the study’s specific systems and accounting methods.

Water management offered another adjustment. In a 2020 cycle, the greenhouse reused 4,650 litres of drainage water, or 31% of total water inputs. The paper calculated that recirculation raised water-use efficiency from 25.4 to 36.8 grams of tomatoes per litre. That result helps show why the authors evaluated irrigation alongside fertilizer and light: changes to one input did not tell the whole story of crop performance.

Why indoor lighting changed the environmental calculation

The study’s life-cycle assessment estimated global-warming impacts of 0.54 kilograms of carbon-dioxide equivalent per kilogram of tomatoes for a 2017 rooftop cycle and 0.94 kilograms for a 2023 cycle. Its one indoor cycle was estimated at 7.06 kilograms per kilogram of tomatoes, with electricity responsible for 93.1% of that impact. These are modeled impacts within the researchers’ defined system boundaries, not direct measurements of every environmental effect.

The indoor comparison was especially limited: it included 18 plants, and the researchers lacked data for energy used to keep the room at stable conditions. They modeled a separate scenario in which all electricity came from solar photovoltaic power; that reduced the indoor estimate to 0.93 kilograms of carbon-dioxide equivalent per kilogram of tomatoes. The lower figure is a projection, not an observed result from an indoor farm powered entirely by solar energy.

Artificial light also presented a trade-off inside the greenhouse. Supplemental LEDs increased yield per plant by 41.6% in one comparison cycle, while modeled global-warming impact rose by 132.5%. Energy use from a nebulization system increased impacts in some 2023 comparisons. Higher output, therefore, did not necessarily mean a lower impact per kilogram under the operating conditions the authors assessed.

How far the Barcelona results can be applied

The researchers say the results may differ in other climates and locations. Some crop cycles had limited samples or repetitions, and the study did not measure every possible influence on yield, including labour, pruning and pollination. Building shadows, varieties, growing practices and electricity sources could also change outcomes elsewhere. Further testing across locations would be needed before treating the Barcelona figures as typical of rooftop or indoor farming generally.

For growers, the study identifies several decisions worth testing in their own systems: monitoring how much light a covering transmits, matching nutrient dose and timing to the crop, and counting the energy used by lighting or climate equipment. Its strongest conclusion is specific to the studied greenhouse: the reported harvests and modeled impacts emerged from successive adjustments over years, with benefits and costs that depended on the production setup.