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Cheng et al. Carbon Footprints 2024;3:10 https://dx.doi.org/10.20517/cf.2023.53 Page 3 of 13

(traditional sunlight greenhouse and artificial light plant factory). The sunlight greenhouse here refers to the
single-span, east-west oriented greenhouse that is widely used in China. The sunlight greenhouse is not
heated but fully relies on sunlight and outer thermal screens in the winter to ensure a suitable temperature
for plant growth. Agricultural machinery was excluded from the analysis to simplify the LCA models and
reduce the uncertainties.

The functional unit of the LCA model was defined as the production of 1 kg of fresh lettuce. The system
boundary was defined as the cradle-to-farm gate [Figure 1]. The cultivating process includes sowing,
seedling raising, planting (process 1), and management and harvesting (process 2). Inputs of raw materials,
electricity, water, fertilizers, and related nitrogen oxide emissions were considered. We excluded the climate
burdens of infrastructure construction and equipment maintenance of greenhouse and plant factory
cultivation from the scope of this study due to their long lifetime.

The traditional sunlight greenhouse is located in the middle and lower reaches of the Yangzi River, covering
2
an area of approximately 667 m and produces about 4 tons of lettuce annually by four rounds of
cultivation. Chemical and organic fertilizers are applied to meet the nutritional requirements of the lettuce
growth. The greenhouse depends on sunlight insulated by polyethylene film, without using artificial
illumination as a supplementary light source. The plant factory with artificial light is owned by Anhui
SANANBIO Biotechnology Co., Ltd, located in Luan City of Anhui Province. The plant factory has an area
2
of about 260,000 m and produces lettuce annually. The plant factory has a fully controllable environment,
using LED spectrum technology and nutrient solution con uration to precisely control the light, water, and
nutrients required for lettuce.

Notably, the two cultivation systems are with such disproportionate production areas. This is because a
sunlight greenhouse can be operated individually per square foot, while a plant factory is a more
collaborative system, where multiple factors such as power control system and nutrient delivery system,
etc., need to be considered collectively as an entire system. All inputs of the two disproportionate cultivation
systems were standardized into the same functional unit to produce 1 kg of fresh lettuce for comparison
purposes.

Life cycle inventory
We built the LCA model using OpenLCA software. The background data mainly came from the dataset of
ecoinvent, facing challenges. Thus, when building flows in each process, we followed the principles below.
Firstly, flows of materials, energy, and auxiliary were directly used if they were available in the background
Ecoinvent database. Otherwise, similar materials were used as alternatives. For example, we used
ammonium hydrogen phosphate and magnesium oxide to replace ammonium phosphate and magnesium
nitrate [18,19] , respectively, to add the flows of the nutrient solution of plant factory cultivations. This
substitution unavoidably introduced uncertainties, however, which were negligible since the mass of the
substituted substance was much less than 5% of the total inputs.

For traditional greenhouse cultivation, the foreground data include the seed, water, fertilizer, and auxiliary
materials (See Supplementary Table 1 in the Supplementary Information for the details). Nitrogen fertilizer
enters the soil to generate nitrate nitrogen through nitrification, and nitrate nitrogen then enters the
anaerobic environment to release N O and other gases through denitrification. Notably, the use of
2
nitrogenous fertilizers produces nitrous oxide with uncertainties. Therefore, when quantifying the nitrous
flow in developing LCA models, we used different emission coefficients of nitrous oxide by referring to
three previous investigations of Hergoualc’h et al. (2019); Wang et al. (2018), and Yue et al. (2019) [20-22] .
Supplementary Table 2 gives details.