Primary production

The cultivation stage contained the entire processes from seeding to harvest. As all the cases sowed and harvested by hands rather than machines, the carbon emissions of sowing and harvest were ignored. The carbon emissions of respiration and the carbon absorption of photosynthesis during the crop growth were also not considered, since these would be negligible compared to the total carbon emissions of the vegetable and fruit production.

Fertilizers, pesticides, and plastic films were the major material inputs in the plastic covering greenhouse production. Data were collected as previously described in Hu et al. (2019). The amounts of nitrogen (N), phosphorus (P) and potassium (K) fertilizers (Tables 3A and ​and3B)3B) were calculated by the dosage of the compound fertilizer and the percentage of each nutrient. According to a study reported on PNAS (Zhang et al., 2013), the carbon emission coefficient of N fertilizer production and application in China was estimated to be 13.5 t CO₂-eq/t. The carbon emission coefficients of China’s P and K fertilizers were obtained from the estimation at the national general level (Chen, Lu & Wang, 2015). The carbon emission factor of manure dry matter was based on a study on the CF of grain production in China (Zhang et al., 2017), while the coefficient of fresh manure was obtained from the Lal’s (2004) review on the carbon emission from farm operations. Similarly, the amount of pesticide (Tables 3A and ​and3B)3B) was calculated by the amount of pesticide products used and the percentage content of the active ingredients. The specific brands of the same type of pesticides were not differentiated. Given the lack of research on carbon emission from pesticide in China, the coefficients of pesticide (insecticide and fungicide) used in this work were determined by the study conducted in the USA (West & Marland, 2002), including the whole process of pesticide formulation. Both greenhouse cover and mulching film used in all cases were plastic films, and the factor was derived from the work of Tian & Zhang (2013) on the agricultural plastic film in China.

In addition to material inputs, the cultivation process also consumes energy, mainly in plowing, irrigation and warmth retention. The plowing machine consumed gasoline or diesel according to the type of equipment. Carbon emission factors of diesel and gasoline were calculated by the conversion factor to standard coal and the carbon emission coefficient of standard coal, which both were from China Energy Statistical Year Book (NBSC, 2017). Since all the cases were not equipped with any independent power supply facilities, and the electricity they used came from the unified electric power system, it was assumed that the electricity consumed in each case is generated from the same source with the same carbon emission coefficient. The coefficient used in this study was the overall carbon emission factor of power supply units in North China Regional Power Grid (including Beijing, Tianjin, Hebei, Shanxi, Shandong and west of Inner Mongolia), which was derived from the Provincial GHG Inventory Guidelines of China (NDRC, 2011). The carbon footprint calculation and the emission parameters selection for the carbon footprint calculation was clearly introduced in the supplement material (SI).