Comparing nutritional value and yield as functional units in the environmental assessment of horticultural production with organic or mineral fertilization

We report the environmental assessment of the cultivation cycle of cauliflower (Brassica oleracea L. var. botrytis), chosen due to its high levels of natural bioactive compounds, using different fertilization practices. The functional units used during the impact assessment were linked with the quantity produced, considering different units of commercialization, or with the cauliflower quality, considering its antioxidant compounds content. Although nutrient content has been described and used as a possible functional unit, using antioxidant compounds as a functional unit has not previously been published.

Three cultivation options with similar dosages of total nitrogen were considered: using mineral fertilizers (M) alone or mineral fertilizers plus compost, with a high (C_H) or a low (C_L) dosage. During the cultivation period, the soil characteristics and nitrogen and moisture content of the fruit were monitored, and the yield and the fruit size were analyzed. In addition, the glucosinolates and the phenolic compounds (sinapic acid, phenols, and flavonoids) content were assessed for the three options. Life cycle assessment (LCA) was used to determine the environmental impacts of the whole cauliflower production cycle, including production of mineral and organic fertilizers, fertilizers transport, and crop stage.

Commercial yields were higher for cultivation options with M and C_L than for option C_H, while higher levels of bioactive compounds were detected in the latter. For C_H and C_L, eutrophication, global warming and ozone layer depletion potentials were generally lower and photochemical oxidation potential was always higher than for the M option, regardless of the functional unit. Regarding functional units involving production (yield, fruit and dry matter harvest), there were higher impacts with the C_H cultivation option than with M for abiotic depletion, acidification, photochemical oxidation, and cumulative energy demand. When the differences in bioactive compounds content (total sinapic acid derivatives and total phenols) were sufficiently high, this was reversed, with C_H having lower impacts for all the environmental categories apart from photochemical oxidation and abiotic depletion.

The differences in the magnitude of individual environmental impacts between cultivation options, and also the order, were highly dependant on the functional unit considered. When functional units associated with production and total phenols content were considered, the C_H cultivation option had the highest impact in four out of seven categories, whereas for the functional unit involving sinapic acid content, this cultivation option had the least impact in five out of seven categories.