Integrating the reuse of contaminated effluents with oil crops production: hints on how economic growth can be decoupled from resource use

The European industry currently seeks crops that may provide feedstocks for energy and materials, capable to substitute fossil-based feedstocks. Oil crops, such as Brassica carinata are being considered promising options. Although this oil crop is less water demanding than other oil crops (e.g. rapeseed), in countries where water resources are scarce, such as those of the Mediterranean, the water use needed for the cultivation may represent a constraint associated with their production. Therefore, this work aimed to evaluate the growth responses, as well as the biomass productivity and the biomass composition of B. carinata, irrigated with wastewaters rich in zinc, in an integrated strategy to minimize freshwater abstraction and use of mineral resources. The trials were established in early January 2019 in pots. Different genotypes of Brassica carinata were sowed and after the establishment of the plants, wastewater rich in ammonium ion and phosphates and artificially contaminated with Zn (10 mg/L; 20 mg/L) was applied every week at a rate to avoid water stress. The crops were also irrigated with tap water as control. At the end of the growing season (late May 2019), the plants were harvested and both the aerial productivity and the number of siliquae were monitored. Throughout the experiment, percolated waters were analysed in terms of zinc to evaluate if the “soil-biomass system” was capable to prevent contamination of the ground and surface waters. In general, the aboveground yield of the several genotypes of B. carinata was not affected by the heavy metal contaminated wastewater. Moreover, it was found that control plants were more prone to be attacked by worms and insects than plants that were being irrigated with contaminated wastewaters. Yet, in most of the genotypes tested, siliquae yield was significantly affected by the irrigation with heavy metal contaminated wastewater. Productivity losses ranged from 40% to 80% in comparison with control. This reduction in the amount of oil produced may be a constraint to the exploitation of this oil crop. Genotype 6091 was the one that showed the most consistent behaviour since it achieved yields over 200 g/m² for all the scenarios, a low ash content on the stems and a lower zinc content on the seeds and stems, making it a very interesting genotype to be exploited by the bioeconomy industries. The system soil-plant effectively depurates the wastewater, preventing the contamination of groundwater with Zn and other pollutants present in the wastewater. No differences were observed among the different genotypes of B. carinata in terms of effluent depuration. However, the irrigation with this wastewater contributed to an increment of Zn in the topsoil, which can also be a constraint to the application of these sorts of effluents in the irrigation of non-food crops.