Assessment of the thermodynamic, environmental and economic output of agro-ecosystems: onion set versus onion production

Farmers sow onion seeds either for the purpose of preparing onion sets or for producing onions. Thermodynamic, economic and environmental analyses were performed to assess the sustainability of these two agro-ecosystems. Thermodynamic analyses indicated more sustainability and renewability of onion production compared to the production of onion sets; however, economic analysis estimated more profitability of onion set production and environmental analysis revealed less greenhouse gas emissions per ton of onion production. The results showed that in both systems, chemical fertilizer had the greatest impact on cumulative energy consumption (CEnC) and greenhouse gas emissions in CO2 equivalents (CCO2E). Human labor input had a significant impact on the cumulative exergy consumption (CExC) and the production costs of both products. The effects of two alternative scenarios, (1) the use of more renewable-based resources in production and (2) the use of wastes for making bioethanol were investigated in both systems. The use of more renewable resources has the greatest impact in driving both systems toward higher sustainability. The results of the renewability indicator (RI) showed that by using more renewable resources, the RI of onions will increase from − 3.00 to − 2.20, and for onion sets, it increases from − 1.43 to − 0.12. The negative sign of RI shows that even if more renewable resources are used, the production system of both products is still not renewable. The reason for this non-renewability is the significant share of human labor required in both onion and onion set farming. The use of onion waste produces 76.67 TJ of energy and reduces CO2 emissions by more than 10.32 × 106 kg CO2 per year.