Assessing the environmental impacts of rice in an anthropized Mediterranean wetland: Towards carbon farming

Abstract

Rice fields in southern Europe are mainly located in wetlands of high ecological value, such as L'Albufera, one of the most important rice-growing areas in Spain. Considering the unique characteristics of this environment, it is crucial to identify those farming practices that minimize the on-field emissions of this crop, one of the main causes of its impacts, and increase soil organic carbon to mitigate climate change. This study assesses the environmental impact of rice grown in L'Albufera from a cradle-to-farm gate perspective, considering current practices of removing straw from the field and a scenario in which the straw is chopped and incorporated into the soil. The functional unit on which the results are expressed is 1 kg of rice. Primary data were collected from a representative farm to develop the life cycle inventory. On-field emissions and annual changes in soil organic carbon were estimated using the Denitrification-Decomposition (DNDC) mechanistic model, and the results were compared with those obtained using conventional Tier 1 and Tier 2 methods. All the impact categories were evaluated following the EF v3.0 characterization method. On-field emissions estimated with DNDC show differences from those calculated by conventional methods, mainly for CH4, NO3− and NH3. The results highlight the great importance of on-field emissions on most impacts, namely climate change, freshwater and marine eutrophication, ozone depletion, terrestrial acidification, ecotoxicity, and human toxicity. The DNDC model estimates CH4 and CO2 emissions to increase when straw is incorporated into the soil. However, the annual change in soil organic carbon decreases by >50 %, and NH3 and N2O are reduced by 13 % and 11 %, respectively. Consequently, differences in impact scores are observed for acidification, terrestrial eutrophication, and particulate matter (about 12 % higher), but also for climate change (15 % lower) and photochemical ozone formation (4 % lower). The need to harmonize methodologies to estimate on-field emissions and the changes in soil organic carbon is highlighted to improve the application of life cycle assessment to agricultural systems, particularly rice cultivation. To effectively promote carbon farming practices related to straw management in paddy rice, it is necessary to conduct long-term field studies to measure on-field emissions associated with each alternative experimentally.