Assessing life cycle impacts from changes in agricultural practices of crop production : Methodological description and case study of microbial phosphate inoculant

PURPOSE: This paper presents an improved methodological approach for studying life cycle impacts (especially global warming) from changes in crop production practices. The paper seeks to improve the quantitative assessment via better tools and it seeks to break down results in categories that are logically separate and thereby easy to explain to farmers and other relevant stakeholder groups. The methodological framework is illustrated by a concrete study of a phosphate inoculant introduced in US corn production. METHODS: The framework considers a shift from an initial agricultural practice (reference system) to an alternative practice (alternative system) on an area of cropland A. To ensure system equivalence (same functional output), the alternative system is expanded with displaced or induced crop production elsewhere to level out potential changes in crop output from the area A. Upstream effects are analyzed in terms of changes in agricultural inputs to the area A. The yield effect is quantified by assessing the impacts from changes in crop production elsewhere. The field effect from potential changes in direct emissions from the field is quantified via biogeochemical modeling. Downstream effects are assessed as impacts from potential changes in post-harvest treatment, e.g., changes in drying requirements (if crop moisture changes). RESULTS AND DISCUSSION: An inoculant with the soil fungus Penicillium bilaiae has been shown to increase corn yields in Minnesota by 0.44 Mg ha⁻¹ (~ 4%). For global warming, the upstream effect (inoculant production) was 0.4 kg CO₂e per hectare treated. The field effect (estimated via the biogeochemical model DayCent) was − 250 kg CO₂e ha⁻¹ (increased soil carbon and reduced N₂O emissions) and the yield effect (estimated by simple system expansion) was − 140 kg CO₂e ha⁻¹ (corn production displaced elsewhere). There were no downstream effects. The total change per Mg dried corn produced was − 36 kg CO₂e corresponding to a 14% decrease in global warming impacts. Combining more advanced methods indicates that results may vary from − 27 to − 40 kg CO₂e per Mg corn. CONCLUSION AND RECOMMENDATIONS: The present paper illustrates how environmental impacts from changes in agricultural practices can be logically categorized according to where in the life cycle they occur. The paper also illustrates how changes in emissions directly from the field (the field effect) can be assessed by biogeochemical modeling, thereby improving life cycle inventory modeling and addressing concerns in the literature. It is recommended to use the presented approach in any LCA of changes in agricultural practices.