Comparative environmental life cycle and agronomic performance assessments of nitrogen fixing rhizobia and mineral nitrogen fertiliser applications for pulses in the Caribbean region

Abstract

Replacing legume fertilisation using mineral nitrogen (N) (conventional technology, CT) with rhizobial/bradyrhizobial inoculation (inoculation technology, IT) is currently encouraged worldwide due to environmental concerns. However, the environmental effect of such inoculants has scarcely been addressed using a systematic methodology like life cycle analysis (LCA). The objective of this work was to assess, in tropical developing countries, the agronomic and environmental performance of rhizobial inoculants. We worked with two contrasting pulses, common bean, which is conventionally grown using an intensive mineral fertilisation strategy, and Pigeon pea, which is subject to a deficit fertilisation strategy. From the agronomic point of view, the replacement of CT by IT improved Pigeon pea yield by 30%, while increasing common bean yield by a not significant 2%. The Global Warming Potential (GWP) and Cumulative Energy Demand (CED) of inoculant production for 1 ha is less than 1% of the GWP and CED corresponding to the production of the mineral N fertiliser used in CT. The high input-high output crop (common bean) caused less environmental damage than the less productive and low input crop (Pigeon pea), indicating that an improvement in the yield would reduce agriculture’s overall environmental impact. Considering the five most impacted categories (CML-IA methodology), the substitution of CT by IT in the common bean resulted in an average reduction of environmental burdens by 19% per ha and 21% per t; in Pigeon pea, the average reduction was 12% per ha and 32% per t. However, phytosanitary treatments and irrigation accounted for higher impacts than N. Thus, improvement in environmental performance must include other technologies to optimise plant protection and irrigation and other strategies to increase yield, such as the optimisation of P, K fertilisation.