Importance of the Bradhyrizobium japonicum symbiosis for the sustainability of a soybean cultivation

Economic development in agriculture is often accompanied by strong environmental pressures that present risks, which could lead to irreversible damage of the associated agro-ecosystem. A form of agricultural management that utilizes natural resources rationally yet maintains ecological stability is necessary. Ecologically sound, yet productive, use of resources requires indicators that assess not only productive and economic factors, but also environmental impact and ecological effects. Thus, to study an agricultural activity and its interaction with the environment, a holistic approach, capable of considering ecological, biophysical and socio-economic aspects is appropriate. Here we use emergy analysis and its indicators as given in Odum (Science 242 (1988) 1132–1139), to evaluate how sustainability of a soybean crop in south Tuscany (Italy) is increased using the specific bacteria inoculation to satisfy, through fixation, the nitrogen need of the crop. Cultivation was studied with two options: (1) utilization of chemical fertilizers to supply nitrogen needs as often done in the past, and as a viable present alternative and (2) the symbiotic activity of Bradyrhizobium bacteria, given as inoculum, to cover all nitrogen needs. This article shows that proper agricultural management should help maintain, and increase, system capacity, i.e. the quality of the system environment (soil, water,…), to sustain biomass, and prevent environmental degradation. For an Italian agro-ecosystem, the results demonstrate that agricultural production is more than doubled and the use of non-renewable and toxic inputs, such as chemical fertilizers, is reduced by use of the inoculum. The good results for soybean suggests development of analogous cultivation methods for other agricultural species, such as gramineous, for which different N-fixing bacteria such as Azospirillum, have been already identified.