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

Economic development in agriculture is often accompanied by strong environm ental pressures that present risks, which could lead to irreversible damage of the associated agro-ecosystem. A form of agricultural management that u tilizes natural resources rationally yet maintains ecological stability is necessary. Ecologically sound, yet productive, use of resources requires in dicators that assess not only productive and economic factors, but also env ironmental impact and ecological effects. Thus, to study an agricultural ac tivity and its interaction with the environment, a holistic approach, capab le of considering ecological, biophysical and socio-economic aspects is app ropriate. 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 inoc ulation to satisfy, through fixation, the nitrogen need of the crop. Cultiv ation was studied with two options: (1) utilization of chemical fertilizers to supply nitrogen needs as often done in the past, and as a viable presen t alternative and (2) the symbiotic activity of Bradyrhizobium bacteria, gi ven as inoculum, to cover all nitrogen needs. This article shows that prope r agricultural management should help maintain, and increase, system capaci ty, i.e. the quality of the system environment (soil, water,...), to sustai n biomass, and prevent environmental degradation. For an Italian agro-ecosy stem, the results demonstrate that agricultural production is more than dou bled and the use of non-renewable and toxic inputs, such as chemical fertil izers, is reduced by use of the inoculum. The good results for soybean sugg ests development of analogous cultivation methods for other agricultural sp ecies, such as gramineous, for which different N-fixing bacteria such as Az ospirillum, have been already identified. (C) 2000 Elsevier Science B.V. Al l rights reserved.