Increasing demand for agricultural production typically results in greater fertilizer application. Nitrogen (N) in fertilizers or manures which is not taken up by crops can be released into the atmosphere as nitrogenous greenhouse gases or leached into ground water, with resulting environmental implications. Rather than relying purely on application of N fertilizer, alternative N sources are needed to help develop more sustainable farming systems. Legumes have the potential to fulfil this requirement due to their unique ability to fix N biologically from the atmosphere, benefiting not only the legumes themselves, but also the intercropped or subsequent crops. In this paper, we explore and compare the environmental conditions influencing complex process of legume biological nitrogen fixation such as temperature, water content, N concentration, root zone рН, plant nutrient status including C and N substrates in roots, and a genetic variation in potential N fixation capacity. The ultimate goal of the research is to develop a model of legume biological nitrogen fixation. In this section, we focus only on how legume biological nitrogen fixation responds to environmental conditions and plant C and N substrates.