This book highlights the latest discoveries about the nitrogen cycle in the soil. It introduces the concept of nitrogen fixation and covers important aspects of nitrogen in soil and ecology such as its distribution and occurrence, soil microflora and fauna and their role in N-fixation. The importance of plant growth-promoting microbes for a sustainable agriculture, e.g. arbuscular mycorrhizae in N-fixation, is discussed as well as perspectives of metagenomics, microbe-plant signal transduction in N-ecology and related aspects. This book enables the reader to bridge the main gaps in knowledge and carefully presents perspectives on the ecology of biotransformations of nitrogen in soil.