Towards a comprehensive understanding of free-living nitrogen fixation

Abstract

Free-living nitrogen fixation (FNF) is a ubiquitous phenomenon that plays a modest role in the (N) economy of an ecosystem. However, sampling difficulties, methodological constraints and environmental controls have presented challenges for predicting the actual rate of FNF. Therefore, a deeper understanding of the accuracy to design models that consider dynamics, heterogeneity, influences, and other limitations is needed. This review presents an overview of the biology and diversity of microorganisms related to FNF as well as various ecological controls that influence these microorganisms. We also discussed contributions of FNF to the N input of various ecosystems. Overall, previous research has shown that considerable spatiotemporal variability exists in microbial types at both biome and microbiome scales, resulting in significant variation in FNF. Beyond this, rate of FNF is controlled by certain factors, such oxygen and metal ion availability, source of energy and soil nutrients, temperature, and pH. Empirical evidence increasingly indicates a significant contribution of FNF to N inputs in natural, agricultural, and aquatic ecosystems. It is inferred from this review that for the expanded exploitation of biological nitrogen fixation (BNF), we must pay additional attention to FNF because it occupies a central role within the process. Finally, we propose a framework for the quantification of FNF alongside a suite of recommendations that would deepen our understanding of FNF.

Keywords

• ecosystem
• environmental controls
• biological nitrogen fixation
• free-living nitrogen fixation
• microorganisms