Bryophyte-cyanobacteria symbioses and their nitrogen fixation capacity—A review

Bryophyte-cyanobacteria symbiosis (BCS) is a key source of nitrogen input into ecosystems in nutrient-poor regions. Investigating BCS relationships and the nitrogen fixation capacity can be a new pathway and window to explore the process and mechanism of nitrogen input and nitrogen cycling. However, BCS relationships and nitrogen fixation/cycling processes and mechanisms remain poorly studied, and most of these studies have only focused on the boreal forest, with no report from Chinese forests. Based on systematic literature search and analysis, this review provides a summary on BCS relationships, the nitrogen fixation capability of BCS, the fate of fixed nitrogen, as well as the environmental factors and driving mechanisms of BCS. Firstly, we synthesized different types of BCS, the mechanisms by which the fixed nitrogen is transferred to and used by other plants within the forest, the rate of fixed nitrogen, the factors influencing the rate of nitrogen fixation. Moreover we point out the existing problems that need to pay close attention to and at least four research directions need to break through. Furthermore, the theoretical basis of BCS is provided for further research, promote and deepen the cognition of BCS and nitrogen-fixing research.