Characteristics of iron cycle and its driving mechanism during the development of biological soil crusts associated with desert revegetation

Iron (Fe) is an essential element for almost all living organisms. In addition, microbial Fe cycling drives the biogeochemical cycles of other elements, although little is known about characteristics of Fe cycling and its driving mechanism in desert ecosystems. Here, we investigated the key microbial functional genes involved in Fe(II) oxidation and Fe(III) reduction and the associated factors affecting the soil during the development of biological soil crusts (BSCs), which are among the most important landscapes in desert ecosystems. We also analyzed the relationships among the genes involved in Fe, carbon (C), and nitrogen (N) cycling. The available Fe content and diversity of genes associated with microbial Fe cycling increased after 61 years of BSC development. Our results suggested that Fe(II) oxidation in BSCs is mainly driven by the iro gene from microaerophilic Fe oxidizing microorganisms (FeOMs), whereas Fe(III) reduction is driven mainly by c-type cytochromes through regulation by the OmcS gene. The presence of both genes significantly correlated with that of genes involved in labile organic C degradation, denitrification, and N reduction. Fungal abundance, soil nutrient content (mainly total N), and the Fe-manganese (Mn) oxide-bound fraction (RED-Fe) were the main soil factors affecting the gene profile associated with microbial Fe cycling in BSCs (explaining 74.8% of the variation). These results indicated that the main Fe cycling process in desert ecosystems can adapt to the local environment. Furthermore, the interaction between Fe and the C or N cycles, as well as the synergistic effects of soil quality improvement, actively promotes effective and dynamic Fe cycling in BSCs.