Unraveling the interaction of iron oxide and microorganisms with internal iron cycling in arid desert riparian forest

Iron (Fe) is crucial for the normal growth and development of plant roots. In arid desert soils, the low availability of Fe poses a significant nutritional obstacle to the growth and development of desert riparian forests. However, the characteristics and mechanisms of microbial Fe cycling in desert riparian forests ecosystems are poorly understood. This study used metagenomics to assess the responses and driving mechanisms of key microbial functional genes associated with Fe cycle in the rhizosphere and bulk soils along a drought stress gradient (mild, moderate, and severe) in a desert riparian forest in northwest China. Generally, the rhizosphere effect lowers soil pH while increasing the availability of Fe-related nutrients. Genes involved in Fe uptake (mbt, pch, ccm) and Fe-Mn transport (sit) were significantly higher in the rhizosphere than in bulk soils (P < 0.001). Genes involved in Fe transport (tro, sit) were significantly higher under severe drought stress (S) than under the mild drought stress (Mi) gradient (P < 0.05). In the Fe cycle network, the complexity of Fe cycling genes and the co-occurrence network increased gradually with an increase in drought stress. Enrichment of Actinomycetes involved in the Fe cycle is a conservative response of plants to drought stress. Iron oxide (Fe_d, Fe_p) is the main composition of soil Fe in desert riparian forests, and on the drought stress gradient, Fe_p is the key influencing factor of Fe cycling where microbes participate, while soil pH plays a leading role in the rhizosphere environment. Our findings highlight that the rhizosphere effect, akin to a magnetic effect, transfers Fe from the bulk soil to the rhizosphere, particularly enhancing Fe absorption and transport. This rapid Fe redox cycle and transport help mitigate Fe deficiency in arid desert forests.