Carbon-Iron coupling drives the shift in nutrient and microbial processes via the iron gate mechanism in dryland paddy fields

Climate change impairs organic matter decomposition and nutrient cycling in arid-region paddy soils, yet the microbial mechanisms underlying exogenous carbon‑iron (C Fe) interactions remain unclear. We investigated how combined additions of exogenous carbon (biochar/straw) and iron (ferrihydrite) modulate soil physicochemical properties, extracellular enzyme activities (EEAs), and microbial communities in paddy fields from the Yellow River irrigation area. Compared with the CK, C Fe coupling significantly reduced soil pH while elevating Fe 2+ and Fe 3+ levels and enhancing the bioavailability of soil organic carbon (SOC) and total nitrogen (TN). The straw + ferrihydrite (SF) treatment most effectively promoted TN accumulation and Fe speciation transformation, and notably increased the activities of β-1,4- N -acetylglucosaminidase (NAG) and catalase (CAT). The biochar + ferrihydrite (BF) treatment induced the highest activities of β-glucosidase (BG) and alkaline phosphatase (ALP), both of which were strongly and positively correlated with SOC and Fe 3+ . C Fe synergy alleviated anaerobic suppression of EEAs via a redox-mediated “iron gate” mechanism, primarily regulated by SOC, Fe 2+ , and Fe 3+ . Both BF and SF significantly increased the α-diversity of the prokaryotic community, as indicated by the Chao1 (increases of 3.34 % and 2.65 %, respectively) and Shannon indices (increases of 12.59 % and 10.60 %, p < 0.05). SF enriched facultative anaerobes (e.g., Chloroflexi), whereas BF favored oligotrophic Patescibacteria. Partial least squares structural equation modeling (PLS-SEM) revealed that microbial diversity was positively driven by EEAs and soil properties, whereas the relative abundances of specific phyla and genera exerted negative effects. Our findings demonstrate that C Fe interactions regulate microbial diversity through direct enzymatic stimulation and indirect shifts in community structure, providing actionable insights for optimizing carbon sequestration and nutrient management in arid agroecosystems. • C–Fe inputs boost SOC, enhance TN availability, and modify Fe speciation. • The 'iron gate' mechanism mitigates enzyme suppression under anoxic conditions. • SOC, Fe(II), and Fe(III) govern enzyme function and diversity. • C–Fe inputs reshape microbial communities via enzyme and redox pathways.