Effects of vegetation modification patterns on iron and aluminum oxides and microbial communities in subtropical ultisol microaggregates

Abstract The stability of soil aggregates in subtropical Ultisol regions is crucial for maintaining ecological functions of soil. However, it is currently unclear how changes in aboveground vegetation affect the mechanisms of “mineral‐microbe” coupling processes regulating soil microaggregate formation and stability. This study used degraded Chinese fir forest stands under three vegetation modification treatments (unmodified control, TP1 replanted with both broad‐leaved tree species and nitrogen‐fixing plants, and TP2 replanted with only broad‐leaved tree species) to analyze the forms of iron and aluminum oxides and the distribution of microbial communities in soil microaggregates. Results indicated that vegetation modification significantly promoted the enrichment of amorphous iron (Fe o ) and aluminum (Al o ) in fine‐grained (<20 µm) microaggregates. Fe o content increased by 45.2%–72.3% in the 0–20 cm soil layer, with TP1 showing a stronger promoting effect and reduced the content of free‐state aluminum (Al d ) in medium‐coarse fractions (20–50 µm, 50–200 µm). Vegetation modification reduced microbial community diversity, with the total content of phospholipid fatty acid (PLFAs) in the topsoil decreasing by 46.2% and 80.8%. Shannon–Wiener and Simpson diversity indices decreased by between 15.3% and 20.7% and between 2.6% and 3.7%, respectively. The microbial community structure underwent reorganization, as bacterial abundance increased in the fine‐grained fraction, while fungi, arbuscular mycorrhizal fungi, and actinomycetes exhibited more pronounced responses in the coarse‐grained fraction. Redundancy analysis indicated that bacteria, arbuscular mycorrhizal fungi, Gram‐negative bacteria, and actinomycetes were the primary microbial groups explaining variations in iron and aluminum oxides. Fe o content exhibited a significant negative correlation with PLFA levels in multiple bacterial species, while the complexed aluminum (Al p ) content demonstrated a positive correlation with microbial diversity. In summary, vegetation modification drove a synergistic response involving mineral transformation and microbial community restructuring within subsurface microaggregates by altering aboveground plant composition. Among these, the combined broad‐leaf tree species and nitrogen‐fixing plant treatment demonstrated greater advantages in promoting amorphous iron‐aluminum enrichment and optimizing mineral cementation. This study provides a scientific basis for gaining an in‐depth understanding of the “plant‐mineral‐microorganism” coupling mechanism for enhancing Ultisol structural stability, as well as for selecting ecological restoration models tailored to local conditions.