The Mongolian Plateau grasslands constitute a vital ecological barrier in inland Asia. Within these ecosystems, the mineral-associated soil fraction (<53 mu m, MASF) is the dominant component, and its iron (Fe) oxides play a pivotal role in mediating carbon (C), nitrogen (N), and phosphorus (P) cycling. Using sequential chemical extraction, we quantified seven Fe fractions (Fe-ex, Fe-carb, Fe-ox1, Fe-ox2, Fe-mag, Fe-prs, and Fe-U) within the MASF across the plateau. The relative abundance of these fractions followed the sequence: Fe-U > Fe-prs > Fe-ox2 > Fe-ox1 > Fe-mag > Fe-carb > Fe-ex. The combined Fe-ox1 and Fe-ox2 fractions, representing highly reactive Fe (Fe-HR), constituted 6.82-55.77% of total iron (FeT), identifying them as the dominant Fe-HR components. Both Fe fraction abundance and inorganic phosphorus extracted by sequential extraction (IPSE) decreased significantly along the grassland gradient: meadow steppe > typical steppe > desert steppe. This parallel decline underscores the key regulatory role of Fe (hydr)oxides in governing P fractionation and bioavailability within the MASF. Multivariate statistical analyses revealed soil physicochemical properties as the primary drivers of Fe-HR variability, explaining 64.52% of the variance, followed by climatic factors (18.6%) and vegetation factors (11.7%). IPSE drivers exhibited a similar hierarchy, suggesting a coupled geochemical cycling mechanism between Fe-HR and IPSE. This study provides fundamental geochemical insights into Fe fractionation within the MASF, advancing analytical approaches for understanding elemental cycling and ecological processes in Mongolian Plateau grasslands.