Fractionation and mechanism of Fe in the mineral-associated soil fraction from the Mongolian Plateau grasslands

The Mongolian Plateau grasslands constitute a vital ecological barrier in inland Asia. Within these ecosystems, the mineral-associated soil fraction (<53 μm, 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 (IP_SE) 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%). IP_SE drivers exhibited a similar hierarchy, suggesting a coupled geochemical cycling mechanism between Fe_HR and IP_SE. 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.