MAOM stoichiometry and Fe-P coupling reveal soil-mediated pathways linking climate to grassland vegetation on the Mongolian Plateau

Background and aims How climate signals propagate through soils to shape grassland vegetation remains poorly resolved, particularly regarding reactive iron (Fe) in mediating phosphorus availability within mineral-associated organic matter (MAOM). We examine how MAOM-bound C-N-P stoichiometry and Fe-P coupling transmit climatic signals across the Mongolian Plateau, and evaluate whether reactive Fe phases constitute an overlooked predictor of dryland grassland functioning. Methods We quantified C, N, P and highly reactive iron (Fe _HR ) concentrations and stoichiometric ratios (C:N, C:P, N:P, Fe _HR :P) in the MAOM fraction (< 53 µm) of 247 topsoil samples spanning meadow, typical and desert steppe. These indicators, with soil pH, cation exchange capacity, climate and topographic variables, were linked to aboveground biomass (AGB), belowground biomass (BGB) and species richness (SR) using redundancy analysis (RDA), variance partitioning, and structural equation modeling (PLS-SEM). Results MAOM C:N:P ratios decreased coherently from meadow to desert steppe, whereas Fe _HR :P peaked in typical steppe with the highest spatial variability (CV = 45.10%). Climate dominated vegetation variation (82.9% in RDA; direct β = −0.60 in SEM), yet the shared climate × soil fraction (32.53%) far exceeded independent effects, and SEM confirmed a strong indirect climate→soil→vegetation pathway (β_indirect = − 0.13). Notably, Fe _HR :P was negatively associated with BGB but positively with SR, ranking above all C:N:P ratios in explanatory power. Conclusion Soil biogeochemistry, particularly Fe-P coupling within MAOM, acts as a critical filter transmitting climate signals to vegetation, underscoring the need to incorporate reactive mineral phases into dryland grassland assessments under global change.