Equal-Carbon Organic Amendments Enhance Soil Multifunctionality Through Divergent Functional Profiles in a Biodegradable Film-Mulched Rainfed Maize Field

Organic amendments influence several soil functions simultaneously, but whether material type matters at a fixed carbon (C) input is unresolved. In a rainfed maize field on the Loess Plateau under uniform biodegradable film, peat (MPT), biochar (MBC), fermented pig manure (MPM) and maize straw (MCS) received equal C (1200 kg C ha−1) with an unamended control (MCK) in a randomised complete block design (n = 3). Seventeen indicators formed a six-dimension soil multifunctionality (SMF) index; metagenomes supplied predictors of diversity, connectivity, Y-A-S investment and C-, N- and P-cycling-gene diversity. All four raised SMF 4.0–4.9-fold without differing detectably; yet, their dimensional composition differed (MBC, carbon storage; MPM, nutrient supply and enzymes; MCS, structure; MPT, water regulation and productivity). Functions were predominantly synergistic, forming two weakly linked modules (nutrient–enzyme and carbon–water–biomass) whose separation, though weaker, persisted after treatment adjustment and the few significant negative correlations reflected the reverse orientation of bulk density rather than functional trade-offs. S/Y stress-tolerance investment and connectivity were the strongest predictors, species richness the strongest correlate of SMF (ρ = 0.66), and cycling-gene diversity tracked both SMF and nutrient supply (ρ = 0.53–0.68). Five SMF-positive genera clustered on two phylogenetic branches and the most connected module; none of the 14 hub and SMF-associated genera changed detectably in absolute abundance, yet seven shifted in relative share. At equal C input, material quality reshapes the combination, not the overall level, suggesting function-targeted material choice, with locally available straw and pig manure-favoured pending multi-year, multi-site validation.