Long-term pig manure application increases soil organic carbon through aggregate protection and Fe-carbon associations in a subtropical Red soil (Udic Ferralsols)

Abstract. Manure is known to improve soil organic carbon (SOC) in Fe-rich red soils, while the underlying stabilization mechanisms remain poorly understood. In this study, four treatments were selected: (1) no amendment (Control), (2) low manure (LM, 150 kg N ha^-1 yr^-1), (3) high manure (HM, 600 kg N ha^-1 yr^-1), (4) high manure with lime (HML, 600 kg N ha^-1 yr^-1 plus 3000 kg Ca (OH)₂ ha^-1 3yr^-1). The quantity and quality of topsoil (0–20 cm) organic carbon were investigated by physical fractionation, ¹³C-nuclear magnetic resonance (NMR) spectroscopy and thermogravimetry (TG) analysis. Manure application increased total SOC by 65.1 %–126.7 % (primarily in the particulate organic matter (POM) fraction), while the mineral-associated organic matter fraction (MAOM), despite its higher C content (4.18–7.09 g C kg⁻¹), contributed less (65.4 %–71.0 %) compared to the control (82.4 %). POM C was stabilized via hierarchical aggregation: fresh manure inputs acted as binding nuclei, increasing macroaggregates (>0.25 mm) while reducing microaggregates (0.05–0.25 mm), physically isolating labile C from microbial decomposition. Concurrently, manure amendments triggered Fe-mediated chemical stabilization. Elevated pH (4.8 to 5.4–7.1) enhanced non-crystalline Fe oxide (Fe_o) content (+25.4 %), which positively correlated with MAOM C (R² = 0.56, P < 0.05). Despite a chemical composition shift toward aliphaticity and reduced aromaticity, thermally stable organic matters increased by 8 %–12 %, revealing critical role of Fe_o (aggregates were destroyed before TG analysis) in offsetting inherent molecular lability. Overall, this study establishes a dual SOC stabilization framework for subtropical red soils, highlighting physical protection through aggregation processes and chemical protection via Fe-carbon associations.