Long-term pig manure application increases SOC through aggregate protection and Fe-C associations in a subtropical red soil (Udic Ferralsols)

Abstract

Manure is known to improve soil organic carbon (SOC) in Fe-rich red soils, but the underlying stabilization mechanisms remain poorly understood. A long-term field experiment was conducted with four treatments: no amendment (Control), low manure (LM, 150 kg N ha−1 yr−1), high manure (HM, 600 kg N ha−1 yr−1), and high manure with lime (HML, 600 kg N ha−1 yr−1 plus 3000 kg Ca (OH)2 ha−1 (3 yr)−1). The quantity and quality of SOC were characterized by physical fractionation, 13C NMR spectroscopy and thermogravimetry (TG) analysis from 0–20 cm depth. 18 year manure application increased total SOC by 65.1 %–126.7 %, with the increase primarily occurring in the particulate organic matter (POM) fraction. Specifically, POM C increased substantially by 208 %–592 %, compared to a 32.0 %–66.8 % increase in MAOM C. POM C was stabilized through the process of hierarchical aggregation. Fresh manure inputs acted as binding nuclei, leading to an increase in macroaggregates (> 0.25 mm) concurrently with a reduction in microaggregates (0.05–0.25 mm), physically isolating labile C from microbial decomposition. Concurrently, manure amendments triggered Fe-mediated chemical stabilization. The elevated pH (4.8 to 5.4–7.1) in the manure treatments enhanced non-crystalline Fe oxide (Feo) content by 25.4 %, which positively correlated with MAOM C (R2 = 0.56, P < 0.05). Despite chemical composition shift toward aliphaticity and reduced aromaticity, thermally stable organic matters increased by 8 %–12 %, revealing the critical role of Feo in offsetting inherent molecular lability when aggregate protection was removed by destruction prior to TG analysis. Overall, this study proposes a dual SOC stabilization framework via physical and chemical Fe-carbon associations in a subtropical red soil.