Chemistry and Associations of Carbon in Water-Stable Soil Aggregates from a Long-Term Temperate Agroecosystem and Implications on Soil Carbon Stabilization

This study aimed to understand the effects of long-term agricultural management on soil organic carbon (SOC) stabilization/storage in aggregates. Carbon near-edge X-ray absorption fine structure spectroscopy (C-NEXAFS), 13C nuclear magnetic resonance (13C NMR), and wet chemical analyses were employed to characterize SOC and organo-mineral associations. Soils were from a corn system (Kansas, USA; > 22 years), comparing till and no-till, with fertilizer treatments (manure/compost and chemical). Results showed high concentrations of SOC/amorphous Fe and enhanced SOC stabilization in manure/compost added soils. Various degrees of decomposition were evident by C-NEXAFS with aromatic (22%–40%) and carboxylic C (20%–45%) being abundant in all aggregates. Microaggregate-associated SOC showed more enhanced aromaticity than other aggregates. Some aggregates from tilled soils had increased aliphaticity in SOC than its counterpart no-till. Humic acid-associated OC (by 13C NMR) showed enhanced aliphaticity with decreasing aggregate sizes. Our results showed that the SOC stabilization/storage occurs due to chemical, mineralogical, and biological mechanisms stimulated by management.