Chemical structure of soil organic carbon governs formation, stability, and carbon accumulation of soil aggregates under contrasting land uses

Purpose: This study aimed to elucidate how the chemical structures of soil organic carbon (SOC) govern the formation, stability, and carbon accumulation of soil aggregates under different land uses. Methods: Soil samples were collected from seven representative land uses: cassava plantation (CVP), orchard (OR), sugarcane plantation (SP), corn plantation (COP), forest (FR), pasture (PT), and abandoned land (AL). Soil aggregates were separated into distinct size classes (> 2000, 500–2000, 250–500, 53–250, and < 53 μm), aggregate stability was assessed using mean weight diameter (MWD) and geometric mean diameter (GMD), and SOC accumulation within aggregate fractions was quantified. The chemical composition of SOC was characterized using ¹³C cross-polarization/magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectroscopy. Results: The results revealed pronounced land use-dependent differences in soil aggregation and SOC distribution. The PT land use exhibited a greater proportion of macroaggregates (> 2000 μm; 70% of total), higher aggregate stability (3.6 and 1.4 mm for MWD and GMD, respectively), and higher SOC accumulation in macroaggregate fractions (13.3 mg C g^{− 1}). Whereas, the CVP resulted in greater SOC accumulation in microaggregates (53–250 μm; 10.9 mg C g^{− 1}). The ¹³C CPMAS NMR results revealed that O-alkyl C was the major C group in the > 2000 μm fraction (23 − 42%), whereas aromatic C was the major C group in the 53 − 250 and < 53 μm fractions (29 − 47% and 35 − 54%, respectively). The formation, stability, and SOC accrual in macroaggregates were positively associated with O-alkyl C and alkyl C components, reflecting the importance of labile organic inputs and chemically hydrophobic features, respectively. Conversely, SOC accumulation in smaller aggregate fractions was positively correlated with aromatic C, suggesting preferential accumulation of recalcitrant compounds. Conclusions: Overall, these findings suggest that land uses with perennial vegetation, such as PT, enhance soil structural stability and SOC accumulation compared to intensively managed croplands, with these advantages being associated with the chemical composition of SOC.