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Increasing the retention and sequestration of plant residue carbon in agricultural soils by incorporating humic acid is the main focus of this study. This study aims to examine the effect of humic acid addition on the decomposition of plant residues of varying degrees of lability. Respiration experiments were conducted to estimate the ability of humic acid to protect plant derived organic compounds from decomposition. Humic acid reduced mineralization from all added residues and this protection effect followed the lability of the residues: vetch> wheat> oak. This could be attributed to the chemical interaction between different plant-derived organic compounds and the humic acid. Lysine was strongly adsorbed to humic acid and mineralization was reduced by 23% as a result of the strong electrostatic interaction. Applying humic acid with vetch reduced all microbial indices as a result of less substrate availability for miroorganisms. On the other hand, applying humic acid with wheat might have simulated the synthesis of extracellular enzymes and the co-metabolism of humic acid (brimming effect), resulting in an enhanced microbial structure toward a higher fungal population. This study suggests: (1) Applying humic acid to ecosystems that receive labile residues (such as vetch) to reduce mineralization and enhance carbon sequestration (2) Applying less labile residues (such as wheat) in combination with humic acid to recover degraded soils and enhance carbon sequestration.

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