Increased soil organic carbon sequestration through hydrophobic protection by humic substances

We studied the effect of humified organic matter (OM) on the mineralization of a representative labile organic compound in soil. In an incubation experiment, a 13C-labeled 2-decanol was added to soil either alone (2-dec∗) or in mixture with two humic acids from compost (HAC∗) and lignite (HAL∗) which had different hydrophobic properties. Isotopic dilution (δ13C) showed that after 3 months of incubation about 58, 40 and 28% of the added 13C was retained in the whole soil treated with HAL∗, HAC∗ and 2-dec∗, respectively. The higher the hydrophobicity of the employed humic material, the larger was the sequestration of organic carbon in soil. Fractionation of incubated samples revealed that the labeled carbon progressively accumulated in the finest particle-size fractions. However, the high hydrophobicity of the lignite HA favored accumulation of 13C also in the sand-sized fraction. The NMR spectra of humic extracts showed that the 13C-methyl group in the original 2-decanol had been oxidized to a 13C-carboxyl group during incubation for all treatments. This indicated that despite its hydrophilicity, the resulting carboxyl carbon was sequestered into the hydrophobic domains of the humic pool in soil. In fact, the residual 13C was larger in humic than in fulvic extracts for the control sample (2-dec∗) and even more so in extracts from soil treated with both exogenous humic acids. Our results suggest that labile organic compounds may be effectively protected in soil by humified OM and their microbial mineralization substantially reduced. Innovative soil management practices employing hydrophobic humic substances may increase the biological stability of soil OM and thus contribute to significantly mitigate CO2 emissions from agricultural soils.