Enhanced extractability of cutin- and suberin-derived organic matter with demineralization implies physical protection over chemical recalcitrance in soil

Soil organic matter (SOM) plays an important role in soil fertility and biogeochemical cycling of carbon. Therefore, it is important to estimate the extent to which SOM components are physically protected in soil. To investigate the role of physical protection with respect to cutin and suberin biomarker extraction from soil, we performed hydrolysis with KOH/MeOH to isolate cutin and suberin biomarkers from four Canadian soils (grassland, agricultural and forest soils; 0-15 cm depth) before and after treatment with HF. Treatment with HF released large amounts of hydrolysable lipids that were assumed to be mineral-associated and/or aggregate-protected. We found that 83-98% of the suberin biomarkers (ω-hydroxy C20-C32 acids, C20-C32 diacids and 9,10-epoxy C18 diacid) and 81-96% of the cutin biomarkers (mid-chain hydroxy C14, C15, C17 acids, mono- and dihydroxy C16 acids and diacids) were protected from extraction. The suberin/cutin ratio for the grassland and agricultural soils increased after HF treatment, indicating that more suberin biomarkers were protected than cutin biomarkers in these soils. Cutin- and suberin-derived compounds together represented 28-58% of the protected ester-bound SOM. The degree of protection differed among the soils and likely reflects differences in the chemical properties of the minerals, availability of mineral surfaces for sorption, presence of multivalent cations, vegetative input and the number of soil aggregates. Land management (e.g. tillage frequency) may have also played a role in the physical protection of suberin- and cutin-derived compounds in the agricultural soil.