Phenolic compounds in NaOH extracts of UK soils and their contribution to antioxidant capacity

Antioxidants are released during the NaOH extraction of soils, and this also releases phenolic compounds from plant and soil material. We hypothesized that phenolics would be important contributors to the antioxidant capacity (AOC) of soils. The concentrations of 12 phenolic compounds and the AOCs in 4 m-NaOH extracts of a range of UK soil types were measured. Samples of both surface and subsurface horizons were taken from 24 sites representing the major soil types (brown soils, gleys, podzols, peats and lithomorphic soils). The land use/vegetation varied from deciduous woodland through heather moorland to agricultural crops. The internal standards method was used to quantify the phenolics, which were detected by gas chromatography. The AOCs of the extracts and of standards of the individual phenolic compounds were measured using the TEAC (Trolox Equivalent Antioxidant Capacity) method. There were differences in the phenolic distributions extracted from soils with different land uses/plant inputs, as well as differences between surface and subsurface samples. A statistically significant linear relationship was found between the AOCs of the extracts and the sum of the phenolic compounds, as both these variables were positively correlated with the organic matter content of the soils. The AOCs of the individual phenols were used to calculate their contribution to the overall AOC of the extracts, and this was found to be small (approximately 1% of the total AOC). We concluded that the measured phenolic compounds were not important contributors to the AOCs of the soil extracts, and therefore that other unidentified antioxidant compounds were probably present.