Sorptive protection or chemical recalcitrance - how important are minerals for soil organic matter stabilization?

Stable soil organic matter (SOM) is important for many environmental issues, including soil-atmosphere carbon exchange and the fate of organic pollutants in soils. It is desirable to understand the chemical composition and the reasons for slow turnover of this organic matter pool. To discern the mechanisms responsible for OM protection and gain insight into the molecular composition of refractory compounds, we operationally divide stable OM into that protected by sorption to mineral phases and that protected by inherent (chemical) recalcitrance. Aggregation or physical protection is generally effective on shorter timescales (i.e., decades) than we are considering. While recalcitrance dominates the preservation of fossil OM in sedimentary rocks, the quantitative importance of intrinsic chemical resistance in soils is unclear. Our objectives were (i) to estimate the relative contribution of recalcitrant SOM to stable SOM; (ii) to characterize the gross chemistry of recalcitrant SOM, and (iii) to elucidate the contribution of lignin to recalcitrant SOM and to SOM stabilized by minerals. We studied 12 samples from acidic soils of varying mineralogical composition. A sequential oxidation-demineralisation treatment (NaOCl-HF) yielded quantitative estimates of the relative sizes of the sorptive protected and recalcitrant SOM pools. We found that the concentration of stable OM, at least in acid sub-surface soils, is governed by poorly crystalline minerals (oxalate-extractable Fe+Al). Stable OM was roughly 75% mineral-protected C and 25% chemically recalcitrant C (with large variations). When isolated, the recalcitrant OM fraction was older than sorptive protected OM and had uniform chemical composition dominated by aliphatic C moieties. Lignin was not an important component of the sorption-protected or chemically recalcitrant OM fractions.