Stabilization of soil organic matter: Association with minerals or chemical recalcitrance?

Soil organic matter ( OM) can be stabilized against decomposition by association with minerals, by its inherent recalcitrance and by occlusion in aggregates. However, the relative contribution of these factors to OM stabilization is yet unknown. We analyzed pool size and isotopic composition (C-14, C-13) of mineral-protected and recalcitrant OM in 12 subsurface horizons from 10 acidic forest soils. The results were related to properties of the mineral phase and to OM composition as revealed by CPMAS C-13- NMR and CuO oxidation. Stable OM was defined as that material which survived treatment of soils with 6 wt% sodium hypochlorite (NaOCl). Mineral-protected OM was extracted by subsequent dissolution of minerals by 10% hydrofluoric acid ( HF). Organic matter resistant against NaOCl and insoluble in HF was considered as recalcitrant OM. Hypochlorite removed primarily C-14-modern OM. Of the stable organic carbon (OC), amounting to 2.4-20.6 g kg(-1) soil, mineral dissolution released on average 73%. Poorly crystalline Fe and Al phases (Fe-o, Al-o) and crystalline Fe oxides (Fed-o) explained 86% of the variability of mineral-protected OC. Atomic C-p/( Fe+ Al)(p) ratios of 1.3 - 6.5 suggest that a portion of stable OM was associated with polymeric Fe and Al species. Recalcitrant OC (0.4 - 6.5 g kg(-1) soil) contributed on average 27% to stable OC and the amount was not correlated with any mineralogical property. Recalcitrant OC had lower Delta C-14 and delta C-13 values than mineral-protected OC and was mainly composed of aliphatic (56%) and O-alkyl (13%) C moieties. Lignin phenols were only present in small amounts in either mineral-protected or recalcitrant OM ( mean 4.3 and 0.2 g kg(-1) OC). The results con. rm that stabilization of OM by interaction with poorly crystalline minerals and polymeric metal species is the most important mechanism for preservation of OM in these acid subsoil horizons.