Stabilization of Soil Organic Matter: Association with Minerals or Chemical Recalcitrance?

Abstract

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 ( 14 C, 13 C) 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 13 C-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 14 C-modern OM. Of the stable organic carbon (OC), amounting to 2.4-20.6 g kg À1 soil, mineral dissolution released on average 73%. Poorly crystalline Fe and Al phases (Fe o , Al o ) and crystalline Fe oxides (Fe dÀo ) 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 À1 soil) contributed on average 27% to stable OC and the amount was not correlated with any mineralogical property. Recalcitrant OC had lower D 14 C and d 13 C 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 À1 OC). The results confirm 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.Abbreviations: CPMAS 13 C-NMR -cross-polarization magic-angle spinning 13 C nuclear magnetic resonance spectroscopy; FR -fluoride reactivity; OC -organic C; OM -organic matter; MOC and MN -mineral-protected organic C and N; ROC and RN -chemically resistant (recalcitrant) organic C and N; SSA -specific surface area; XRD -x-ray diffraction; TEM -transmission electron microscopy

Texture and organic carbon contents do not impact amount of carbon protected in Malagasy soils

“…As soil Al or Fe oxides are generally associated with clay+fi ne silt fraction contents (Zinn et al, 2007), our results showed a correlation of clay+fi ne silt fraction with SOC ( Figure 1A) and with macroaggregates ( Figure 1F). The formation of organo-mineral complexes which protect SOC from biodegradation (Wiseman and Püttemann, 2005;Mikutta et al, 2006) and the adsorption of SOC on the high specifi c surface area resulting from Al-Fe substitution (Barthes et al, 2008) might explained this positive relationship between clay+fi ne silt contents and SOC content ( Figure 1A). Therefore, these results confi rm SOC physicochemical stabilization for SOC accumulation mechanisms in such soils.…”

Storage and Turnover of Organic Matter in Soil

“…In two comparisons, NaOCl appears to cause less mineral alteration and yield an older OM fraction than does HCl (Mikutta et al 2006;Zimmermann et al 2007). These approaches use chemical reactivity of organic matter as a proxy for readiness to microbial degradation, rather than attempting to separate material of different chemistry per se.…”

“…Within the profile, the δ 13 C of bulk soil increases by 1-3‰ with increasing depth and decreasing particulate SOM. Comparisons of 100 y old and modern soil profiles shows that the increase in 13 C with depth is not due to fossil fuel effects (Torn et al 2002), but there is ongoing discussion regarding whether these trends are caused by microbial discrimination or selective preservation of plant compounds (Dijkstra et al 2006;Mikutta et al 2006) and resolution of these questions may be difficult using only natural abundance stable isotopes and non-manipulated systems.…”

“…This is useful for studying very stable C pools in soils, since they reside long enough for significant decay of 14 C to occur. In this case, the more 14 C-depleted a soil fraction is, the slower the turnover of the C is (even if the soil fraction also contains some faster-cycling components; Mikutta et al 2006;Sollins et al 2006). …”

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater

“…However, recent direct observations indicate the OC in the subsurface exists in smaller, simpler molecular structures than originally thought based on bulk extractions, and that often these reactive molecules persist in sedimentary systems much longer than previously believed (10,11). This different perspective suggests that the molecular properties do not in themselves control reaction rates but OC decomposition rates are affected by ecosystem properties such as nutrient limitation, energy scarcity, or carbon bioavailability due to physical limitation such as reactions with mineral surfaces (12)(13)(14). These recent observations suggest that sedimentary organic matter or older OC pools may also be available for microbial respiration.…”