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International audience; Clay-sized soil minerals are known to protect organic carbon (OC) from mineralisation by formation of organo-mineral associations limiting its availability to microorganisms. The impact of soil fauna on these processes is poorly known. The aim of this study was to investigate the effect of earthworms on organic matter (OM) decomposition and association with minerals during a laboratory experiment. We used a model system consisting of fresh OM incubated with and without epigeic earthworms (Eisenia andrei and foetida) in presence of different types and amounts of phyllosilicates (kaolinite, montmorillonite) and an iron oxide (goethite) and combinations of these minerals. Our experimental setup included a high OM:mineral ratio to represent the soil-litter interphase. We monitored OC mineralisation during 196 days. Additionally, we investigated physicochemical parameters and chemical OM characteristics of decomposition products by determination of water-soluble OC (WSOC) and acquisition of solid-state 13C NMR spectra. We also analysed microscale organisation of the organo-mineral associations produced with and without earthworms by transmission electron microscopy (TEM).Earthworms enhanced OC mineralisation in all treatments. They also led to greater reductions of OC emissions in the presence of minerals as compared to the mineral-free control, depending on the type and amount of minerals added. The presence of earthworms affected microbial biomass, the concentration of WSOC and increased the contribution of aromatic compounds to OM decomposition products. Microscale analyses by TEM showed that earthworms favoured association of minerals with partly degraded OM along with completely degraded material, while in absence of earthworms only completely degraded OM was associated with minerals. We conclude that earthworms impact OM decomposition through (1) their effect on microbial biomass and the physicochemical parameters of microbial habitat and (2) the formation of OM associations by changing the OM types associated to minerals and possibly by creating closer association of partly degraded OM and iron oxides. The stability of these associations remains to be investigated.

Biochar (BC) amendments may be suitable to increase the ecosystems resistance to drought due to their positive effects on soil water retention and availability. We investigated the effect of BC in situ ageing on water availability and microbial parameters of a grassland soil. We used soil containing 13C labeled BC and determined its water holding capacity, microbial biomass and activity during a 3 months incubation under optimum and drought conditions. Our incubation experiment comprised three treatments: soil without BC (Control), soil containing aged BC (BCaged) and soil containing fresh BC (BCfresh), under optimum soil water (pF 1.8) and drought conditions (pF 3.5). Under optimum water as well as drought conditions, soils containing BC showed higher soil organic carbon (SOC) mineralization as compared to control soil. Moreover, BC effects on the soil water regime increase upon in situ aging. Native SOC mineralization increased most for soils containing BCaged. The BCaged led to improved C use under drought as compared to the other treatments. We conclude that BC addition to soils can ameliorate their water regime, especially under drought conditions. This beneficial effect of BC increases upon its aging, which also improved native substrate availability.

Organic matter (OM) amendments, originating from waste materials, could be used to enhance soil organic carbon (SOC) storage and fertility of cropland soils. However, there is a limited understanding about the long-term effect of different urban waste compost amendments on soil organic matter (SOM) formation processes and their impact on SOC storage. Accordingly, the long-term effects of different OM amendments on the amount and composition of particulate and mineral associated SOM were investigated. Surface soils were sampled from a Luvisol under cropping rotation, which received biannually 0.4 kg organic carbon m 2 in form of three different urban composts or cattle manure for a period of 15 years. Despite similar C input, different urban waste compost amendments resulted in contrasting C storage. While there were no C stock changes for municipal solid waste compost amended soils, composts from organic waste and green waste and sewage sludge increased SOC stocks in a similar range as conventional farmyard manure. In bulk soils, SOC stocks were increased by approximately 30%, in occluded particulate OM

International audience; Despite their importance for C sequestration, especially in the subsoil, little is known about decomposition and stabilisation processes affecting root litter in soil horizons at different depths. In particular the influence of specific conditions at depth on molecular alterations of degrading root litter is unknown. We took advantage of a decomposition experiment, which was carried out at different soil depths under field conditions and sampled litterbags with C-13-labelled wheat roots, incubated in subsoil horizons at 30, 60 and 90 cm depth for up to 36 months. Changes of bulk root chemistry were studied by solid-state C-13 NMR spectroscopy, and lignin content and composition was assessed after CuO oxidation. Compound-specific isotope analysis allowed assessment of the role of root lignin for soil C storage at the different soil depths. Results indicated that decomposition proceeded in a similar way at all three depths, but at a different rate. The alkyl/O-alkyl C ratio was a meaningful indicator to assess the degree of root litter degradation within the mineral soil. After three years, the greatest increase of this ratio, corresponding to the most advanced degradation degree, occurred at 30 cm compared to the lower depths despite a similar carbon loss. The greater proportion of O-alkyl C persisting in deeper subsoil horizons was consistent with their higher clay content. Root derived lignin-C concentration decreased at all soil depths and soil lignin content reached a similar level after 12 months, suggesting that microbial communities in all subsoil depths had capability to degrade lignin. However, the intensity of degradation appeared to be different at different soil depths, with lignin being less transformed at 60 and 90 cm depth. We conclude that chemistry of subsoil organic matter is determined by horizon-specific conditions, which have to be fully understood in order to explain the long residence times of subsoil C. In our study physico-chemical parameters only partly explained the observations.

International audience; Our aims were to characterize the fate of leaf-litter-derived nitrogen in the plant-soil-microbe system of a temperate beech forest of Southern Germany and to identify its importance for N nutrition of beech seedlings. N-15-labelled leaf litter was traced in situ into abiotic and biotic N pools in mineral soil as well as into beech seedlings and mycorrhizal root tips over three growing seasons. There was a rapid transfer of N-15 into the mineral soil already 21 days after tracer application with soil microbial biomass initially representing the dominant litter-N sink. However, N-15 recovery in non-extractable soil N pools strongly increased over time and subsequently became the dominant N-15 sink. Recovery in plant biomass accounted for only 0.025 % of N-15 excess after 876 days. After three growing seasons, N-15 excess recovery was characterized by the following sequence: non-extractable soil N >> extractable soil N including microbial biomass >> plant biomass > ectomycorrhizal root tips. After quick vertical dislocation and cycling through microbial N pools, there was a rapid stabilization of leaf-litter-derived N in non-extractable N pools of the mineral soil. Very low N-15 recovery in beech seedlings suggests a high importance of other N sources such as root litter for N nutrition of beech understorey.

International audience; C-13 contents of organic matter are changing during decomposition of plant material and stabilization as soil organic carbon (SOC). In this context, several studies showed C-13 enrichment in soil as compared to vegetation for C-3 forests, whereas depletion of C-13 was frequently reported for C-4 grassland soil as compared to C-4 vegetation. These changes were often attributed to selective preservation and/or stabilization of specific organic compounds. This study investigates if changes in the chemical composition of OC and specifically lignin may explain the observed shifts in delta C-13 values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C-4 grasslands and C-3 Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope (C-12/C-13) composition was analyzed for total organic carbon (OCtot) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state C-13 NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the C-13/C-12 isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OCtot C-13 depletion in C-4 grassland soil and C-13 enrichment in C-3 forest soil compared to the corresponding vegetation). In this regard, the isotopic difference between roots and aboveground biomass was not relevant, but may become more important at greater soil depths. C-13 depletion of VSC lignins relative to Qat was higher in C-3-biomass and C-3-derived SOC compared to the C-4 counterparts. As lignin contents of heavy fractions were low, in particular for those with C-4 isotopic signature, the influence of lignin on OCtot delta C-13 values in grassland topsoils is presumably low. Rather, the presence of charred grass residues and the accumulation of alkyl C in heavy fractions as revealed by C-13 NMR spectroscopy contribute to decreasing delta C-13 values from grass biomass to C-4-derived heavy fractions. In forest topsoils, the accumulation of C-13 depleted VSC lignin residues in heavy fractions counteracts the prevailing C-13 enrichment of OCtot from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high C-13 contents like microbial-derived OC have a stronger influence on delta C-13 values of OCtot in forest soils than lignins or aliphatic biopolymers. The mineral-associated SOC is in a late phase of decomposition with large contributions of microbial-derived carbohydrates, but distinct structural and isotopical alterations of lignin between C-4- and C-3-derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C-4 grassland and C-3 forest resulting from other kind of decomposer communities in association with distinct types and amounts of plant input as source of SOM and thus, carbon source for microbial transformation. Our results indicate that the importance of lignin for delta C-13 values of OCtot was overestimated in previous studies, at least in subtropical C-4 grassland and C-3 forest topsoils. (C) 2012 Elsevier Ltd. All rights reserved.

12 pages, 7 figures, 4 tables, 59 references., This study investigates if Araucaria forest (C3 metabolism) expansion on frequently burnt grassland (C4 metabolism) in the southern Brazilian highland is linked to the chemical composition of soil organic matter (SOM) in non-allophanic Andosols. We used the 13C/12C isotopic signature to group heavy organo-mineral fractions according to source vegetation and 13C NMR spectroscopy, lignin analyses (CuO oxidation) and measurement of soil colour lightness to characterize their chemical compositions. Large proportions of aromatic carbon (C) combined with small contents of lignin-derived phenols in the heavy fractions of grassland soils and grass-derived lower horizons of Araucaria forest soils indicate the presence of charred grass residues in SOM. The contribution of this material may have led to the unusual increase in C/N ratios with depth in burnt grassland soils and to the differentiation of C3- and C4 -derived SOM, because heavy fractions from unburnt Araucaria forest and shrubland soils have smaller proportions of aromatic C, smaller C/N ratios and are paler compared with those with C4 signatures. We found that lignins are not applicable as biomarkers for plant origin in these soils with small contents of strongly degraded and modified lignins as the plant-specific lignin patterns are absent in heavy fractions. In contrast, the characteristic contents of alkyl C and O/N-alkyl C of C3 trees or shrubs and C4 grasses are reflected in the heavy fractions. They show consistent changes of the (alkyl C)/(O/N-alkyl C) ratio and the 13C/12C isotopic signature with soil depth, indicating their association with C4 and C3 vegetation origin. This study demonstrates that soils may preserve organic matter components from earlier vegetation and land-use, indicating that the knowledge of past vegetation covers is necessary to interpret SOM composition., This study was funded by the Deutsche Forschungsgemeinschaft (DFG) under project Ko 1035/24-1.

International audience; Soil organic matter (SOM) stabilisation in subsoil horizons received much attention in recent years, due to the presence of compounds with very long residence times. The reasons for enhanced organic carbon stabilisation in subsoil horizons are poorly understood. In this study, we characterised SOM in adjacent soil compartments with different pedological functioning. We sampled SOM in visually identifiable zones in form of tongues and the adjacent soil matrix from deep soil horizons (60–140 cm depth) of 3 profiles under agricultural land. The samples were analysed for elemental and isotopic composition, radiocarbon age, chemical composition and lignin signature. The objective of the study was to examine if the tongues are characterised by contrasting carbon amounts and composition with regards to the soil matrix. Our results indicate that tongues have two times higher carbon content and are depleted in 15N with regards to the adjacent soil matrix. SOM in the tongues is characterised by up to modern radiocarbon ages, whereas SOM in the adjacent soil matrix is several thousand years old. Twenty percent more HF soluble carbon in the soil matrix suggest that more mineral bound, highly mobile SOM is present compared to tongues. Differences in chemical composition concern the lignin component, which seems to be preserved in the soil matrix. These data may be explained by different functioning in the two parts of the soil profile. In tongues, fresh carbon input by preferential flow and/or roots may lead to higher SOM turnover compared to the soil matrix. This heterogeneous distribution of stabilised SOM must be taken into account, when studying carbon sequestration in deep soil horizons.

The vegetation of the southern Brazilian highlands in Rio Grande do Sul State is a mosaic of grassland (C 4 ) and deciduous forests (C 3 ) with the conifer Araucaria angustifolia . It was uncertain, whether the grasslands represent relics of drier periods in the Holocene or if they are the result of deforestation in recent times. We analyzed plant tissues from gramineous and woody species, organic surface layers, as well as soil organic matter of 13 Andosols and Umbrisols in grassland, shrubland, pine plantations and Araucaria forest for stable carbon isotope ratios ( δ 13 C) and 14 C activity. The soil organic matter was separated into a free particulate organic matter (fPOM) and a heavy, organo-mineral fraction by density fractionation. All grassland soils have consistently δ 13 C values of − 18.7 to − 14.3‰ typical for C 4 grasses. In Araucaria forests and forest patches within grassland the δ 13 C values of both, the fPOM throughout the soil and the organic surface layers, are characteristic for the present below- and above-ground input from C 3 trees. The C 3 - and C 4 -derived SOC stocks reflect expansion of Araucaria forest on grassland, which started after 1300 yr BP. The youngest forests are found at the forest border and in forest patches. Grassland soils lose their typically black colour from the top downwards after shrub encroachment or establishment of forest as indicated by increasing melanic indexes which are closely related to the δ 13 C values. The natural 13 C depletion with depth in grassland soils counteracts the enrichment of 13 C in the subsoils of present Araucaria forest. The results clearly indicate that current grasslands represent relics at least from the early and mid Holocene period (6000–8000 yr BP) and are not the result of recent deforestation.

Hydrofluoric acid (HF) is able to dissolve most minerals, so it is often used to enrich organic matter (OM) in soils and sediments, though significant OM losses sometimes occur. The objective of this study was to examine these carbon losses and the hypothesis that HF dissolves the mineral-associated OM along with its mineral carrier. We compared composition, radiocarbon activity and particle morphology of untreated and HF-treated soil samples, using bulk samples, density fractions > 2 g cm � 3 and fine particle size fractions < 6.3 lm from a Haplic Podzol and a Dystric Cambisol. A positive correlation between HF-soluble organic carbon and organic carbon in the dense fraction confirms a linkage between the dissolved fraction and the mineral-associated OM fraction. However, treatment of heavy soil fractions shows that removal of the mineral-associated OM fraction is not complete. In the topsoil carbon losses amount to only 20–30% of the mineral-associated C, compared to 40–55% for the sub-soil of the Dystric Cambisol and 70–85% for the sub-soil of the Haplic Podzol. Compositional OM change as a consequence of HF treatment was not detected in upper soil horizons, but was substantial for the lower horizons. With depth or age of soil horizon, the HF-soluble OM becomes enriched in O-alkyl C, aryl C, and carbonyl C compared to the HF-resistant OM. The HF-soluble material in all horizons of the Dystric Cambisol is dominated by high alkyl C content, whereas O-alkyl C dominates in deeper horizons of the Haplic Podzol. In A horizons, radiocarbon ages of the HF-resistant OM fraction were similar, but slightly younger than for bulk soil radiocarbon ages. In contrast, in the subsoil horizons the HF-resistant fraction was up to 2000 yr older than the bulk soil age. � 2007 Elsevier Ltd. All rights reserved.

International audience; The effect of demineralisation using hydrofluoric acid (HF) treatment on the elemental, isotopic and chemical composition of organic matter with increasing degree of humification was examined using four horizons of a forest humus profile. The aim was to determine whether HF treatment alters pristine organic matter or not. The conceptual approach included elemental and isotopic analyses of the L, Of, Oh and Ah horizons of a Dystric Cambisol under forest before and after treatment with 2% and 10% HF. The chemical composition of the bulk organic matter was analysed using 13C CPMAS NMR spectroscopy, Fourier transformed infrared (IR) spectroscopy and pyrolysis coupled to gas chromatography and mass spectrometry (GC/MS). The lignin and non-cellulosic sugar components were characterised by wet chemical analysis. Carbon loss after HF treatment was between 7% and 23% of initial C and nitrogen loss between 14% and 27% of initial N. The HF concentration (2% and 10%) influenced the C and N concentration of the HF residue but in most cases not the C and N loss. No change in the isotopic signatures (δ13C and δ15N) were apparent. The bulk chemical composition as seen by 13C CPMAS NMR spectroscopy did not change after HF treatment. (FTIR) spectra of organic horizons were also not affected but analysis of organic matter of mineral rich Oh and Ah horizons became possible only after the treatment. These two methods were not sensitive enough to record changes in OM composition induced by the HF treatment. Pyrolysis GC/MS showed, in agreement with the wet chemical analysis of non-cellulosic sugars, that some change occurred in the quantity and composition of these saccharides. Lignin analysis suggested that some changes occurred in the composition of the molecule.

Research on soil organic matter (SOM) has attracted the increasing interest of scientists over recent decades. Undoubtedly, a major driving force for this progress is the search for soils as a sink for CO2, a policy-driven research area that arises as a consequence of the Kyoto Protocol. This is further emphasized by publication of the resolution of the European Parliament (published by the European Commission) of the communication Towards a Thematic Strategy for Soil (see: http://forum. europa.eu.int/Public/irc/env/soil/home), in which decline in soil organic matter status is identified as a major environmental threat. For a long time, research on soil organic matter was research on classical humic fractions. Current characterization of SOM has largely moved away from operational definitions based solely on chemical extraction procedures, which give ‘humic’ and ‘fulvic’ acids. Instead, definitions based on physical fractionations are preferred as physical separation of SOM is related to the role that organic matter plays in soil structure and soil function. These fractionation procedures aim to partition SOM into components that differ in their longevity (‘turnover time’), chemistry (structure and mass of molecules, types of functional groups), and origin (plant derived versus microbially derived). The study of the role of soils within the biogeochemical C cycle has made it obvious that we have an increasingly great need for better understanding and quantification of the reactions and mechanisms that control sources and sinks of nutrients, and of the biological, chemical and physical factors that regulate transformation processes, spatially and temporally, among these components. This extends to the all-important current debate about the contribution that soil organic carbon might make to the stabilization of atmospheric CO2 status in the long term (see also, Bellamy, P.H., Loveland, P.J., Bradley, R.I., Lark, R.M. & Kirk, G.J.D. 2005. Carbon losses from all soils across England and Wales 1978–2003. Nature, 437, 245–248

The aliphatic components of soil organic matter (SOM) are supposed to be of major importance in terms of carbon stabilisation and may be part of the soil’s stable carbon pool. Base hydrolysis releases ester-bound material, which represents a significant amount of OM. The aim was to evaluate the effect of this treatment on the chemical composition of (a) OM with increasing degree of humification and (b) OM with a considerable mineral matter content. Samples were taken from the litter layers (L and Of horizon) of a Dystric Cambisol. In addition, the humified Oh horizon containing considerable amounts of soil minerals and the mineral top soil (A horizon) were sampled. Samples were analysed for elemental and isotopic composition and chemical structure using 13C CPMAS NMR spectroscopy and pyrolysis GC mass spectrometry before and after base hydrolysis. Hydrolysis released 20% of carbon from horizons L and Of. Horizons containing OM with a high degree of decomposition and high mineral matter content (Oh and Ah) lost up to 41%. This may be related to (1) accumulation of aliphatic material with increasing humification and/or (2) solubilisation of other humic material due to the lack of specificity of the treatment. Structural analysis indicated that the material released by base hydrolysis contained similar amounts of aliphatic material in all four horizons. The contribution of O-alkyl carbon, assigned as polysaccharide-type material, to the solubilised OM was more important in the Oh and Ah horizons than in the L and Of horizons. In the former, the δ13C value of the remaining OM increased and the OC/O ratio decreased, giving evidence that the material is either highly humified and/or depleted in compounds with a low 13C content. The organic matter remaining after base hydrolysis still contained 10–18% alkyl carbon. These results were corroborated by analytical pyrolysis, which indicated a preservation of alkane/alkene pairs in the Oh and Ah horizons. Most of the aliphatic material remained in the Oh and Ah horizons and may be explained either by a large contribution of non-ester bound compounds and/or interaction of aliphatic material with soil minerals, thereby rendering it non-extractable.

In the Lusatian mining district, rehabilitated mine soils contain substantial amounts of lignite in addition to recent carbon derived from plant litter. The aim of this study was to examine the importance of the two organic matter types as substrates for soil microbial biomass in mine soils containing organic matter with a contrasting degree of humification. Samples were taken from the lignite-containing overburden material, from a mine soil under 14-year-old black pine (Pinus nigra) and from a mine soil under 37-year-old red oak (Quercus rubra). Overburden material was ameliorated with alkaline ash and incubated in an identical manner as the 14-year-old and 37-year-old mine soils for 16 months. Carbon mineralisation was monitored throughout. After 0, 3, 6, 12 and 16 months, samples were removed and analysed for chemical parameters and for microbial biomass. In addition, 14C activity measurements in bulk soil and microbial biomass were used to estimate their lignite content. Despite the high content of organic carbon in lignite-rich overburden material, low contents of microbial biomass were recorded. Ash-amelioration led to high pH values in the overburden material, resulting in high concentrations of dissolved organic carbon most likely derived from lignite. Development of the microbial community was subsequently stimulated by presence of an easily available carbon source. In older mine soils, larger amounts of microbial biomass are most likely related to the presence of recent organic matter. Radiocarbon analysis of the microbial biomass extracted from the 14-year-old mine soil indicated higher lignite carbon contribution than recorded for microbial biomass of the 37-year-old mine soil. The highest concentration of lignite C present in microbial biomass as indicated by the Cmic/Corg ratio was, however, observed in the ameliorated overburden material. Therefore, we conclude that the importance of lignite as a carbon source for micro-organisms decreases when recent organic matter is present in the older stages of mine soil development.

Aliphatic C most probably derived from ester-bound moieties was found to be present in sandy subsoil horizons. The hydrophobic nature of such compounds may increase their stabilization potential. Therefore, the aim of this study was to investigate the potential of aliphatic compounds in mineral soil horizons along a Dystric Cambisol profile under beech forest to increase hydrophobicity. The conceptual approach included the analyses of soil samples before and after solvent extraction and base hydrolysis for elemental and isotopic composition. Additionally, the advancing contact angle was measured to quantify hydrophobicity. Curie-point pyrolysis GC/MS was carried out to characterize the nature of alkyl C present in subsoil samples. A close correlation between the 14C activity and the stable-C-isotope ratio (δ13C) indicates isotopic fractionation upon C stabilization in subsoils. Free lipids contributed less than 10% to the organic C found in subsoil horizons. Base hydrolysis revealed very high amounts of hydroxyalkanoic acids in the B horizons of the acid forest soil. Hydrophobicity of SOM was not found to be correlated to esterified- or free-lipid content. The contact angle was in a similar range for all bulk soil horizons, suggesting greater hydrophobicity of organic matter in subsoil horizons considering their very low concentrations of organic C compared to the A horizon. The quantity and nature of pyrolysis products change with increasing depth in the soil profile. Aliphatic products cannot be detected in B and C horizons by Curie-point pyrolysis GC/MS.

Recent carbon inventories have shown that significant amounts of soil organic matter (SOM), even though at low concentrations, can be stored in the subsoil (B and C horizons). Its quantity, turnover and chemical composition are largely unknown. The objective of the present study was to analyse the organic matter in the mineral horizons of two forest soils with different pedogenetic history and to assess the quantity, turnover and chemical composition of SOM stored in the subsoil compared to the topsoil (A horizon and litter layers). Samples were taken from a Dystric Cambisol under beech and a Haplic Podzol under spruce forest down to a depth of 140 and 80 cm, respectively. They were analysed for elemental composition, carbon storage and chemical structure of SOM by 13C CPMAS NMR spectroscopy, radiocarbon age by accelerator mass spectrometry and plant derived phenols as well as hydroxyalkanoic acids by CuO oxidation. Special attention was drawn to the contribution of phenols and hydroxyalkanoic acids, because they are major contributors of root litter. Up to 75% of the organic carbon present in the mineral soil of the two profiles was found below the A horizon. Radiocarbon measurements showed that the organic carbon in the subsoil had an apparent age of several thousand years. The structural analyses indicated a similar chemical composition of the organic matter present in the litter layers of both soils. In the mineral soils, the chemical composition of the SOM differed according to the pedogenetic processes operating at the two sites. A high contribution of alkyl carbon was recorded in the B horizons of the Dystric Cambisol which may be partly explained by the contribution of hydroxyalkanoic acids which are preserved preferentially in this soil compared to phenols. In the Haplic Podzol, spectra of the B horizons indicate a higher contribution of O-alkyl and carboxylic carbon due to carbon leaching during podzolisation. In the C horizons of both soils, most of the organic carbon was mobilised after demineralisation by treatment with 10% hydrofluouric acid (HF) and may therefore be adsorbed to the soil minerals. Our data indicated that there is, apart from root litter, a strong influence of soil-forming processes on the composition of organic carbon in subsoils.

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle-size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM-EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF-soluble, or located in the density fraction d >1.6 g cm—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm—3 were closely correlated to the contents of oxalate and dithionite-citrate-bicarbonate-extractable Fe (r2 = 0.93 and 0.88, P 1.6 g cm—3 und in den Tonfraktionen angereichert. Die C-Anteile in der Dichtefraktion d >1.6 g cm—3 waren eng mit den Gehalten an oxalat- und dithionitloslichem Eisen korreliert (r2 = 0.93 bzw. 0.88, P

12 pages.