Micropore characteristics of organic matter pools in cemented and non‐cemented podzolic horizons

In Podzols, organic matter (OM) is stabilized mainly by interaction with minerals, as a direct consequence of pedogenic processes. Metal–organic associations strongly affect OM surface features, particularly microporosity. Cemented ortstein horizons (CM) may form during podzolization, accompanied by a spatial arrangement of OM on mineral surfaces, which differs from that in non‐cemented horizons (N‐CM). To investigate the metal–organic associations and their changes during pedogenesis, we selected both N‐CM and CM podzolic horizons, isolated NaClO‐resistant OM and compared the specific surface area (SSA) before and after OM oxidation. The SSA was assessed by using N₂, to detect the pores in the range of micropores (< 2 nm) and mesopores (2–50 nm), and CO₂, to measure a smaller microporosity (< 0.5 nm), which is not accessible to N₂. Only the N‐CM samples showed the typical increase in N₂‐SSA after the removal of labile OM, while a decrease was found in all CM horizons. The CO₂‐SSA revealed a large number of small micropores characterizing OM, both before and after oxidation. The smallest micropore classes (< 0.5 nm) were, however, more abundant in NaClO‐resistant OM, which had therefore a larger number of N₂‐inaccessible surfaces than the labile pool. The N₂‐SSA data thus indicated a more homogeneous coverage of mineral surfaces by stabilized OM in CM samples. Because of the abundance of small micropores, OM in these podzolic B horizons had extremely large CO₂‐SSA values (about 800 m² g^?1), with sharp differences between the NaClO‐labile OM (290–380 m² g^?1) and the NaClO‐stabilized pool (1380–1860 m² g^?1), thus indicating very reactive illuvial organic materials.     

Buffer capacities of podzolic and peat gleyic podzolic soils to sulfuric and nitric acids

Soil samples from the main genetic horizons of pale podzolic and peat gleyic podzolic soils from the Central Forest Reserve were subjected to a continuous potentiometric titration by sulfuric and nitric acids. The sulfate sorption capacity was determined in soil mineral horizons. The buffer capacity of mineral horizons of both soils to sulfuric acid was found to be higher than that to the nitric acid. This is explained by the sorption of sulfates via the mechanism of ligand exchange with the release of hydroxyl groups from the surfaces of Fe and Al hydroxide particles and edge faces of clay crystallites. The buffer capacity of organic horizons of the pale podzolic soil to sulfuric acid proved to be higher than that to nitric acid; in organic horizons of the peat gleyic podzolic soil, the buffer capacity to sulfuric acid was lower than that to nitric acid. The reasons for this phenomenon have yet to be investigated.     

Mineral surfaces and soil organic matter

The organic carbon content of soil is positively related to the specific surface area (SSA), but large amounts of organic matter in soil result in reduced SSA as determined by applying the Brunauer–Emmett–Teller (BET) equation to the adsorption of N₂. To elucidate some of the controlling mechanisms of this relation, we determined the SSA and the enthalpy of N₂ adsorption of separates with a density > 1.6 g cm^?3 from 196 mineral horizons of forest soils before and after removal of organic matter with NaOCl. Likewise, we investigated these characteristics before and after sorption of increasing amounts of organic matter to four mineral soil samples, oxides (amorphous Al(OH)₃, gibbsite, ferrihydrite, goethite, haematite), and phyllosilicates (kaolinite, illite). Sorption of organic matter reduced the SSA, depending on the amount sorbed and the type of mineral. The reduction in SSA decreased at larger organic matter loadings. The SSA of the mineral soils was positively related to the content of Fe oxyhydroxides and negatively related to the content of organic C. The strong reduction in SSA at small loadings was due primarily to the decrease in the micropores to which N₂ was accessible. This suggests preferential sorption of organic matter at reactive sites in or at the mouths of micropores during the initial sorption and attachment to less reactive sites at increasing loadings. The exponential decrease of the heat of gas adsorption with the surface loading points also to a filling or clogging of micropores at early stages of organic matter accumulation. Desorption induced a small recovery of the total SSA but not of the micropore surface area. Destruction of organic matter increased the SSA of all soil samples. The SSA of the uncovered mineral matrix related strongly to the amounts of Fe oxyhydroxides and the clay. Normalized to C removed, the increase in SSA was small in topsoils and illuvial horizons of Podzols rich in C and large for the subsoils containing little C. This suggests that micropores preferentially associate with organic matter, especially at small loadings. The coverage of the surface of the soil mineral matrix as calculated from the SSA before and after destruction of organic matter was correlated only with depth, and the relation appeared to be linear. We conclude that mineralogy is the primary control of the relation between surface area and sorption of organic matter within same soil compartments (i.e. horizons). But at the scale of complete profiles, the surface accumulation and stabilization of organic matter is additionally determined by its input.     

The phenanthrene‐sorptive interface of an arable topsoil and its particle size fractions

Sorption of organic chemicals in soil is affected by the properties and availability of surfaces. These surfaces are composed of diverse mineral, organic and biological components, forming a soil's ‘biogeochemical interface’. Phenanthrene was used to probe the hydrophobic sorptive capacity of the interface of an arable soil. Batch sorption experiments were carried out with the bulk soil as well as the fine (0.2–6.3 µm) and coarse (6.3–63 µm) particle size fractions of two arable topsoil samples with different organic matter (OM) contents from a Eutric Cambisol. The specific surface area (SSA) of the bulk soil and particle size fractions was determined by BET‐N₂ and EGME sorption. OM composition was characterized by solid‐state ¹³C NMR spectroscopy. No clear relationship was found between phenanthrene sorption and SSA. We conclude that phenanthrene probes a specific fraction of the soil interface that is not well represented by the traditional methods of SSA detection such as BET‐N₂ and EGME sorption. The sorption behaviour of phenanthrene may therefore provide a useful additional tool to characterize the specific affinity of the soil biogeochemical interface for hydrophobic molecules. Sorption capacity for phenanthrene increased after particle‐size fractionation, indicating that the reduced availability of the interface caused by the aggregated structure is important for the sorptive capacity of a soil. This should be considered when projecting data obtained from extensively treated and fractionated samples to the actual interaction with biogeochemical interfaces as they are present in soil.     

Stabilization of dissolved organic matter by sorption to the mineral soil

The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We quantified mineralized C by frequent CO₂ measurements in the headspace of the incubation vessels and calculated mean residence times by a double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence) and isotope (¹³C, ¹⁴C) measurements after hot-water extraction of OM.The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and ¹³C measurements indicated that aromatic and complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The ¹³C and ¹⁴C data showed that degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about 24 Mg C ha⁻¹ highlighting the importance of sorption for accumulation and preservation of OM in soil.     

Sorptive stabilization of organic matter by microporous goethite: sorption into small pores vs. surface complexation

Stabilization of organic matter (OM) by sorption to minerals is thought to be due to (i) sorption into small pores (Ø < 50 nm) that prevents hydrolytic enzymes approaching and decomposing the organic substrate or (ii) reduced availability of organic molecules after formation of strong multiple bonds by complexation of organic ligands at mineral surfaces. We tested these two potential mechanisms by studying the binding of dissolved OM to microporous goethite (α‐FeOOH). The size of organic molecules dissolved prior to and after equilibration with goethite was determined using atomic force microscopy (AFM). The goethite–OM complexes were analysed for bulk and surface elemental composition (by X‐ray photoelectron spectroscopy, XPS), specific surface area (SSA) and mesopore and micropore volumes (by N₂ adsorption/desorption), by scanning electron microscopy (SEM), and by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The absolute density of goethite–OM complexes was determined by gas pycnometry and the sorbed OM’s apparent density was calculated by assuming no major changes in the volumes of the goethite upon sorption of OM. The stability of the OM–mineral interactions was tested in desorption experiments and by treatment with NaOCl. Surface accumulation of OM by sorption decreased the N₂‐accessible SSA of the goethite, mostly because micropores (Ø < 2 nm) were rendered inaccessible to N₂. The decrease in accessibility of micropores was most pronounced at small surface OM concentrations. The majority of dissolved organic molecules detected with AFM prior to interaction with goethite were globular with a diameter of 4–10 nm, the rest were mainly linear, 20–100 nm long and 4–8 nm thick. After contact with goethite, the latter type of molecules dominated, which suggests preferential sorption of globular molecules. Their size exceeded or equalled the size of micropores and small mesopores (Ø < 10 nm) and so sorption therein is unlikely. Also, the changes in volumes of pores with a size of 2–50 nm were smaller than the estimated volume of the OM sorbed. The apparent density of sorbed OM always exceeded that of the freeze‐dried OM and was largest at small surface concentrations. DRIFT spectroscopy showed that most carboxyl groups at the goethite surface were in their complexed form. The proportion of complexed carboxyl groups dropped at larger surface concentrations, parallel to the decrease in micropore volume. Thus, micropores seem to favour the formation of multiple complex bonds per molecule. Scanning electron microscopy showed that at small surface concentrations, OM coated the goethite crystals and crystallites tightly, while at larger surface concentrations bulky accumulations of OM were more abundant. Even strongly desorbing reagents such as NaOH and Na pyrophosphate released only part of the sorbed OM. Treatment with NaOCl removed mainly bulky accumulations of OM; the OM tightly bound to goethite crystals was hardly affected by NaOCl. We conclude that molecules tightly bound via multiple complex bonds, probably at the mouths of small pores, are barely desorbable and resist the attack of chemical reagents and probably also of enzymes.     

An improved method for determining the specific surface areas of topsoils with varied organic matter content, texture and clay mineral composition

Measuring the specific surface area (SSA) of soils that contain much organic matter (OM) is problematic. The adsorption of p-nitrophenol (pNP) from xylene at room temperature yielded realistic values for the SSA of a wide range of clays, oxides and subsoils. Here we have extended the same measurement to some topsoils with varied OM content, texture and clay mineral composition. Specifically, we have compared the surface areas measured by adsorption of N₂, and, applying the BET equation, with the values obtained by adsorption of pNP, before and after treatment of the samples with hydrogen peroxide. In all instances, the removal by H₂O₂ of organic matter – albeit in part only – led to a marked increase in the SSAs measured by nitrogen because of the exposure of micropores previously blocked or covered by OM. The surface areas measured by pNP were appreciably larger than those obtained by the standard BET equation, and showed little change after removal of organic matter. However, the surface area of two smectite-rich samples measured by pNP increased substantially after peroxidation, presumably because smectite crystals decomposed during treatment with H₂O₂. The results suggest that, under the experimental conditions used, pNP could diffuse without hindrance into and through organic matter, enabling it to adsorb on to micropore surfaces within clay aggregates (domains). In keeping with this suggestion, the relation between the surface areas measured by pNP and the corresponding values calculated from the clay and OM contents, and clay mineral composition, of the soils was close to 1:1. An even stronger relation was observed between the measured and calculated values for cation exchange capacity.     

Assessing soil carbon lability by near infrared spectroscopy and NaOCl oxidation

The feasibility of near infrared (NIR) spectroscopy for quantifying labile organic matter (OM) in arable soils and for predicting soil refractory OM fractions was tested on 37 soils varying in texture and soil carbon (C) content. Three sets of arable soils (0-20 cm depth) were sampled from 1) long-term field experiments with different OM inputs, 2) individual sites with inherent with-in field gradients in soil texture and/or C content, and 3) from a range of different sites covering variations in management and geological origin. The labile OM fraction was defined by the CO₂ evolved from the soils incubated for 34 weeks while refractory OM was obtained by NaOCl oxidation.The labile fraction of the soil C accounted for 2-12% of the total soil C content. No systematic relationship between labile C content and total soil C or clay was found, but NIR spectra could be correlated well with the labile C fraction. A distinct, close linear relationship was found for C in soil before and after the NaOCl oxidation, indicating that this method was unable to reveal any additional information not contained in the total soil C measurement. NIR was also correlated with the NaOCl resistant C fraction, but this was considered to relate to the ability of NIR to predict total soil C contents. Thus NIR seemed to have the potential to estimate labile OM determined under laboratory incubations, while it still remains open how to identify and quantify refractory pools of soil OM.     

Retention of dissolved organic matter by illitic soils and clay fractions: Influence of mineral phase properties

The dependency of the retention of dissolved organic carbon (DOC) on mineral phase properties in soils remains uncertain especially at neutral pH. To specifically elucidate the role of mineral surfaces and pedogenic oxides for DOC retention at pH 7, we sorbed DOC to bulk soil (illitic surface soils of a toposequence) and corresponding clay fraction (< 2 μm) samples after the removal of organic matter and after removal of organic matter and pedogenic oxides. The DOC retention was related to the content of dithionite‐extractable iron, specific surface area (SSA, BET‐N₂ method) and cation exchange capacity (pH 7). The reversibility of DOC sorption was determined by a desorption experiment. All samples sorbed 20–40 % of the DOC added. The DOC sorption of the clay fractions explained the total sorption of the bulk soils. None of the mineral phase properties investigated was able to solely explain the DOC retention. A sorption of 9 to 24 μg DOC m^–2 indicated that DOC interacted only with a fraction of the mineral surface, since loadings above 500 μg m^–2 would be expected for a carbon monolayer. Under the experimental conditions used, the surface of the silicate clay minerals seemed to be more important for the DOC sorption than the surface of the iron oxides. The desorption experiment removed 11 to 31 % of the DOC sorbed. Most of the DOC was strongly sorbed.     

Organo‐mineral associations in temperate soils: Integrating biology,mineralogy, and organic matter chemistry

We summarize progress with respect to (1) different approaches to isolate, extract, and quantify organo‐mineral compounds from soils, (2) types of mineral surfaces and associated interactions, (3) the distribution and function of soil biota at organo‐mineral surfaces, (4) the distribution and content of organo‐mineral associations, and (5) the factors controlling the turnover of organic matter (OM) in organo‐mineral associations from temperate soils. Physical fractionation achieves a rough separation between plant residues and mineral‐associated OM, which makes density or particle‐size fractionation a useful pretreatment for further differentiation of functional fractions. A part of the OM in organo‐mineral associations resists different chemical treatments, but the data obtained cannot readily be compared among each other, and more research is necessary on the processes underlying resistance to treatments for certain OM components. Studies using physical‐fractionation procedures followed by soil‐microbiological analyses revealed that organo‐mineral associations spatially isolate C sources from soil biota, making quantity and quality of OM in microhabitats an important factor controlling community composition. The distribution and activity of soil microorganisms at organo‐mineral surfaces can additionally be modified by faunal activities. Composition of OM in organo‐mineral associations is highly variable, with loamy soils having generally a higher contribution of polysaccharides, whereas mineral‐associated OM in sandy soils is often more aliphatic. Though highly reactive towards Fe oxide surfaces, lignin and phenolic components are usually depleted in organo‐mineral associations. Charred OM associated with the mineral surface contributes to a higher aromaticity in heavy fractions. The relative proportion of OC bound in organo‐mineral fractions increases with soil depth. Likewise does the strength of the bonding. Organic molecules sorbed to the mineral surfaces or precipitated by Al are effectively stabilized, indicated by reduced susceptibility towards oxidative attack, higher thermal stability, and lower bioavailability. At higher surface loading, organic C is much better bioavailable, also indicated by little ¹⁴C age. In the subsurface horizons of the soils investigated in this study, Fe oxides seem to be the most important sorbents, whereas phyllosilicate surfaces may be comparatively more important in topsoils. Specific surface area of soil minerals is not always a good predictor for C‐stabilization potentials because surface coverage is discontinuous. Recalcitrance and accessibility/aggregation seem to determine the turnover dynamics in fast and intermediate cycling OM pools, but for long‐term OC preservation the interactions with mineral surfaces, and especially with Fe oxide surfaces, are a major control in all soils investigated here.     

A warmer climate reduces the bioreactivity of isolated boreal forest soil horizons without increasing the temperature sensitivity of respiratory CO2 loss

Changes in the carbon (C) balance of boreal forest ecosystems may impact the global C cycle and climate. The degree to which antecedent temperature regime and mineral protection of soil organic matter (OM) influence the temperature response of boreal soil C pools remains unknown, however. To investigate these phenomena on time scales relevant to anthropogenic climate change, we quantified the temperature response of four soil C pools (L, F and H organic horizons and B mineral horizon) within soil profiles collected from replicated sites representing two regions along a climate transect (“regional warming”) during a 480-day incubation at 5 and 15 °C (“experimental warming”). We hypothesized that 1) warmer region soils would exhibit reduced bioreactivity, a measure of C lability assessed via cumulative soil C mineralization, relative to colder region soils, paralleling a decrease in bioreactivity with depth in both regions, and 2) temperature sensitivity of C mineralization (denoted as Q₁₀) would increase with decreasing bioreactivity congruent with the “C quality-temperature” (CQT) hypothesis, with a smaller effect in mineral soil where physico-chemical protection likely occurs. Cumulative C mineralization decreased from surface L to deeper horizons and from the cold to warm region for organic F and H horizons only. This decrease in soil bioreactivity with depth was paralleled by an increase in Q₁₀ with depth as expected, except in mineral soil where Q₁₀ was similar to or lower relative to the overlying organic layer. The lower bioreactivity in F and H horizons of the warm relative to the cold region was not, however, associated with a greater Q_10. A warmer regional climate in these otherwise similar forests thus resulted in reduced bioreactivity of isolated soil C pools without increasing the temperature sensitivity of soil C mineralization. This suggests that assumptions about temperature sensitivity of C mineralization based on the propensity for isolated organic C pools to undergo mineralization may not be valid in some organic-rich, boreal forest soils.     

Short-term controls over inorganic phosphorus during soil and ecosystem development

Geochemical sorption and biological demand control phosphorus (P) retention and availability in soils. Sorption and the biota predominantly utilize the same inorganic form of P, from the same soil pool, on the same time scale, and thus are likely to compete for P as it flows through the available pool. In tropical soils, P availability is typically quite low and soil geochemical reactivity can be quite high. We tested whether greater P sorption strength in tropical soils resulted in lower biological uptake of available P. Since the strength of soil sorption and biological demand for P change as ecosystems develop and soils age, we used soils from the two upper horizons from three sites along a 4.1 million-year-old tropical forest chronosequence in the Hawaiian archipelago. We evaluated the strength of geochemical sorption, microbial demand, and the partitioning of added available P into biological versus geochemical soil pools over 48 h using a ³²PO₄ tracer. Soil sorption strength was high and correlated with soil mineral content. The amount of added phosphate geochemically sorbed versus immobilized by microbes varied more between the organic and mineral soil horizons than among soil ages. Microbial activity was a good predictor of how much available P was partitioned into biological versus geochemical pools across all soils, while sorption capacity was not. This suggests that microbial demand was the predominant control over partitioning of available P despite changes in soil sorption strength.     

Pedogenetische Differenzierung von Bodeneigenschaften auf Aggregatebene

Pedogenetic differentiation of soil properties in aggregates Besides the pedogenetic differentiation of soils in horizons a differentiation within horizons across aggregates seems possible. The objective of this study is to check if there is a differentiation of soil properties across aggregates. From a Braunerde, a Podzol-Braunerde, and 2 Podsols from Bavaria and Slovakia aggregates of 10–30 mm in diameter were selected manually from both topsoil and subsoil horizons and mechanically fractionated into a core and a surface fraction. In the aggregate fractions C_org, Al_o, and Fe_d were determined. C_org is generally depleted in the surface fractions of the A-horizons compared to the core fractions. This may be due to favoured microbial degradation of organic matter compared to the aggregate core and preferential leaching of organic C. In the subsoil horizons of the Braunerde C_org is lower in the aggregate surface fraction, in the Podzol, however, it is higher. In Podzols preferential C-input and sorption to aggregate surfaces seems to dominate. Lower Al_o? and Fe_d?concentrations in the aggregate surface fractions of all A-horizons may be explained by preferential acidification of aggregate surfaces as the aggregate surfaces mainly buffer the proton input into structured mineral soils. In the B-horizons only in Braunerde Al_o and Fe_d are lower in the aggregate surface fractions than in the core fractions. The Podzol B-horizons show preferential illuvial enrichment of sesquioxides at aggregate surfaces. Thus, pedogenesis results in the differentiation of soil properties not only between horizons but also within horizons on the level of aggregates. The resulting different chemical properties of aggregate surface and core fractions may affect the sorption capacity of structured soils.     

Droplet infiltration and organic matter composition of intact crack and biopore surfaces from clay‐illuvial horizons

The organic matter (OM) in biopore walls and aggregate coatings may be important for sorption of reactive solutes and water as well as for solute mass exchange between the soil matrix and the preferential flow (PF) domains in structured soil. Structural surfaces are coated by illuvial clay‐organic material and by OM of different origin, e.g., earthworm casts and root residues. The objectives were to verify the effect of OM on wettability and infiltration of intact structural surfaces in clay‐illuvial horizons (Bt) of Luvisols and to investigate the relevance of the mm‐scale distribution of OM composition on the water and solute transfer. Intact aggregate surfaces and biopore walls were prepared from Bt horizons of Luvisols developed from Loess and glacial till. The mm‐scale spatial distribution of OM composition was scanned using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The ratio between alkyl and carboxyl functional groups in OM was used as potential wettability index (PWI) of the OM. The infiltration dynamics of water and ethanol droplets were determined measuring contact angles (CA) and water drop penetration times (WDPT). At intact surfaces of earthworm burrows and coated cracks of the Loess‐Bt, the potential wettability of the OM was significantly reduced compared to the uncoated matrix. These data corresponded to increased WDPT, indicating a mm‐scaled sub‐critical water repellency. The relation was highly linear for earthworm burrows and crack coatings from the Loess‐Bt with WDPT > 2.5 s. Other surfaces of the Loess‐Bt and most surfaces of the till‐derived Bt were not found to be repellent. At these surfaces, no relations between the potential wettability of the OM and the actual wettability of the surface were found. The results suggest that water absorption at intact surface structures, i.e., mass exchange between PF paths and soil matrix, can be locally affected by a mm‐scale OM distribution if OM is of increased content and is enriched in alkyl functional groups. For such surfaces, the relation between potential and actual wettability provides the possibility to evaluate the mm‐scale spatial distribution of wettability and sorption and mass exchange from DRIFT spectroscopic scanning.     

Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model

Based on recent findings in the literature, we developed a process‐oriented conceptual model that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co‐action of various processes that are active under specific pedogenetic conditions. To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo‐mineral interactions for OM stabilization in the passive pool is well‐known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site‐ and horizon‐specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo‐mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo‐mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool.     

Air-drying changes the distribution of Hedley phosphorus pools in forest soils

Hedley labile phosphorus(P)pools in soil tend to be several times larger than annual forest requirements,even in highly weathered soils characterized by P limitation.The discrepancy between plant and soil P status could be partly attributable to the frequently adopted air-drying pretreatment that tends to increase soil P solubility.In this study,the effects of air-drying on the distribution of Hedley P fractions were examined using soils collected under 4 forest types at Gongga Mountain,southwestern China.The results showed that the microbial biomass P(Pmic)in the organic horizon decreased markedly after air-drying.The concentrations of Hedley labile P in the air-dried samples were 31%–73%more than those in the field-moist samples.Consequently,the air-drying-induced increments of Hedley labile P pools in the surface soil horizons were 0.8–3.8 times the annual plant P requirements.The organic horizon was more susceptible to the air-drying-induced increases in Hedley labile P than the mineral horizon,probably because of the stronger release of Pmicand disruption of soil organic matter.The quality of P,indexed by the ratio of Hedley labile P to slowly cycling P,shifted in favor of the Hedley labile fractions after air-drying,further revealing that air-drying changed the distribution of Hedley P pools in forest soils.These indicated that the effects of air-drying could not be ignored when interpreting the discrepancy between the P status of plants and the Hedley labile P pools in forest soils.To more efficiently evaluate the P status in forest soils via the Hedley fractionation procedure,the use of field-moist soils is recommended.     

Thermal oxidation does not fractionate soil organic carbon with differing biological stabilities

Thermal analysis techniques have been used to differentiate soil organic carbon (SOC) pools with differing thermal stability. A correlation between thermal and biological stability has been indicated in some studies, while others reported inconsistent relationships. Despite these controversial findings and no standardized method, several recently published studies used thermal analysis techniques to determine the biological stability and quality of SOC in mineral soils. This study examined whether thermal oxidation at temperature levels between 200°C and 400°C, combined with evolving gas analysis and isotope ratio mass spectrometry, is capable of identifying SOC pools with differing biological stability in mineral soils. Soil samples from three sites being under Miscanthus (C₄‐plant) cultivation for more than 17 years following former agricultural cropland (only C₃‐plant) cultivation were used. Due to natural shifts in ¹³C content, young and labile Miscanthus‐derived SOC could be distinguished from stable and old C₃‐plant‐derived SOC. The proportion of Miscanthus‐derived SOC increased significantly with increasing temperatures up to 350°C in bulk soil samples, indicating increasing oxidation of labile and young SOC with increasing temperatures. Use of density fractions to validate the thermally oxidized SOC from bulk soil samples revealed that the thermal oxidation patterns did not reflect the biological stability of SOC. The suggested biologically labile particulate organic carbon (light fraction from density fractionation) was clearly enriched in Miscanthus‐derived young SOC. The thermal oxidation patterns, however, revealed preferential oxidation of these biologically labile fractions not at low temperatures, but rather at higher temperatures. The reverse was found for the biologically stable mineral‐associated density fraction (heavy fraction). Based on different soil types, it was concluded that the thermal stability of SOC between 200°C and 400°C is not a suitable indicator of the biological stability of SOC and, thus, thermal oxidation is not capable of fractionating SOC pools with differing biological stability.     

Analyzing organic matter composition at intact biopore and crack surfaces by combining DRIFT spectroscopy and Pyrolysis‐Field Ionization Mass Spectrometry#

In the clay‐illuvial horizons (Bt) of Luvisols, surfaces of biopores and aggregates can be enriched in clay and organic matter (OM), relative to the bulk of the soil matrix. The OM composition of these coatings determines their bio‐physico‐chemical properties and is relevant for transport and transformation processes but is largely unknown at the molecular scale. The objective of this study was to improve the interpretation of spectra from Fourier transform infrared spectroscopy in diffuse reflectance mode (DRIFT) by using thermograms and released ion intensities obtained with pyrolysis‐field ionization mass spectrometry (Py‐FIMS) for a more detailed analysis of the mm‐scale spatial distribution of OM components at intact structural surfaces. Samples were separated from earthworm burrow walls, crack coatings, uncoated cracks, root channels, and pinhole fillings of the Bt‐horizons of Luvisols. The information from Py‐FI mass spectra enabled the assignment of OM functional groups also from spectral regions of overlapping DRIFT signal intensities to specific OM compound classes. In particular, bands from C=O and C=C bonds in the infrared range of wave numbers between 1,641 and 1,605 cm^?1 were related to heterocyclic N‐compounds, benzonitrile, and naphthalene. The OM at earthworm burrow walls was composed of chemically labile aliphatic C‐rich and rather stable lignin and alkylaromatic compounds whereas the OM of thick crack coatings and pinholes was dominated by heterocyclic N and nitriles and high‐molecular compounds, likely originating from combustion residues. In combination with Py‐FIMS, DRIFT applications to intact samples seem promising for generating a more detailed mm‐scale spatial distribution of OM‐related sorption and wettability properties of crack and biopore surfaces that may serve as preferential flow paths in structured soils.     

Selenium retention in the organic matter of Swedish forest soils

Fractions of selenium present in the soil profiles of three Swedish podzols were analysed using a sequential extraction scheme to characterize Se distribution among the organic and inorganic fractions. The process by which selenite deposited from the atmosphere is retained in a podzolic profile rich in organic matter was studied in a column experiment. Selenium present in organic fractions accounted for most of the Se extracted by Na₄P₂O₇/NaOH. All soil organic matter fractions, particularly those in the B horizons, were considerably enriched with Se as compared with plant biomass. The most enriched fraction was that containing hydrophobic fulvates which had C to Se ratios ranging from 33 000 to 80 000. The distribution of Se among the organic fractions differed markedly from that of sulphur. Selenite applied to columns continuously for 67 d was fixed very rapidly upon entering the forest floor layers, with 77% being recovered in the top 2 cm of the forest floor after the experiment. In column leachates from the surface layers, C to Se ratios decreased progressively following Se application. No effect specifically related to Se application was observed for leachates and soil horizons underlying Bs1. The mechanism responsible for the efficient and rapid Se immobilization by organic matter is unknown.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.