Organo-mineral Interactions Research Articles

Corrigendum: Organo-mineral interactions in contrasting soils under natural vegetation

Corrigendum: Organo-mineral interactions in contrasting soils under natural vegetation

An adapted sequential chemical fractionation coupled with UV and fluorescence spectroscopy for calcareous soil organic matter study after compost amendment

This paper aims to improve the understanding of organic amendments distribution on different fractions of soil organic matter (SOM) and the knowledge of processes controlling the temporal evolution of organic matter (OM) in the first layer of a calcareous soil of two adjacent plots, one control and one treated with 3kgm−2 of organic amendment. An appropriate fractionation procedure of SOM, consisting in sequential extractions by water, hydrochloric acid, sodium hydroxide and potassium pyrophosphate extractions, was applied. Dissolved organic carbon (DOC), UV and fluorescence spectroscopic properties of organic matter reveal quantitative and qualitative aspects. Spectral indexes such as SUVA254 (specific UV absorbance), E2/E3 and FI (fluorescence index) were used to examine annual changes of SOM in the different fractions.Extractions with pyrophosphate, and in a lesser amount with sodium hydroxide, represented the major part of DOC, for both control and treated soils. Yields of sequential extractions ranged from 16.1% to 16.4% of TOC for these two soils. Compost inputs resulted in evolution of quantitative and qualitative results, especially for water extractable organic matter (WEOM) and pyrophosphate extracts mainly during the first 3months of this field experiment. Study of annual changes of DOC, SUVA254 and E2/E3 index values in treated soil indicated a priming effect during the weeks after compost spreading, in relation with a revitalizing effect of microbial activity. Contributions of organo-mineral interactions and rainfall regulated WEOM amount throughout the year, and the activity of microbial communities in soil seemed to play a significant role on WEOM production. FI index values demonstrated microbial origin for WEOM, and for OM extracted either with acid or with sodium hydroxide. At the contrary, pyrophosphate extracts revealed that OM came essentially from terrestrial origin, which corresponds to the most complex and aromatic compounds. It was found that sodium hydroxide extracted less aromatic SOM, but higher molecular weight compounds. 3D fluorescence spectra revealed the presence of four main types of fluorophores according to the different extracts: humic, fulvic and marine-like compounds (A, C and M) common to all extracts, and lignin-like compounds (H) specific to pyrophosphate extracts only in treated soil at the beginning of the experiment due to initial compost effect. PCA on quantitative and qualitative data for WOEM fraction revealed essentially that meteorological aspects, with temperature and precipitation variations, were an important parameter over the year of experimentation, and that compost input impacted significantly WEOM amounts at the beginning of the experiment.

Fate of humic acids isolated from natural humic substances

Composition of humic acids (HA) is a function of plant-derived inputs, degradation processes regulated by microorganisms, organo-mineral interactions and age. Characterization of different origin humic substances is important for evaluation of their contribution to stabile and labile carbon pool in the environment. The relative abundance of chemical components in HA isolated from soils, compost, commercial lignohumates, alginite, acadiane and lignite was studied with aim to quantify content of important biomarkers such as amino acid, lipids and polyphenols. HA were considered as a heterogeneous complex and high concentration of peptides, polyphenols and lipids was determined in acadian-HA to compare with soil-HA. Compost-HA contained much more amino acids to compare with soil-HA samples. Alginite-HA and lignite-HA were similar in biomarkers content to soil-HA. Fourier transform infrared spectroscopy confirmed that chemical composition and functional groups content differs with the origin, humification degree and the age of studied samples. Soil-HA are typically composed of a variety of −OH, COOH−, C–O, C–H2, (aliphatic and aromatic) groups, quinines, lignin fragments, polysaccharide, monosaccharide and proteins fragments, which are linked together by −O−, −NH−, −H=, >C=O, metal ions and –S− groups. 13C NMR spectroscopy showed that aromatic carbon content was the highest in lignite-HA and soil-HA.

Evaluation of seasonal variability of soil biogeochemical properties in aggregate-size fractioned soil under different tillages

An augment of soil organic matter (SOM) in agricultural lands is mandatory to improve soil quality and fertility and to limit greenhouse gases emissions. A better protection of SOM from degradation is seconded to its inclusion in aggregates and to the formation of organo–mineral interactions with the clay fraction within the soil matrix. Under Mediterranean conditions, conservation agriculture (CA) has been widely related with macro-aggregates formation, SOM protection, and to an improvement of soil fertility and crop yields.The objective of this work was to evaluate the biogeochemical properties of five aggregate-size fractions obtained by dry sieving of a Calcic Fluvisol of an experimental farm managed under three different tillages. Soil aggregates distribution, total organic carbon (TOC), labile carbon pools, and enzymatic activities were measured in 2 different periods of the same agricultural campaign. CPMAS 13C NMR analyses were also performed to elucidate the structure of preserved SOM.The results evidenced seasonal variability in aggregate distribution, labile carbon pools and dehydrogenase activity (DHA), whereas TOC, permanganate oxidizable carbon (POxC), and β-glucosidase activity demonstrated to be reliable soil quality indices for soil fractions. The NMR analyses showed a better SOM preservation under conservation tillages, due to higher plant litter inputs and/or higher amount of necromass derived compounds if compared with traditional tillage. Particularly interesting are the results of the Ø 0.5–1mm fraction, in which different trends were found for β-Glu and several organic compound classes if compared with the other fractions. Possibly, in this fraction are concentrated most of the products from cellulose depolymerization stabilized by organo–mineral interactions.

Analytical pyrolysis as a tool to probe soil organic matter

The environmental importance of soil organic matter (SOM) in the ecosystems and in the C biogeochemical cycle is well established. Indeed, it represents the main terrestrial carbon pool and due to its vulnerability, it plays a key role in the global carbon cycle. However, as SOM is mainly composed of products resulting from microbial and physicochemical transformations of vegetal, microbial and animal biomass, it results in a heterogeneous mixture. This complexity, along with organo-mineral interactions, makes challenging the characterization of SOM composition at the molecular scale. Nevertheless, its precise characterization is essential to determine its fate in the environment and eventually to provide recommendations on sustainable practices. Among the available techniques to analyse SOM, thermal degradations appear as especially efficient as they are less selective than some chemical ones, leading to a larger view of the SOM chemical structure. Analytical pyrolysis was thus used in a wide range of soil science fields including studies on pedogenesis and anthropic effects. It allows to characterize SOM at the molecular level, including identification of biomarkers, and to compare different soils and/or different horizons in a given soil profile under various impacts (land use, evolution, etc.). The review of recent developments in data acquisition and/or processing leads us to provide guidelines to select the most appropriate method and to avoid possible pitfalls. Examples will illustrate the wide range of soil science applications and show the potential and limitations of this approach.

Decoupled carbon and nitrogen mineralization in soil particle size fractions of a forest topsoil

To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long-term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000–20 μm), silt (20–2 μm), and clay (<2 μm). Bulk soil and particle size fractions were incubated in replicate, allowing periodic destructive sampling of triplicates at day 0, 14, 42, 84, 140, 210, and 280. We monitored CO2–C respiration, NH3–N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.01 M CaCl2) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. The chemical composition of selected samples was further characterized by 13C-NMR spectroscopy. Fractionation did not influence carbon mineralization (∑ fractions ≈ bulk soil), which decreased in the order sand > clay > silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure (∑ fractions < bulk soil) and followed the order clay > silt > sand. Fractionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N-alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach.

Effects of Surface Properties of Organic Matters on Cation Adsorption in Solution Phase

Organic substances have been commonly added to mine tailings to improve physical and chemical conditions and facilitate plant establishment. Apart from their nutritional benefits, aggregation and physical improvements may be resulted from organo-mineral interactions between their functional groups and cations of variable charges on mineral surfaces. Organic matters (OM), e.g., woodchips (WC) and sugarcane residue (SR), were initially evaluated for their affinities for Cu2+, Ca2+, and Fe3+ in relation to their surface properties at constant or different pH conditions in aqueous phase, before examining the formation of organo-mineral association in solid phase. Results indicated that WC and SR had different adsorption kinetics for these cations. Their surface negative charges indicated by zeta potentials were positively related to the adsorption capacities. 13C NMR analysis of the OM before and after adsorption revealed that carboxylic and phenolic groups were the major contributors to cation adsorption in solution. SR was found to have more carboxylic groups and thus had higher cation exchange capacity than WC. Therefore, organic materials with high density of functional anion groups on the surfaces may be advantageous when stimulating organo-mineral interactions in mine tailings which are rich in primary and secondary minerals with positively charged surfaces. The organo-mineral association between the OM and base metal mine tailings will be examined in the solid phase in a follow-up experiment.

Soft X-ray spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions.

There is a growing acceptance that associations with soil minerals may be the most important overarching stabilization mechanism for soil organic matter. However, direct investigation of organo-mineral associations has been hampered by a lack of methods that can simultaneously characterize organic matter (OM) and soil minerals. In this study, STXM-NEXAFS spectroscopy at the C 1s, Ca 2p, Fe 2p, Al 1s, and Si 1s edges was used to investigate C associations with Ca, Fe, Al, and Si species in soil clay fractions from an upland pasture hillslope. Bulk techniques including C and N NEXAFS, Fe K-edge EXAFS spectroscopy, and XRD were applied to provide additional information. Results demonstrated that C was associated with Ca, Fe, Al, and Si with no separate phase in soil clay particles. In soil clay particles, the pervasive C forms were aromatic C, carboxyl C, and polysaccharides with the relative abundance of carboxyl C and polysaccharides varying spatially at the submicrometer scale. Only limited regions in the soil clay particles had aliphatic C. Good C-Ca spatial correlations were found for soil clay particles with no CaCO3, suggesting a strong role of Ca in organo-mineral assemblage formation. Fe EXAFS showed that about 50% of the total Fe in soils was contained in Fe oxides, whereas Fe-bearing aluminosilicates (vermiculite and Illite) accounted for another 50%. Fe oxides in the soil were mainly crystalline goethite and hematite, with lesser amounts of poorly crystalline ferrihydrite. XRD revealed that soil clay aluminosilicates were hydroxy-interlayered vermiculite, Illite, and kaolinite. C showed similar correlation with Fe to Al and Si, implying a similar association of Fe oxides and aluminosilicates with organic matter in organo-mineral associations. These direct microscopic determinations can help improve understanding of organo-mineral interactions in soils.

Novel contribution on the diagenetic physicochemical features of bone and teeth minerals, as substrates for ancient DNA typing.

The extraction of DNA from skeletal remains is a major step in archeological or forensic contexts. However, diagenesis of mineralized tissues often compromises this task although bones and teeth may represent preservation niches allowing DNA to persist over a wide timescale. This exceptional persistence is not only explained on the basis of complex organo-mineral interactions through DNA adsorption on apatite crystals composing the mineral part of bones and teeth but is also linked to environmental factors such as low temperatures and/or a dry environment. The preservation of the apatite phase itself, as an adsorption substrate, is another crucial factor susceptible to significantly impact the retrieval of DNA. With the view to bring physicochemical evidence of the preservation or alteration of diagenetic biominerals, we developed here an analytical approach on various skeletal specimens (ranging from ancient archeological samples to recent forensic specimens), allowing us to highlight several diagenetic indices so as to better apprehend the complexity of bone diagenesis. Based on complementary techniques (X-ray diffraction (XRD), Fourier transform infrared (FTIR), calcium and phosphate titrations, SEM-EDX, and gravimetry), we have identified specific indices that allow differentiating 11 biological samples, primarily according to the crystallinity and maturation state of the apatite phase. A good correlation was found between FTIR results from the analysis of the v3(PO4) and v4(PO4) vibrational domains and XRD-based crystallinity features. A maximal amount of information has been sought from this analytical approach, by way of optimized posttreatment of the data (spectral subtraction and enhancement of curve-fitting parameters). The good overall agreement found between all techniques leads to a rather complete picture of the diagenetic changes undergone by these 11 skeletal specimens. Although the heterogeneity and scarcity of the studied samples did not allow us to seek direct correlations with DNA persistence, the physicochemical parameters described in this work permit a fine differentiation of key properties of apatite crystals among post mortem samples. As a perspective, this analytical approach could be extended to more numerous sets of specimens so as to draw statistical relationships between mineral and molecular conservation.

Using settling behaviour to study mesostructural organization of cement pastes and superplasticizer efficiency

Superplasticizers are polymer admixtures which have been used for decades in the building industry. Their role is to increase the workability of fresh concrete by dispersing cement particles through an electrostatic or steric repulsion mechanism. Their efficiency is generally related to their adsorption ability and intrinsic rheological behaviour of mineral materials. However, at an intermediate scale, mesostructural organization is a major parameter that controls not only the fluidity but also the homogeneity and the stability of a suspension. The aim of this paper is to highlight the relationship between mesostructure and organo-mineral interactions occurring between superplasticizers and mineral particles. The influence of the molecular structure of polycarboxylates (PCEs) on the destabilization phenomena was characterized. Stability ranges were assessed using Turbiscan MA 2000 and quantified by a Phase Separation Index (PSI) inspired by previous works. A relationship with mesostructural organization was established by correlation with the dispersion state characterized by laser granulometry. The affinity of PCEs with cement particles as a function of molecular structure was characterized by Total Organic Carbon (TOC) measurements.

Sorption affects amino acid pathways in soil: Implications from position-specific labeling of alanine

Organo-mineral interactions are the most important mechanisms of long-term C stabilization in soils. Nevertheless, a part of the sorbed low molecular weight organic substances (LMWOS) remains bioavailable. Uniformly labeling of substances by 14C or 13C reflects only the average fate of C atoms of a LMWOS molecule. The submolecular tool of position-specific labeling allows to analyze metabolic pathways of individual functional groups and thus reveals deeper insight into mechanisms of sorption and microbial utilization.Alanine labeled with 14C in the 1st, 2nd or 3rd position was adsorbed to five sorbents: two iron oxides with different crystalline structure: goethite and haematite; two clay minerals with 2:1 layers – smectite, and 1:1 layers – kaolinite; and activated charcoal. After subsequent addition of these sorbents to a loamy haplic Luvisol, we analyzed 14C release into the soil solution, its microbial utilization and 14CO2 efflux from individual C positions of alanine.All sorbents bound alanine as an intact molecule (identical sorption of 1st, 2nd or 3rd positions). The bioavailability of sorbed alanine and its microbial transformation pathways depended strongly on the sorbent. Goethite and activated charcoal sorbed the highest amount of alanine (∼45% of the input), and the lowest portion of the sorbed alanine C was microbially utilized (26 and 22%, respectively). Mineralization of the desorbed alanine peaked within the first 5 h and was most pronounced for alanine bound to clay minerals. The initial mineralization to CO2 of bound alanine was always highest for the C-1 position (–COOH group). Mineralization rates of C-2 and C-3 exceeded the C-1 oxidation after 10–50 h, reflecting the classical biochemical pathways: 1) deamination, 2) decarboxylation of C-1 within glycolysis, and further 3) oxidation of C-2 and C-3 in the citric acid cycle. The ratio between two metabolic pathways – glycolysis (C-1 oxidation) versus citric-acid cycle (oxidation of C-2 and C-3) – was dependent on the microbial availability of sorbed alanine. High availability causes a peak in glycolysis C-1 oxidation followed by an abrupt shift to oxidation via the citric acid cycle. Low microbial availability of sorbed alanine, in turn, leads to a less pronounced, parallel oxidation of all three positions and to a higher relative incorporation of alanine C into microbial compounds. Modeling of C fluxes revealed that a significant portion of the sorbed alanine was incorporated in microbial biomass after 78 h and was further stabilized at the sorbents' surfaces.Position-specific labeling enabled determination of pathways and rates of C utilization from individual molecule positions and its dependence on various sorption mechanisms. We conclude that position-specific labeling is a unique tool for detailed insights into the submolecular transformation processes, mechanisms and rates of C stabilization in soil.

Long-term influence of biochar on native organic carbon mineralisation in a low-carbon clayey soil

Biochar can influence native soil organic carbon (SOC) mineralisation through “priming effects”. However, the long-term direction, persistence and extent of SOC priming by biochar remain uncertain. Using natural 13C abundance and under controlled laboratory conditions, we show that biochar-stimulated SOC mineralisation (“positive priming”) caused a loss of 4 to 44 mg C g−1 SOC over 2.3 years in a clayey, unplanted soil (0.42% OC). Positive priming was greater for manure-based or 400°C biochars, cf. plant-based or 550°C biochars, but was trivial relative to recalcitrant C in biochar. From 2.3 to 5.0 years, the amount of positively-primed soil CO2-C in the biochar treatments decreased by 4 to 7 mg C g−1 SOC. We conclude that biochar stimulates native SOC mineralisation in the low-C clayey soil but that this effect decreases with time, possibly due to depletion of labile SOC from initial positive priming, and/or stabilisation of SOC caused by biochar-induced organo-mineral interactions.

Organo-mineral interactions in contrasting soils under natural vegetation

Organo-mineral interactions are important for the cycling and preservation of organic carbon (OC) in soils. To understand the role of soil mineral surfaces in organo-mineral interactions, we used a sequential density fractionation procedure to isolate 2.6 g cm-3 density fractions from topsoils (0-10 cm) of contrasting mineralogies. These soils were under natural vegetation of four major Australian soil types - Chromosol, Ferrosol, Sodosol and Vertosol. The soils and their organic matter (OM) contents were found to be partitioned in four distinct pools: i) particulate organic matter 2.6 g cm-3; and iv) Fe oxides dominant >2.0 g cm-3 (in the Ferrosol.) X-ray photoelectron spectroscopy was used to investigate organic C and N bonding environments associated within each density fraction. Mineral pools were shown to be enriched in distinct organic functional groups: phyllosilicate dominant fractions were enriched with oxidized OC species (C-O, C=O, O-C=O) and protonated amide forms; quartz and feldspar dominated fractions were enriched in aliphatic C and protonated amide forms; Fe oxides dominant fractions had the greatest proportions of oxidized OC species and were low in protonated amide forms. The enrichment of different C species was related to the interaction of functional groups with the mineral surfaces. These results demonstrate the potential of mineral surfaces in influencing the chemical composition of OM bound in surfaces reactions and subsequently the stability of OM in organo-mineral interactions.

Soil organic matter stability in organo-mineral complexes as a function of increasing C loading

Soil carbon (C) saturation behavior predicts that soil C storage efficiency observed under field conditions decreases as a soil approaches C saturation. This may be due to a decline in soil organic matter (SOM) stability as the result of changes in the type, strength or turnover time of organo-mineral interactions with increasing organic C input. The goal of this study was to test whether the stability of organic matter bound to soil minerals decreases as organo-mineral complexes approach C saturation with increasing C loading. A series of batch sorption experiments with natural dissolved organic matter (DOM) and soil mineral components was conducted to obtain organo-mineral complexes with a range of organic C loadings. The relative stability of C in these organo-mineral complexes was subsequently assessed using evolved CO2 gas analysis during thermal analyses and laboratory incubations. Results indicated that differences in dissolved organic C before and after sorption overestimated the amount of sorbed C when compared to differences in solid-phase C concentrations. Values of C/N, δ13C, and δ15N of the organo-mineral complexes were significantly smaller or more negative than initial soil samples or the stock DOM solution, consistent with the concept of molecular fractionation by sorption to minerals and suggesting that the composition of the organic matter in the organo-mineral complexes may have changed as the amount of sorbed organic matter increased. Observations that organic C loadings at maximum sorption did not substantially exceed 1 mg C m−2 and linear rather than asymptotic relations between sorbed C and initial DOM concentrations suggest that organo-mineral complexes may not have reached C saturation under the conditions in the batch sorption experiments of this study. The temperature at which half of the CO2 evolved during thermal analysis (i.e., CO2-T50) increased with increasing C loading, suggesting the sorbed C required greater energy input for combustion. Results of the laboratory incubations to determine relative biological stability of sorbed C were not consistent with the initial hypothesis. The size of potentially mineralizable C pool relative to the total sorbed C decreased with increasing C loading, even though this pool was being degraded more rapidly. Overall, the results did not support the hypothesis that SOM stability decreases with increasing C loading. In spite of generating a wide range of C loadings substantially greater than previous studies, the conditions of the DOM sorption experiments conducted in this study appeared unable to generate organo-mineral complexes exhibiting C saturation behavior. We speculate that measurable decreases in SOM stability may occur only once the threshold of C saturation is reached.

Modeling the vertical soil organic matter profile using Bayesian parameter estimation

Abstract. The vertical distribution of soil organic matter (SOM) in the profile may constitute an important factor for soil carbon cycling. However, the formation of the SOM profile is currently poorly understood due to equifinality, caused by the entanglement of several processes: input from roots, mixing due to bioturbation, and organic matter leaching. In this study we quantified the contribution of these three processes using Bayesian parameter estimation for the mechanistic SOM profile model SOMPROF. Based on organic carbon measurements, 13 parameters related to decomposition and transport of organic matter were estimated for two temperate forest soils: an Arenosol with a mor humus form (Loobos, the Netherlands), and a Cambisol with mull-type humus (Hainich, Germany). Furthermore, the use of the radioisotope 210Pbex as tracer for vertical SOM transport was studied. For Loobos, the calibration results demonstrate the importance of organic matter transport with the liquid phase for shaping the vertical SOM profile, while the effects of bioturbation are generally negligible. These results are in good agreement with expectations given in situ conditions. For Hainich, the calibration offered three distinct explanations for the observations (three modes in the posterior distribution). With the addition of 210Pbex data and prior knowledge, as well as additional information about in situ conditions, we were able to identify the most likely explanation, which indicated that root litter input is a dominant process for the SOM profile. For both sites the organic matter appears to comprise mainly adsorbed but potentially leachable material, pointing to the importance of organo-mineral interactions. Furthermore, organic matter in the mineral soil appears to be mainly derived from root litter, supporting previous studies that highlighted the importance of root input for soil carbon sequestration. The 210Pbex measurements added only slight additional constraint on the estimated parameters. However, with sufficient replicate measurements and possibly in combination with other tracers, this isotope may still hold value as tracer for SOM transport.

Strategies and Mechanisms of Building up and Stabilizing Organic Matter Stocks in Soils

Soil organic matter (SOM) has very important functions in the soil. It affects the soil physical, chemical and biological properties, and eventually affecting the overall soil and crop productivity. Increase in SOM matter is associated with an increase in soil and crop productivity. It also contributes to climate change mitigation through soil carbon sequestration. This paper discusses various soil management and/or farming strategies that contribute to the building up of SOM. The paper also highlights mechanisms that stabilize organic matter in the soil and protect it from rapid decomposition and its loss from the soil. Through reviewing of various research papers, literature shows that a number of strategies provide substantial contributions to building up of SOM. These include: conservation agriculture, crop rotations, cover cropping, agroforestry and afforestration, improved fallows, well managed pastures and organic farming. Various physical, chemical and biochemical mechanisms contribute to stabilization of organic matter and protect the accumulated SOM from rapid decomposition. Quantity and quality of organic materials, soil matrix and clay minerals, organo-mineral interactions and soil management practices are all important factors in SOM stabilization. From this review it can be pointed out that research based knowledge of both SOM accumulation strategies

Production and elimination of water extractable organic matter in a calcareous soil as assessed by UV/Vis absorption and fluorescence spectroscopy of its fractions isolated on XAD-8/4 resins

Water extractable organic matter (WEOM) in soil refers to a broad mixture of water-soluble structures recoverable in laboratory through predetermined procedures. In this work, we investigate the mechanisms controlling the occurrence of WEOM fractions isolated on XAD-8/4 resins in a calcareous soil profile as a function of soil depth and seasonal climatic changes. WEOM was roughly collected on a bi-weekly basis during eight consecutive months from an experimental field in the Mediterranean region, south-east of France. Each fraction was analysed for organic carbon content and further characterised using UV/Vis absorbance and 3D fluorescence spectroscopy.The concentrations of hydrophobic (HPOA), transphilic (TPIA) and hydrophilic (HPIA) acids fractions isolated from WEOM followed similar decreasing trends with increasing soil depth, but HPOA decreased most sharply. Specific UV absorbance at 254nm (SUVA254) indicated that HPOA's distribution along the soil column is strongly influenced by the selective removal of aromatic structures, which could be controlled by adsorption mechanisms involving organo-mineral interactions and/or by a shift towards less aromatic precursors in deeper soil layers. 3D spectra of fluorescence revealed the presence of four types of fluorophores: two humic-like (A and C) common to all fractions, one lignin-like (H) specific to HPOA particularly in the upper soil layer, and two protein-like (B1 and B2) specific to TPIA. The importance of the humic component in all fractions was verified by the highly significant correlations between their carbon content and the intensity of humic-like peaks. The aging index (IA/IC) and the fluorescence index (FI), two indicators derived from 3D fluorescence spectra, showed an increase in the microbial input and the degree of humification in HPOA as a function of increasing soil depth. By comparison, IA/IC and FI values in HPIA and TPIA indicated that both fractions were composed almost exclusively of microbial-derived structures, regardless of soil depth.Principal component analysis (PCA) showed the association of temperature (T) to the carbon content in all fractions extracted from the soil layers 0–30 and 30–60cm, suggesting possible role of microbial activity on their occurrence. The hydraulic regime in soil, controlled by the frequency of rainfall events, affected most significantly HPIA across the entire soil column. PCA, however, failed to identify any consistent trends relating the qualitative properties of WEOM fractions to climatic variables.

Clay fractions from a soil chronosequence after glacier retreat reveal the initial evolution of organo–mineral associations

Interactions between organic and mineral constituents prolong the residence time of organic matter in soils. However, the structural organization and mechanisms of organic coverage on mineral surfaces as well as their development with time are still unclear. We used clay fractions from a soil chronosequence (15, 75 and 120years) in the foreland of the retreating Damma glacier (Switzerland) and from mature soils outside the proglacial area (>700 and <3000years) to elucidate the evolution of organo–mineral associations during initial soil formation. The chemical composition of the clay-bound organic matter (OM) was assessed by solid-state 13C NMR spectroscopy while the quantities of amino acids and neutral sugar monomers were determined after acid hydrolysis. The mineral phase was characterized by X-ray diffraction, oxalate extraction, specific surface area by N2 adsorption (BET approach), and cation exchange capacity at pH 7 (CECpH7). The last two methods were applied before and after H2O2 treatment. We found pronounced shifts in quantity and quality of OM during aging of the clay fractions, especially within the first one hundred years of soil formation. The strongly increasing organic carbon (OC) loading of clay-sized particles resulted in decreasing specific surface areas (SSA) of the mineral phases and increasing CECpH7. Thus, OC accumulation was faster than the supply of mineral surfaces and cation exchange capacity was mainly determined by the OC content. Clay-bound OC of the 15-year-old soils showed high proportions of carboxyl C and aromatic C. This may point to remnants of ancient OC which were inherited from the recently exposed glacial till. With increasing age (75 and 120years), the relative proportions of carboxyl and aromatic C decreased. This was associated with increasing O-alkyl C proportions, whereas accumulation of alkyl C was mainly detected in clay fractions from the mature soils. These findings from solid-state 13C NMR spectroscopy are in line with the increasing amounts of microbial-derived carbohydrates with soil age. The large accumulation of proteins, which was comparable to those of carbohydrates, and the very low C/N ratios of H2O2-resistant OM indicated strong and preferential associations between proteinaceous compounds and mineral surfaces. In the acid soils, poorly crystalline Fe oxides were the main providers of mineral surface area and important for the stabilization of OM during aging of the clay fractions. This was indicated by (I) the strong correlations between oxalate soluble Fe and both, SSA of H2O2-treated clay fractions and OC content, and (II) the low formation of expandable clays due to small extents of mineral weathering. Our chronosequence approach provided new insights into the evolution of organo–mineral interactions in acid soils. The formation of organo–mineral associations started with the sorption of proteinaceous compounds and microbial-derived carbohydrates on mineral surfaces which were mainly provided by ferrihydrite. The sequential accumulation of different organic compounds and the large OC loadings point to multiple accretion of OM in distinct zones or layers during the initial evolution of clay fractions.

Effect of green and farmyard manure on carbohydrates dynamics of salt-affected soil

In Pakistan, salinization of agricultural lands is a serious problem prevailing in arid and semi-arid regions of the country. The main objective of the present study was to investigate the carbohydrates dynamics of the salt-affected soil, amended with different organic matter combinations and incubated for varying time periods (0, 15, 30 and 60 days) and temperatures (15 and 37 oC). Replenishment of soil organic matter (SOM) was made with Green manure (GM) and Farmyard manure (FYM). The soil pH, EC, Na+ and K+ contents were evaluated through standard methods. An improvement in the soil organic carbon (OC) and nitrogen (N) contents ranging from 0.10 to 1.40% and 0.05 to 0.55% was achieved through the selected soil amendment practices. Total carbohydrates were found to be enhanced with organic treatments alone or in combination, accounting for 13.20% to 26.30% of the total soil. Non-glycosidic interactions of soil carbohydrates with minerals were investigated by applying powdered X-Ray diffraction (XRD) analysis using a Rigaku wide range goniometer (PW 1050/23). Semi-quantitative analysis of the soil organo-mineral interactions, based on their relative frequency abundance, was expressed in terms of a system of nine components. X-Ray diffraction (XRD) analysis revealed interactions of varying magnitude namely major, minor, trace and rare between the soil carbohydrates and minerals.

Distribution of chemical compartments of soil organic matter and c stocks of a cambisol from south Brazil as affected by Pinus afforestation

Distribution and stocks of soil organic matter (SOM) compartments after Pinus monoculture introduction in a native pasture area of a Cambisol, Santa Catarina, Brazil, were investigated. Pinus introduction increased soil acidity, content of exchangeable Al+3 and diminished soil nutrients. Nevertheless, soil C stock increased in all humic fractions of the 0-5 cm layer after Pinus afforestation. In the subsurface, the vegetation change only promoted SOM redistribution from the NaOH-extractable humic substances to a less hydrophobic humin fraction. Under Pinus, soil organo-mineral interactions were relevant up to a 15 cm depth, while in pasture environment, this mechanism occurred mainly in the surface layer.