Humification Of Soil Organic Matter Research Articles

The addition of labile carbon alters litter fungal communities and decreases litter decomposition rates

Many field and laboratory studies have demonstrated that the addition of easily available carbon can increase rates of decomposition of plant litter or humified soil organic matter, though opposite trends are also common. In boreal forests, a large part of labile carbon enters soil via tree roots and ectomycorrhizal (EM) network. The influence of increased C availability and the presence of EM roots on litter decomposition was studied in a long-term field experiment in a 50-year-old Picea abies plantation. Litterbags containing litter of three species ( Populus tremula, Quercus robur, or P. abies) were buried to the depth of ca. 2.5 cm. The experiment was set up in a full-factorial design with carbon availability and presence of roots as factors. Carbon was added as an aqueous sucrose solution (50 g C m −2) every second week during snow-free period. Spruce roots around and underneath experimental plots were cut once a month. Subsets of litterbags were analysed after 12, 21 and 24 months. The serial dilution technique was used to assess the fungal community composition. The addition of labile carbon led to a strong decrease in litter decomposition rates, associated with consistent shifts in the composition of saprotrophic microfungal communities, and to an increase in the overall density of culturable fungal species ( r-strategists), but not of specialized cellulolytic microfungi ( K-strategists), isolated on Hutchinson agar. Sucrose addition did not affect the number of species isolated but the dominance structure was shifted toward the domination of the 2–4 most abundant species. The presence of EM roots significantly reduced moisture content in decomposing litter but had only weak and inconsistent effect on the decomposition rates and on the community composition of saprotrophic microfungi; these effects did not depend on the level of carbon availability.

Use of mathematical models for assessing the pool and dynamics of carbon in forest soils

The contribution of forest soils to the total carbon budget and to the emission of greenhouse gases is an important problem involved in many international programs, including the Kyoto Protocol. Direct measurements of the carbon pool in forest soils and its changes are slow and expensive; therefore, mathematical models are proposed in different countries for describing the dynamics of soil organic matter (SOM). The models differ in complexity and consider different processes of SOM mineralization and humification. The input parameters include model coefficients (these are usually the rates of decomposition and humification of different SOM compartments) and the initial values for different SOM pools. The coefficients can be estimated in special laboratory and field experiments, but the characteristics of the initial values for different SOM pools are usually absent. In this case, some assumptions about the character of SOM accumulation, which depends on forest vegetation, are used. The most realistic is the use of databases on the pools of carbon and other elements related to the types of forest or habitat conditions, including the primarily water regime and soil fertility. Under some suppositions, the agreement conditions between the main parameters of the SOM and forest vegetation can be formulated to assess the initial SOM pools in the forest litter and mineral horizons of the soil. An example of assessing the prediction of forest soil dynamics in Leningrad oblast was considered.

Mn oxide as a contaminated-land remediation product

Natural Mn oxides are an important component of biogeochemical cycles in many environmental settings. Mn oxides are strong oxidizing agents, facilitating the breakdown of organic contaminants and enhancing humification of soil organic matter. Interactions with metals and radionuclides, including surface adsorption, sequestration and oxidation can lead to incorporation of metals into insoluble mineral phases and a consequent reduction in bioavailability of toxic contaminants. Because of these properties, addition of Mn oxides may prove an effective treatment method for land contaminated by a range of organic and inorganic contaminants.

The mineralisation of fresh and humified soil organic matter by the soil microbial biomass

Soil organic matter comprises all dead plant and animal residues, from the most recent inputs to the most intensively humified. We have found that traces of fresh substrates at microg g(-1) soil concentrations (termed 'trigger molecules') activate the biomass to expend more energy than is contained in the original 'trigger molecules'. In contrast, we suggest that the rate limiting step in soil organic matter mineralisation is independent of microbial activity, but is governed by abiological processes (which we term the Regulatory Gate theory). These two findings have important implications for our understanding of carbon mineralisation in soil, a fundamental process in the sequestration of soil organic matter.

Organic carbon balance in Chilean volcanic soils after human intrusion and under different management practices

It is well known that humified soil organic matter (SOM) plays a central role in soil fertility due to its chemical and physical properties, especially by sustaining soil biological and related ecological equilibrium. Soil organic matter quantity and quality needs to be preserved in order to maintain soil productivity. In this study total organic carbon contents and their distribution were determined in several volcanic ash derived soils of Chile. Carbon balances are expressed in terms of a labile fraction (alkali-extractable less than 10.000 Dalton and polysaccharide compounds) and a stable fraction (fulvic, humic and humine compounds). The effects of different agronomic management on soil organic matter (SOM) quality and quantity are estimated and compared between agricultural and virgin forest soil sites. The results indicate that soils under native forest show a C level higher than agricultural soils. After human intrusion the stable C (m.w.>10,000 Daltons), viewed as a measurement of the humification grade, shows a mean value of 70% over the total C and follows the order: ultisols (‘Rojo arcillosos’)>andisols-placandepts (‘Ñadis’)>andisols-distrandepts (‘Trumaos’). C balance demonstrates that in agricultural soils the soluble C, or ‘labile C’ is increased and accounts for 12–30% of the total C; these organic compounds contribute largely to the solubilization, mobility and availability of plant nutrients but also enhance losses of organic carbon by lixiviation. By using the carbon balance methodology discussed in this work, changes of quality and quantity of SOM can be followed up. The SOM of Chilean volcanic soils is being degraded after human intrusion; this needs to be considered in cultivation of these soils in order to preserve their fundamental properties.

Simulated mechanisms of soil N feedback on the forest CO2 response

AbstractAn improved understanding of the response of forest ecosystems to elevated levels of CO2 in the atmosphere is crucial because atmospheric CO2 concentration continues to increase at an accelerating rate and forests are an important sink in the global carbon cycle. Several CO2‐enrichment experiments have now been running for more than 10 years, with highly variable short‐term results after the first decade. Responses to rising [CO2] over the next few decades will depend on several plant and ecosystem feedbacks that are inadequately understood. In this study, we conduct a sensitivity analysis, within the context of the simulated CO2 response, using a new version of the G'DAY ecosystem model, with an improved decomposition submodel, applied to a nitrogen‐limited Norway spruce forest site in the north of Sweden. The new decomposition model incorporates important modifications to soil processes, including some that constitute negative feedbacks on an ecosystem's growth response to elevated [CO2]. The sensitivity analysis reveals key parameters and processes that are important for the simulated CO2 response on the short term and others that are more important on the long term. A process that has a strong impact on the short‐term response is a change in decomposer composition, potentially in response to altered litter quality. Parameters that become increasingly important in the long term are carbon allocation to root exudates that are directly or indirectly associated with atmospheric N2 fixation, and the rate of humification of soil organic matter. We identify factors intrinsic to species and site (microbes and resources) and ecosystem nutrient supply that determine the duration of the enhanced simulated growth response to elevated [CO2].

Black (pyrogenic) carbon: a synthesis of current knowledge and uncertainties with special consideration of boreal regions

Abstract. The carbon (C) cycle in boreal regions is strongly influenced by fire, which converts biomass and detrital C mainly to gaseous forms (CO2 and smaller proportions of CO and CH4), and some 1–3% of mass to pyrogenic C (PyC). PyC is mainly produced as solid charred residues, including visually-defined charcoal, and a black carbon (BC) fraction chemically defined by its resistance to laboratory oxidation, plus much lower proportions of volatile soot and polycyclic aromatic hydrocarbons (PAHs). All PyC is characterized by fused aromatic rings, but varying in cluster sizes, and presence of other elements (N, O) and functional groups. The range of PyC structures is often described as a continuum from partially charred plant materials, to charcoal, soot and ultimately graphite which is formed by the combination of heat and pressure. There are several reasons for current interest in defining more precisely the role of PyC in the C cycle of boreal regions. First, PyC is largely resistant to decomposition, and therefore contributes to very stable C pools in soils and sediments. Second, it influences soil processes, mainly through its sorption properties and cation exchange capacity, and third, soot aerosols absorb solar radiation and may contribute to global warming. However, there are large gaps in the basic information needed to address these topics. While charcoal is commonly defined by visual criteria, analytical methods for BC are mainly based on various measures of oxidation resistance, or on yield of benzenepolycarboxylic acids. These methods are still being developed, and capture different fractions of the PyC structural continuum. There are few quantitative reports of PyC production and stocks in boreal forests (essentially none for boreal peatlands), and results are difficult to compare due to varying experimental goals and methods, as well as inconsistent terminology. There are almost no direct field measurements of BC aerosol production from boreal wildfires, and little direct information on rates and mechanisms for PyC loss. Structural characterization of charred biomass and forest floor from wildfires generally indicates a low level of thermal alteration, with the bulk of the material having H/C ratios still >0.2, and small aromatic cluster sizes. Especially for the more oxidation-resistant BC fraction, a variety of mainly circumstantial evidence suggests very slow decomposition, with turnover on a millennial timescale (in the order of 5–7 ky), also dependent on environmental conditions. However, there is also evidence that some PyC may be lost in only tens to hundreds of years due to a combination of lower thermal alteration and environmental protection. The potential for long-term PyC storage in soil may also be limited by its consumption by subsequent fires. Degraded, functionalized PyC is also incorporated into humified soil organic matter, and is transported eventually to marine sediments in dissolved and particulate form. Boreal production is estimated as 7–17 Tg BC y−1 of solid residues and 2–2.5 Tg BC y−1 as aerosols, compared to global estimates of 40–240 and 10–30 Tg BC y−1, respectively. Primary research needs include basic field data on PyC production and stocks in boreal forests and peatlands, suitable to support C budget modeling, and development of standardized analytical methods and of improved approaches to assess the chemical recalcitrance of typical chars from boreal wildfires. To accomplish these goals effectively will require much greater emphasis on interdisciplinary cooperation.

Simulating the dynamics of carbon and nitrogen in litter-removed pine forest

The model TREEDEV is developed based on the TREEDYN and TREEGROW models. In TREEDEV model the characteristics and available data of Masson pine ( Pinus massoniana) forest in Dinghushan Biosphere Reserve are taken into account as a demonstration. The model was validated by plantation forest and natural forest data. Calculation of tree growth is based on carbon and nitrogen balance, which depends on the photoproduction of its needles, respiration, nitrogen content of all organisms and that in soil, and other losses due to respiration, litterfall and renewal of stem, branch, needle and root. The model includes four aspects: (1) Exogenous driving force solar radiation, which is a function of latitude, specific meteorological conditions and seasonal time. (2) Canopy photoproduction, in this submodel the influence of temperature, solar radiation, seasonal time and nitrogen content of the soil are taken into account. (3) Biomass compartment into needles, fine roots, coarse roots, branches and stems, enabling us to calculate the appropriate respiration demands for the maintenance of the living biomass fractions, fine roots and needles renewal more accurately. (4) The cycle of carbon and nitrogen in soil, which is related to the decomposition and humification of soil organic matter and litter, nitrogen mineralization from litter, and nitrogen leaching. The dynamics of nitrogen include nitrogen uptake demand of the tree, the loss of nitrogen, and nitrogen relocation before litter shedding and the nitrogen supply from soil. Combined with field condition, scenarios involving litter removal and protective fencing were taken into account. Sensitivity analysis was performed in which 43 model independent variables were varied and analyzed in respect to their influence on 22 dependent variables describing forest state. Simulation results showed that litter removal has an influence on the growth of vegetation, which changes the environment, and retards the renewal of litter and reduces the uptake and fixation of available nitrogen, especially to needle and fine root. The forest with low intensity disturbance can be restored if it is fenced within an appropriate time (30 years in this paper), but this retards the maturation time of stands and depreciates the economic value of the wood. In this paper carbon and nitrogen cycles are analyzed in view of physiological processes with the goal of improving forest management planning.

Rayleigh distillation and the depth profile of 13C/ 12C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC (δ 13C SOC) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC ( C) and δ 13C SOC. Within 1 m of the soil surface, δ 13C SOC in the coarse-textured soil increases by 1.3 to 1.6‰, while δ 13C SOC from the fine-textured soil increase by as much as 3.8 to 5.5‰. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of δ 13C SOC in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of 13C in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of δ 13C SOC in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios.

Surface charge evolution of mineral-organic complexes during pedogenesis in Hawaiian basalt

Changes in surface charge of soil particles that accompany mineral transformations during soil formation were measured for a humid tropical chronosequence in Hawaiian basalt ranging in lava flow age from 0.3 to 4100 kiloyears (ky). Parent mineralogy is dominated by glass, olivine, pyroxene, and feldspar, whereas poorly crystalline (PC) weathering products (allophane, microcrystalline gibbsite, ferrihydrite) accumulate in early to intermediate weathering stages (through 400 ky), and crystalline secondary minerals (kaolinite, gibbsite, goethite) are dominant in the oldest (1400 and 4100 ky) soils. Detailed characterization of the solid phase was accomplished with chemical extractions, X-ray diffraction analysis, and molecular spectroscopy (FTIR and 13C MAS NMR). Simultaneous proton titration and background ion adsorption measurements were made on LiCl saturated soils over a range in pH (2–9) and ionic strength (0.001 and 0.01 M LiCl). Dependence of variable surface charge on solution composition reflects the changing nature of mineral-organic interactions over the course of pedogenesis. Points of zero net proton charge (PZNPC) ranged from 3.4 to 6.2 and 2.0 to 5.8 at 0.001 and 0.01 M ionic strength ( I), respectively. Intermediate-aged soils containing the highest mass concentration of humified soil organic matter (SOM) and its complexes with PC minerals gave rise to the steepest charging curves (largest pH dependence) and highest PZNPC values. Surface charge properties of these soils most closely reflected their weakly acidic Al and Fe hydroxide constituents, which is consistent with metal hydroxide saturation of organic functional groups, rather than organic coating of mineral surfaces. Charging curves were less steep and PZNPC values were lower for the older soils, consistent with SOM coating of more crystalline goethite, kaolinite, and gibbsite surfaces in a soil system less impacted by labile Al and Fe.

Nitrogen Mineralization from Humic Acid Fractions in Rice Soils Depends on Degree of Humification

Although the chemical nature of soil organic matter (SOM) is thought to affect the mineralization rate of N bound in SOM, little direct evidence exists for such effects. To test the hypothesis that the N mineralization rate is affected by the degree of SOM humification, we added equivalent amounts of humic acid (HA) N as either the labile mobile humic acid (MHA) fraction or the more humified calcium humate (CaHA) fraction to two lowland rice (Oryza sativa L.) soils, which were subsequently incubated under anaerobic conditions for 6 wk. The HA fractions had been chemically extracted from seven irrigated lowland rice soils from Vietnam and the Philippines. In both incubation soils, the amount of N mineralized from the added HA fractions decreased exponentially as the optical density of the HA at 465 nm (E4, index of humification) increased (R2 = 0.94–0.98). Mineralization of humic N was also strongly negatively associated with the amount of N or C contained in each HA fraction per kilogram of soil extracted, suggesting a more recalcitrant nature of the fractions in those soils where conditions allowed them to accumulate. Nitrogen mineralization was less for both the MHA (by 26%) and CaHA (by 41%) when incubated in the International Rice Research Institute (IRRI) soil compared with the Tanhoi soil. The most plausible explanation for this decrease is increased stabilization of the added HA by the relatively abundant Ca+2 in the IRRI soil. We conclude that the degree of humification of the MHA and CaHA fractions plays an important role in governing the rate of N mineralization in lowland rice soils because (i) N mineralization from these fractions was affected by their degree of humification, and (ii) the fractions are a small but important component of total soil N.

Increased N deposition retards mineralization of old soil organic matter

The aim of this study was to investigate the effects of increased N deposition on new and old pools of soil organic matter (SOM). We made use of a 4-yr experiment, where spruce and beech growing on an acidic loam and a calcareous sand were exposed to increased N deposition (7 vs. 70 kg N ha −1 yr −1) and to elevated atmospheric CO 2. The added CO 2 was depleted in 13C, which enabled us to distinguish between old and new C in SOM-pools fractionated into particle sizes. Elevated N deposition for 4 yr increased significantly the contents of total SOM in 0–10 cm depth of the acidic loam (+9%), but not in the calcareous sand. Down to 25 cm soil depth, C storage in the acidic loam was between 100 and 300 g C m −2 larger under high than under low N additions. However, this increase was small as compared with the SOM losses of 600–700 g C g C 0.25 m −1 m −2 from the calcareous sand resulting from the disturbance of soils during setting up of the experiment. The amounts of new, less than 4 yr old SOM in the sand fractions of both soils were greater under high N deposition, showing that C inputs from trees into soils increased. Root biomass in the acidic loam was larger under N additions (+25%). Contents of old, more than 4 yr old C in the clay and silt fractions of both soils were significantly greater under high than under low N deposition. Since clay- and silt-bound SOM consists of humified compounds, this indicates that N additions retarded mineralization of old and humified SOM. The retardation of C mineralization in the clay and silt fraction accounted for 60–80 g C m −2 4 yr −1, which corresponds to about 40% of the old SOM mineralized in these fraction. As a consequence, preservation of old and humified SOM under elevated N deposition might be a process that could lead to an increased soil C storage in the long-term.

N ISOTOPE FRACTIONATION AND MEASURES OF ORGANIC MATTER ALTERATION DURING DECOMPOSITION

Most soil organic matter (SOM) derives from plant material, but there are substantial gaps in our understanding of its transformation mechanisms. Alterations that occur as SOM decays and is stabilized have proved difficult to study owing largely to its diverse initial chemical composition and stable isotope values. We examined SOM stable isotope ratios in relation to composition using solid-state 13C Cross Polarization Magic Angle Spin (CPMAS) nuclear magnetic resonance (NMR) in sequentially deeper organic and mineral horizon soil samples collected from an unpolluted ecosystem in southeast Alaska, USA, dominated by C3 vegetation. We found that, as humified carbon (C not respired as CO2 during decomposition) increased in aliphaticity (ratio of unsubstituted aliphatics to carbohydrates), it also increased in 15N but showed no clear trend in 13C. These results imply that humification (and the concomitant stabilization of soil C) at our site resulted from microbial alteration of organics rather than from accumulation of recalcitrant compounds. The strong, and previously unreported, relation between 15N and SOM composition found at our study site suggests that degree of SOM humification may be correlated with an increase in δ15N SOM values relative to δ15N for fresh litter and other source material.

Characterization of soil organic matter in different particle-size fractions in humid subalpine soils by CP/MAS 13C NMR

The particle-size separation of subalpine soils was used for delineating the different functional C pools of soil organic matter (SOM) by solid-state cross-polarization/magic-angle-spinning (CP/MAS) 13C NMR spectroscopy. The soils collected from hemlock forest and dwarfed bamboo grassland along the slope positions at elevations from 2550 to 3200 m in central Taiwan, received high precipitation (>3000 mm). The obtained data show that organic matter fractions differ according to particle-size distribution and vegetations. Being more aliphatic, clay-size fractions were significantly different from bulk soils. Sand-size fractions generally gave very similar results to bulk soils. Alkyl-C (mainly polymethylene) increased from coarse sand (>250 μm) to clay (<2 μm) particles size in all sites, suggesting an accumulation of recalcitrant material in fine particle-size fraction. O-Alkyl-C (mainly carbohydrate) content decreased from coarse to fine particle size under the hemlock forest, whereas no consistent trend was found in dwarfed bamboo grassland soils. High amounts (>35%) of O-alkyl-C in all particle-size fractions in dwarfed bamboo grassland soils were attributed to that of the linkage of phenolic acids to cell-wall polysaccharides in bamboo litter. Based on a humification index of alkyl/ O-alkyl-C, the humification degrees in particle-size fractions are in the order: clay>fine silt>coarse silt>fine sand>coarse sand in both of hemlock forest and dwarfed bamboo grassland soil. The alkyl/ O-alkyl-C ratios in hemlock forest soil were consistently higher than those in dwarfed bamboo grassland soils, suggesting that humification of SOM is higher in the forest soil than in the dwarfed bamboo grassland soils. The 13C NMR analysis and the change of C/N ratio of the particle-size fractions indicate that the accumulation of recalcitrant material in fine particle size favors alkyl-C rather than aromatic-C in these subalpine soils. The low content of aromatic-C in soil might be due to the high precipitation in this subalpine area.

HUMIFICATION DEGREE OF SOIL HUMIC ACIDS DETERMINED BY FLUORESCENCE SPECTROSCOPY

The humification process increases semiquinone-type free radical concentrations (SFRC) in humus. Their quantification by electron spin resonance spectroscopy (ESR) has been a good indicator of the degree of humification of soil organic matter. As an alternative to relatively complex and expensive ESR spectroscopy, fluorescence spectroscopy was used in this study to evaluate the humification degree of 18 humic acids (HA) extracted from four Brazilian soils under different land use, tillage, or cropping systems. Two fluorescence humification indexes of HA were calculated based on work done by Zsolnay et al. and Kalbitz et al. in 1999, and a third new fluorescence index was proposed. Our proposal is to use the blue wavelength (465 nm) as the HA fluorescence excitation source. As this wavelength is more resonant with the humificated groups present in soil HA samples, our hypothesis is that the resultant fluorescence will provide information about these structures and, therefore, on the degree of humification of the HA. The SFRC varied from 1.90 × 10(17) to 14.75 × 10(17) spins/g HA, characterizing a wide range of the degree of humification of soil HA. The lowest SFRC occurred in native forests (1.90 × 10(17) to 7.50 × 10(17) spins/g HA) and pasture soils (2.30 × 10(17) to 4.64 × 10(17) spins/g HA). In cultivated sites, soil HAs from no-tillage soil were less humified (2.41 × 10(17) to 13.30 × 10(17) spins/g HA) than those in conventionally tilled soil (5.68 to 14.75 × 10(17) spins/g HA). The three fluorescence indexes show the same tendency as SFRC. Our fluorescence method was closely related to procedures found in the works of Zsolnay et al. and Kalbitz et al. (R∼0.9, P < 0.0001) and with SFRC (R∼0.85, P < 0.02), showing their potential as a simple and sensitive technique for evaluating the effect of land use and soil management systems on the humification degree of soil humic acids.

Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants.

Equilibrium sorption isotherms were measured for five different low-polarity organic compounds (benzene, trichloroethene, 1,2- and 1,4-dichlorobenzene, and phenanthrene) over a wide concentration range. The investigated sorbents can be grouped into the following three classes: (1) humic soil organic matter, which shows linear sorption isotherms (solely partitioning, as observed in the peat sample); (2) carbon materials, which were thermally altered (due to their natural history or industrial production) and thus contain a high specific surface area and exhibit nonlinear isotherms, and (3) pure engineered microporous materials (e.g., zeolites and activated carbon), where adsorption is solely due to a pore-filling process. Sorption of all compounds was fitted very well by the Polanyi-Dubinin-Manes (PDM) model, which for sorbents containing humic organic matter (e.g., peat) was combined with linear partitioning. Both the partitioning and the Polanyi-Dubinin-Manes model predict unique sorption isotherms of similar compounds if the solubility-normalized aqueous concentration is used. In addition, an inverse linear relationship between the distribution coefficient (Kd) and water solubility, which was very well confirmed by the data, is obtained. This also leads to unit-equivalent Freundlich sorption isotherms and explains the often observed apparent correlation between sorption capacity at a given concentration (e.g., Freundlich coefficient) and sorption nonlinearity (Freundlich exponent).

Carbon stocks in organic matter fractions as affected by land use and soil management, with emphasis on no-tillage effect

Land use and soil management may affect both labile and humified soil organic matter (SOM) fractions, but the magnitude of these changes is poorly known in subtropical environments. This study investigated effects of four land use and soil management systems (forest, native pasture, and conventional tillage and no-tillage in a wheat/soybean succession) on (i) total soil organic carbon (SOC) stocks (0 to 250mm depth) and on (ii) carbon (C) stocks in labile (coarse, light) and humified (mineral-associated, humic substances) SOM fractions (0 to 25mm depth), in a Hapludox soil from southern Brazil. In comparison to the adjacent forest site, conventionally tilled soil presented 36% (46.2Mg ha-1) less SOC in the 0 to 250mm depth and a widespread decrease in C stocks in all SOM fractions in the 0 to 25mm depth. The coarse (&gt;53 mum) and light (&lt;1kg dm-3) SOM fractions were the most affected under no-tillage, showing 393% (1.22Mg C ha-1) and 289% (0.55Mg C ha-1) increases, respectively, in relation to conventional tillage. Similar results were observed for mineral-associated SOM and humic substance C pools (34% and 38% increases, respectively) under no-tillage. Compared with labile SOM fraction results, the percentual increments on C stocks in humified fractions were smaller; but in absolute terms this C pool yielded the highest increases (3.06 and 2.95Mg C ha-1, respectively). These results showed that both labile and humified organic matter are better protected under the no-tillage system, and consequently less vulnerable to mineralization. Humified SOM stabilization process involving interactions with variable charge minerals is probably important in maintaining and restoring soil and environmental quality in tropical and subtropical regions.

Humic substances along the profile of two Typic Haploxerert

In Vertisols, organic matter contributes to soil colour by formation of organo-mineral complexes and affects morphological, physico-chemical, biological and biochemical properties. Turbation may affect the chemical and structural composition of the most stabilised fractions of soil organic matter (SOM), i.e., humic substances (HS). The objectives of this study were to: (1) characterise SOM in two Vertisols (V1 and V2) developed under Mediterranean climate in Italy, using some HS characteristics as indicators of SOM turnover in Vertisols, and (2) explore possible differences related to the pedomorphologic conditions of the area under which two soils have formed. SOM evolution along the soil profile was investigated by applying different techniques to chemically extracted HS. Humification parameters, thermal methods, such as Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) together with isoelectric focusing (IEF) were used. The results showed a significant difference in the chemical characteristics of SOM in the considered Vertisols. A higher amount of total and humified carbon was found in pedon V2. In particular, the results indicated an accumulation of humified compounds in deeper horizons of V2: this effect is particularly significant since both soils were under the same agricultural management and the same climatic conditions, but were different in physiografic position, slope and colour. The higher amount of not humified SOM in the pedon V1 could explain the differences found in the shape and size of soil structure between the two pedons. The different SOM evolution occurring in the two Vertisols apparently influenced the structure and composition of the HS fraction. Thermal analysis and isoelectric focusing data provided evidence of a different level of stabilisation of HS from the two Vertisols. The HS from pedon V2 have a higher molecular weight structure, which would imply a higher level of SOM stabilisation. Conversely, HS in the pedon V1 appeared to be richer in aliphatic and phenolic structures, which may imply the presence of less stabilised HS fractions. These findings agree well with the different water dynamics occurring in the two sites, which confirmed the influence exerted by water on chemical and structural composition of HS and, consequently, their effect in determining Vertisols colour. In conclusion, thermal and isoelectric focusing characterisation of HS appear to shed some light in understanding the structural complexity of Vertisols. The results obtained in the present study, if confirmed on a larger number of Vertisols, suggest the possibility of using HS as indicators of SOM turnover in relation to pedomorphology, use and management of soils.

Stocks and humification degree of organic matter fractions as affected by no-tillage on a subtropical soil

Assessment of particulate (>53-μm) and mineral-associated (<53-μm) soil organic matter (SOM) fractions is a useful approach to understand the dynamic of organic matter in soils. This study aimed to compare the long-term (9-yr) effects of no-tillage (NT) and conventional tillage (CT) on C and N stocks in the two above mentioned organic fractions in a Brazilian Acrisol. The degree of SOM humification, which has been associated with the concentration of semiquinone-type free radicals (`spin') determined by electron spin resonance (ESR), was also evaluated. Soil under no-tillage had 7.55 Mg ha−1 (25%) more C and 741 kg ha−1 (29%) more N than conventionally tilled soil in the 0–175-mm depth. Both particulate and mineral-associated SOM increased in the no-tilled soil. The increase of C and N stocks in the mineral-associated SOM accounted for 75% and 91% of the difference in total soil C and N stocks between NT and CT, respectively. Averaged across tillage systems, C and N stocks were respectively 4.6 and 16.8 times higher in the mineral-associated SOM than in particulate SOM. The higher C and N stocks were associated with greater recalcitrance of mineral-associated SOM to biological decomposition, resulting, probably, from its interaction with variable charge minerals. This is corroborated by a positive relationship between concentrations of C and iron oxides and kaolinite in the 53–20, 20–2 and <2-μm particle size classes, of the 0–25-mm soil layer. The degree of SOM humification, assessed by ESR, decreased in both the 53–20 and 20–2-μm fractions under NT. However, it was unaffected by tillage in the <2-μm fraction, which normally presented the lowest `spin' concentration. Since quality as well as quantity of SOM improved in the no-tillage soil, adoption of this system is highly recommended for amelioration of degraded tropical and subtropical soils.

Temporal and vertical organic matter differentiation along a vegetation succession as revealed by pyrolysis and thermally assisted hydrolysis and methylation

The origin and chemical nature of soil organic matter (SOM) in the ectorganic (L, F and H) horizons from four profiles from different stages of a primary vegetation succession on drift sand was studied to evaluate the impact of the vegetation on organic matter formation. In addition, the main sources of SOM, i.e. needles, leaves and (fine) roots of the covering species were analysed. Using conventional pyrolysis, virtually no differences were found between profiles in the same pine forest but with (chemically) different undergrowths ( Deschampsia flexuosa vs. Empetrum nigrum) that clearly delivered leaf and root litter to SOM. Thermally assisted hydrolysis and methylation (THM) of the same samples released compounds that were not detected by conventional pyrolysis without methylation. These compounds included methyl ethers and methyl esters of ((poly)hydroxy)alkanoic acids and were identified as building blocks of cutin and suberin. Both biopolymers appeared to be better tracers of the various inputs of plant material than lignin, particularly in case of the most humified SOM. Although there was evidence of alteration of cutin and suberin compositions in the course of the succession in the beech forest, an inventory was still possible of above- and below ground litter as sources of SOM.