Soil Clay Fraction Research Articles

The potential to regain organic carbon in degraded soils: A boundary line approach

There is a physical limit to the storage capacity of soil organic carbon (SOC) that is dependent on fine particles in arable soils. Here, it is demonstrated that a ceiling of SOC storage can be identified utilizing a boundary line approach relating SOC to the clay fraction of soils in Ontario. The method can determine the deficit of SOC to establish the potential to regain SOC and ameliorate eroded landscape positions.

Model-Based Evaluation of Signal-to-Clutter Ratio for Landmine Detection Using Ground-Penetrating Radar

A regression model is developed in order to estimate in real time the signal-to-clutter ratio (SCR) for landmine detection using ground-penetrating radar. Artificial neural networks are employed in order to express SCR with respect to the soil's properties, the depth of the target, and the central frequency of the pulse. The SCR is synthetically evaluated for a wide range of diverse and controlled scenarios using the finite-difference time-domain method. Fractals are used to describe the geometry of the soil's heterogeneities as well as the roughness of the surface. The dispersive dielectric properties of the soil are expressed with respect to traditionally used soil parameters, namely, sand fraction, clay fraction, water fraction, bulk density, and particle density. Through this approach, a coherent and uniformly distributed training set is created. The overall performance of the resulting nonlinear function is evaluated using scenarios which are not included in the training process. The calculated and the predicted SCR are in good agreement, indicating the validity and the generalization capabilities of the suggested framework.

Distributions of cadmium and lead in peri-urban wetlands as influenced by soil organic matter, clay fraction, and moisture content

AbstractEnvironmental distribution of cadmium (Cd) and lead (Pb) were studied in two wetlands (alluvial plain and inland depression) in Ibadan, Nigeria. Mean and median values of variables measured across the wetlands showed that soil properties varied considerably. Hydraulic conditions, soil organic matter (SOM), and particle size distribution contributed to the distribution of heavy metals recorded. Profile distribution of Cd and Pb maintained two patterns: (1) linear distribution pattern, where concentrations of Cd and Pb increase and decrease respectively, with increase in soil depth and (2) middle depth enrichment. Active carbon (AC), SOM, and clay fraction affected the surface and profile distribution of Cd and Pb. Cadmium showed significant correlation (R2 = −0.61* and 0.64*) with AC and SOM, respectively, while Pb had R2 values of 0.77* and 0.57 with SOM and clay content, respectively. The non-residual fractions of the heavy metals increased with increasing metal loading and SOM. This study is use...

Soil acidification under long-term tobacco plantation results in alterations of mineralogical properties in an Alisol

ABSTRACTIn the present study, soil samples collected from 20-year (1993‒2013) long-term tobacco plantation and perennial fallow were analyzed to study the influence of soil acidification on soil physicochemical and mineralogical properties. Results showed a significant decrease of soil pH, Ca2+, Mg2+, Na+ and total exchangeable cations, except K+, under tobacco plantation than under fallow land. Further X-ray diffraction (XRD) analysis for 1 mm soil size demonstrated an increase of vermiculite and kaolinite, but an obvious decrease of original potassium-bearing mica and potassium feldspar, under tobacco plantation than under fallow. For the <2 μm soil clay fraction, the XRD patterns showed an obvious desilication and accumulation of Al and Fe under tobacco plantation than under fallow. Other potassium-bearing minerals, such as illite and chlorite, had also visibly weathered and slaked under tobacco plantation than under fallow, resulting in an increase of vermiculite and hydroxy-Al-vermiculite, and an accelerated decomposition of 2:1 minerals and formation of 1:1 minerals. These results demonstrated that long-term continuous tobacco plantation has exerted negative effects on soil acidification and maintenance of soil potassium-bearing minerals.

Microwave Acid Extraction to Analyze K and Mg Reserves in the Clay Fraction of Soils

ABSTRACT: Extraction of K and Mg with boiling 1 mol L-1 HNO3 in an open system for predicting K and Mg uptake by plants is a method of low reproducibility. The aim of this study was to compare the extraction capacity of different acid methods relative to hydrofluoric acid extraction for K and Mg. A further objective was to develop a chemical extraction method using a closed system (microwave) for nonexchangeable and structural forms of these nutrients in order to [...]

Fluoride in soil and plant

Fluorine is unique chemical element which occurs naturally, but is not an essential nutrient for plants. Fluoride toxicity can arise due to excessive fluoride intake from a variety of natural or manmade sources. Fluoride is phytotoxic to most plants. Plants which are sensitive for fluorine exposure even low concentrations of fluorine can cause leave damage and a decline in growth. All vegetation contains some fluoride absorbed from soil and water. The highest levels of F in field-grown vegetables are found up to 40 mg kg -1 fresh weight although fluoride is relatively immobile and is not easily leached in soil because most of the fluoride was not readily soluble or exchangeable. Also, high concentrations of fluoride primarily associated with the soil colloid or clay fraction can increase fluoride levels in soil solution, increasing uptake via the plant root. In soils more than 90 percent of the natural fluoride ranging from 20 to 1,000 μg g -1 is insoluble, or tightly bound to soil particles. The excess accumulation of fluorides in vegetation leads to visible leaf injury, damage to fruits, changes in the yield. The amount of fluoride taken up by plants depending on the type of plant, the nature of the soil, and the amount and form of fluoride in the soil should be controlled. Conclusively, fluoride is possible and long-term pollution effects on plant growth through accumulation of the fluoride retained in the soil.

Characterization of products of the early stages of pedogenesis in ornithogenic soil from Maritime Antarctica

SummaryLimited pedogenesis on the Antarctic continent results in unusual, but important, short‐range order inorganic phases and organo‐mineral complexes (both with broad peaks from X‐ray diffraction) in the clay fraction, which can be studied by sequential chemical extractions. We aimed to develop a better sequential extraction procedure to obtain more detailed discrimination of the short‐range order phases (SROP) of the clay fraction of soil from Maritime Antarctica. Selected chemical properties of the soil and clay fraction were related to different phases of these products. The total amount of SROP and organo‐mineral complexes in the clay fraction ranged from 15.1 to 43.9%, with greater occurrence in ornithogenic soil. The continuous incorporation of guano from penguins into the soil profile has contributed to increasing amounts of organo‐mineral complexes. Short‐range order phases have an important effect on soil chemical properties, such as an increase in non‐exchangeable acidity, quantity of hydroxyl groups on the surface of the colloidal fraction, the buffering capacity of Al3+ from non‐exchangeable and structural forms of the element, and a decrease in soil CEC. The greater number of sequential extraction steps enabled the separation of SROP and organo‐mineral complexes into two phases of stability, with a clear predominance of the more soluble forms.

Dust emission parameterization scheme over the MENA region: Sensitivity analysis to soil moisture and soil texture

The mineral dust emissions from arid/semiarid soils were simulated over the MENA (Middle East and North Africa) region using the dust parameterization scheme proposed by Alfaro and Gomes (2001), to quantify the effect of the soil moisture and clay fraction in the emissions. For this purpose, an extensive data set of Soil Moisture and Ocean Salinity soil moisture, European Centre for Medium-Range Weather Forecasting wind speed at 10 m height, Food Agricultural Organization soil texture maps, MODIS (Moderate Resolution Imaging Spectroradiometer) Normalized Difference Vegetation Index, and erodibility of the soil surface were collected for the a period of 3 years, from 2010 to 2013. Though the considered data sets have different temporal and spatial resolution, efforts have been made to make them consistent in time and space. At first, the simulated sandblasting flux over the region were validated qualitatively using MODIS Deep Blue aerosol optical depth and EUMETSAT MSG (Meteosat Seciond Generation) dust product from SEVIRI (Meteosat Spinning Enhanced Visible and Infrared Imager) and quantitatively based on the available ground-based measurements of near-surface particulate mass concentrations (PM10) collected over four stations in the MENA region. Sensitivity analyses were performed to investigate the effect of soil moisture and clay fraction on the emissions flux. The results showed that soil moisture and soil texture have significant roles in the dust emissions over the MENA region, particularly over the Arabian Peninsula. An inversely proportional dependency is observed between the soil moisture and the sandblasting flux, where a steep reduction in flux is observed at low friction velocity and a gradual reduction is observed at high friction velocity. Conversely, a directly proportional dependency is observed between the soil clay fraction and the sandblasting flux where a steep increase in flux is observed at low friction velocity and a gradual increase is observed at high friction velocity. The magnitude of the percentage reduction/increase in the sandblasting flux decreases with the increase of the friction velocity for both soil moisture and soil clay fraction. Furthermore, these variables are interdependent leading to a gradual decrease in the percentage increase in the sandblasting flux for higher soil moisture values.

Ecological peculiarities of cadmium dispersal at the urbanized terrain edaphotopes of the Steppe Dnieper region (shown by Dniprodzerzhinsk as an example)

The objective of this research is the clarification of ecological peculiarities of cadmium dispersal at the urbanized terrain edaphotopes of Dniprodzerzhinsk. Heavy metals have been widely discussed in a huge number of researches. Special attention should be paid to cadmium as one of the most hazardous environmental toxic agents. There exist multiple data on cadmium content in various soil types of Ukraine, but until the present time there’s no full information as for the accurate geochemical behavior of cadmium in the soils, as well as for the priority impact of one or another soil property upon its concentration in certain soil types – natural ones or anthropogenically modified ones. The applicability of such researches is first of all determined by the necessity to decrease the hazardous environmental consequences of soil contamination with cadmium, as well as to improve the living conditions of the population. Evaluation of the technology-related impact upon the city soil layer is of current concern in terms of monitoring that is corresponding to the modern tasks for the environmental condition control. The article highlights the issue of ecological peculiarities of cadmium dispersal at the urbanized terrain edaphotopes of Dniprodzerzhinsk: variety of cadmium concentration being formed under the natural and anthropogenous influence in the soils of Dniprodzerzhinsk makes difficult to solve the problem of soil contamination with such chemical element. Soils with various content of cadmium can be found within the limits of the city, underneath the steppe vegetation. Content differentiation is attributed to rather diverse set of processes in the soil and to various properties of the soils. Edaphotopes of all studied right-bank urbanized terrains are cadmium contaminated to any extent. Cadmium content in the soil layer of the administrative districts of Dniprodzerzhinsk is ascending as follows: Dniprovsky district (0.6–9.9 mg/kg) – Bagliy district (1.0–10.5 mg/kg) – Zavodsky district (1.5–10.8 mg/kg). Evaluation of intensity of the technology-related geochemical abnormalities in the city soils has been provided by the abnormality level factor. The lowest (weak) soil contamination level can be observed within the edaphotopes of the left-bank area of Dniprovsky district of the city. High and very high soil contamination levels can be observed within the urbanized terrains of the right-bank part of the city at Bagliy and Zavodsky districts (eastern, central and western areas). Results of the statistical processing of the experimental material show the ratios of correlation of cadmium content and mechanical composition of the soil and humus. Absence of cadmium correlation with physical clay-fraction of soil, as well as close significant relation with humus enable us to associate its presence in the soils with biogenous accumulation, being also the evidence of the anthropogenous impact upon cadmium content in the soil layer.

Plant–soil interactions in lower–upper montane systems and their implications in a warming world: a case study from the Maloti-Drakensberg Park, southern Africa

The potential impacts of climate change on mountain environments are gaining increasing attention given the inherent fragile nature and high biodiversity value of mountain biota. We pay particular attention to the soil–plant–climate interface given that such interactions and trade-offs may exert a strong influence on how plant species will respond to climate change in mountain environments. Our study site in the Maloti-Drakensberg Park, Drakensberg Mountains (southern Africa), is located at the juncture of two climato-ecological belts with relatively well-partitioned vegetation communities that should therefore allow vegetation shifts to be easily detected. We believe that this treeline ecotone is a key temperature threshold area and is therefore an ideal boundary to use for climate change research. Soils sampled from this ecotone revealed a broad suite of high nutrient levels, indicative of soils derived from nutrient-rich flood basalts, with the exception of key growth-limiting nutrients, i.e. available phosphorus and nitrogen. Together with other soil and site characteristics such as high soil organic matter and clay fractions, low temperatures, high rainfall and steep topography resulting in nutrient leaching and low pH soils, we reason that the soil nutrient economy in the Maloti-Drakensberg Park is more constrained than previously thought. This is also supported by the plant communities sharing strong links to the Cape Floristic Region, characterised by intrinsically nutrient-poor soils. Given the increased pool of soil nutrients potentially available under a warming regime, we propose a research framework outlining key areas of investigation given the potentially disastrous implications for plant biota adapted to a nutrient-poor soil environment.

Influence of Olea europea L. and Ficus Carrica L. fine root activity on the K biodisponibility and clay mineralogy of the rhizosphere

The objective of this study was to compare the effect of fine root activity of Olea europea L. and Ficus carrica L. of soil in its immediate vicinity (in the so-called rhizosphere zone). The study was conducted on two stations in Northern Algeria: Guendoul and Bouira. Olea europea L. and Ficus carrica L. roots significantly modified some chemical properties of rhizosphere soil. Increases of soil carbon, KNH 4 + and KHNO 3 - were observed in the Olea europea L. and Ficus carrica L rhizosphere soil at both stations. Bulk and rhizosphere soil clay mineralogy was similar. Interstratified illite-smectite, smectite-illite and illite were predominant in the clay fraction. Chlorite and kaolinite were less represented. The decomposition of XRD diffractograms of two soil clay fractions using the Decomp program revealed that Olea europea L. roots promote nK + storage in interlayer position. Indeed, the lower abscissa position of the gravity center (cg) of the X-ray patterns, the peak displacement of clays populations PCI, I/S, S/I toward illite peak position indicates an increase of “illite-like” layer content in the vicinity of Olea europea L. roots. Olea europea L. roots appeared to have more influence on the rhizosphere soil than Ficus carrica L. roots probably because of its higher root biomass and the greater activity of the tree in winter (contrary to Ficus Carrica L., Olea europea L. keep their leaves in winter). The two species underground activity seems to be well reflected in their respective rhizosphere.

Insights into tetrabromobisphenol A adsorption onto soils: Effects of soil components and environmental factors

Concerns regarding tetrabromobisphenol A (TBBPA), the most widely utilized brominated flame retardant in the world, are growing because of the wide application and endocrine-disrupting potential of this compound. To properly assess its environmental impacts, it is important to understand the mobility and fate of TBBPA in soil environments. In this study, the effects of soil components, dissolved organic carbon (DOC) and heavy metal cations on TBBPA adsorption onto two Chinese soils (red soil and black soil) were investigated using batch sorption experiments. The desorption behavior of TBBPA when the two soils are irrigated with eutrophicated river water was also investigated. The results showed that pH greatly affects the adsorptive behavior of TBBPA in soils. Iron oxide minerals and phyllosilicate minerals are both active surfaces for TBBPA sorption, in addition to soil organic matter (SOM). DOC (50mgOCL−1) exhibited a limited effect on TBBPA sorption only under neutral conditions. TBBPA sorption was only minimally affected by the heavy metals (Cu2+, Pb2+ and Cd2+) in the studied pH range. Eutrophicated river water significantly enhanced the desorption of TBBPA from red soil due to the change in soil solution pH. These findings indicate that mobility of TBBPA in soils is mainly associated with soil pH, organic matter and clay fractions: it will be retained by soils or sediments with high organic matter and clay fractions under acidic conditions but becomes mobile under alkaline conditions.

Effects of Soil Bulk Density on Gas Transport Parameters and Pore‐Network Properties across a Sandy Field Site

The gas diffusion coefficient, air permeability, and their interrelations with air‐filled porosity are essential for characterization of diffusive and convective transport of gases in soils. Variations in soil bulk density can affect water retention, air‐filled pore space, and pore‐network connectivity and tortuosity and, thereby, control gas diffusion and air permeability. Considering 86 undisturbed core samples with variable bulk densities that were extracted on a 15 by 15 m grid from the top layer of a sandy field, the effects of soil bulk density on gas transport parameters and the soil water characteristic were investigated. Interactions with soil organic matter, sand, and clay fractions were also examined. To evaluate bulk density effects, two constitutive parameters were derived from each of the three measured relationships. The Campbell pore‐size distribution index (b) and the air‐entry matric potential (ψae) were derived from the soil water characteristic; the diffusive percolation threshold (εDPT), the air‐filled porosity where gas diffusivity ceases to almost zero because of interconnected water films creating isolated–inactive air content, and a pore‐network connectivity index (A2) were derived from the gas diffusivity curve, and the analogous parameters convective percolation threshold (εCPT) and convective pore‐network connectivity index (B2) from the air permeability curve. All six parameters showed significant negative correlations with bulk density. To further account for the effects of both bulk density and macroporosity in parametric gas transport models, a diffusive‐analog macroporosity–dependent model (DAMP) for gas diffusivity and a generalized Kawamoto et al. model (GK) for air permeability, which yielded improved predictive capabilities when compared with previous models, were developed. Both new models apply a reference point of prediction at −100 cm H2O matric potential (macroporosity drained), corresponding to the point where analysis of pore‐network tortuosity (T) and equivalent pore diameter for gas transport (dg) showed diminishing effects of water blockage on gas transport in the sandy soil.

Direct Nitrous Oxide Emissions Related to Fertilizer-Nitrogen, Precipitation, and Soil Clay Fraction: Empirical Models

AbstractDirect nitrous oxide (N2O) emissions (DNEs) from croplands are required in national inventories of greenhouse gases. The Intergovernmental Panel on Climate Change (IPCC) guidelines provide an approach using direct emission factors (EFds) to estimate DNEs, which are constants for large regions. The goal of this paper is to establish empirical models to account for the temporal and spatial variations of EFds, which, apart from the nitrogen addition rate, also vary with a range of environmental factors, so as to enhance the accuracy of regional/national DNE estimates. Therefore, the seasonal/annual DNEs (n = 71) from upland croplands, which are the differences in N2O emissions between fields with and without fertilizer-nitrogen addition, were used to statistically relate DNEs to regulating factors including the fertilizer-nitrogen addition rate (FN), and environmental (climate and soil) factors. The multivariate stepwise linear regression results showed positive combined effects of Fn and clay fracti...

Interaction of humic acids with soil minerals: adsorption and surface aggregation induced by Ca2+

Environmental context Humic acids, important components of natural organic matter in soils, sediments and aquatic media, can interact with the surface of minerals affecting key environmental processes. In the presence of calcium, humic acids can also interact among themselves leading to molecular aggregates. We demonstrate that a solid mineral surface facilitates the formation of humic acid aggregates, and thus surface aggregation occurs under conditions where normal aggregation in solution does not occur. Abstract Humic acids (HAs) interact with the surface of mineral particles leading to the formation of clay–humic complexes that affect the transport of nutrients and contaminants in the environment, soil structure, soil erosion and carbon sequestration by soils. The interaction is influenced by the presence of multivalent ions, such as Ca2+, which enhances the uptake of HAs by the particles. This article reports the effects of Ca2+ on the interaction between a HA and a soil clay fraction, both obtained from the same soil sample. The study was performed by using zeta potential measurements, HA adsorption isotherms, Ca2+ adsorption isotherms and microscopy. The results show that at low HA concentrations and low Ca2+ concentrations HA adsorption takes place, but that at high concentrations surface aggregation and precipitation also takes place, a process that is seldom reported or analysed in the literature. HA adsorption isotherms only give the overall HA uptake by the solid but they do not allow differentiation of HA adsorption from surface aggregation. However, HA adsorption v. Ca2+ concentration plots and Ca2+ adsorption isotherms at different HA concentrations can distinguish these two processes quite clearly. In addition, surface aggregation could be undoubtedly observed with optical microscopy. Surface aggregation starts to take place at a 0.7mM Ca2+ concentration, which is lower than the Ca2+ concentration needed to start HA aggregation in solution. This indicates that the surface of soil minerals acts as a nucleation centre for HA aggregation.

Funções de pedotransferência para retenção de água e condutividade hidráulica em solo submetido a subsolagem

This experiment aimed to develop pedotransfer functions, based on soil physical-hydric attributes, to estimate the water retention and hydraulic conductivity. The experiment was carried out in area with a Conilon (Coffea canephora Pierre) coffee crop irrigated and subjected to subsoiling in the row of planting. Undeformed samples were collected on the row (P1) and inter row (P2) of the crop from the layer 0 to 0.80 m, to determine the water retention curve in the soil, and soil conductivity by using Guelph permeameter. The pedotransfer functions showed underestimates for the potentials 0, -500 and -1500 kPa and -6, -10, -33 and -100 kPa, respectively in P1 and P2. In P2 the average relative error for the potential percentage was 8.42%, which is higher than the P1 (4.87%), resulting in higher error in the estimates of height of water stored in the soil. The water retention curve for the subsoiled soil may be estimated with error less than 5%, by clay, total porosity, macroporosity, microporosity and soil density attributes, and the hydraulic conductivity by soil clay fraction. For non-subsoiled soil, the retention curve may be estimated with error less than 9%, by the total pore volume and macro and micropores and the hydraulic conductivity by the coarse sand fraction.

The variations in soil microbial communities, enzyme activities and their relationships with soil organic matter decomposition along the northern slope of Changbai Mountain

We present a comprehensive analysis of the changes in the structure and functioning of the soil microbial communities, as well as their driving factors along the northern slope of Changbai Mountain by analyzing soil phospholipid fatty acids (PLFAs) profiles and enzymatic activities. The four vertical zones of vegetation selected in this study were as follows: (1) mixed coniferous and broad-leaved forest (MCB) located below 1100m (site A and B); (2) dark-coniferous spruce-fir forest (DCF) located between 1100–1700m (site C); (3) Ermans birch forest (EB) located between 1700 and 2000m (site D); and (4) alpine tundra (AT) located above 2000m (sites E and F). The results showed that the total viable microbial PLFAs varied with altitude, with the highest value detected in the EB (site D) (76.8nmolg−1). The fungal/bacterial (F/B) and the Gram positive/Gram negative (G+/G−) ratios increased with elevation. The lower values of F/B obtained in the MCB (site A) and in DCF (site C) for G+/G－. The soil microbial community structure observed in the MCB (sites A and B) was different from that observed in the other four sites, with lower fungal biomass. The soil microbial activities such as β-glucosidase (βG), N-acetylglucosaminidase (NAG), acid phosphatase (AP) and leucine aminopeptidase (LAP) in the MCB (sites A and B) were significantly higher than those documented in the other four sites. The substantial differences in the soil microbial community composition were significantly correlated with the mean annual temperature (MAT), the mean annual precipitation (MAP) and soil temperature, silt and clay fraction, total phosphorus, nitrate nitrogen. However, the variations in soil enzyme activities were significantly correlated with soil nutrients. The MAP and nitrate nitrogen were the significant predictors of the variance in the soil microbial community and enzyme activity, respectively. The soil organic matter (SOM) decomposition rate showed a significant positive relationship with the total microbial, bacterial, actinomycetes PLFAs and the soil enzyme activities. Our results suggest that studies incorporating the microbial community structure and soil enzyme activity with the SOM decomposition may enhance our understanding of the mechanisms of biogeochemical processes.

Quantifying nitrogen loading from a paddy field in Shanghai, China with modified DNDC model

Nitrogen (N) contamination from rice paddy cultivation has threatened the quality of water resources in the upper reaches of the Huangpu River. In this study, a 3-year experiment (2009–2011) was conducted at a typical rice field in the Huangpu River watershed. The rates of N loading via surface runoff and subsurface leaching from the field were measured with a permanently installed lysimeter system. In the experiment, four treatments were tested, including applications of only chemical fertilizer (CT), only organic manure (OT), a mixture of the two types of fertilizers (MT) and a control (CK). The field data indicated that the average rates of the seasonal N loading induced by both runoff and leaching for CK, CT, MT and OT were 3.38, 16.79, 15.07 and 7.14kgN/ha, respectively. However, OT decreased the rice yields to 5482kg/ha, whereas MT maintained the optimal yields of 6818kg/ha. A process-based model, denitrification–decomposition or DNDC, was adopted to assist with interpreting and integrating the experimental results. DNDC has been calibrated and validated against the field data and accurately simulated the N losses via surface runoff and subsurface leaching from the paddy field. The modeled seasonal N loading rates for CK, CT, MT and OT were 3.25, 18.84, 15.54 and 6.46kgN/ha, respectively, which were in accordance with field observations. The modeled rice yields (4126–7365kg/ha) across the treatments were also in agreement with the observations (3788–6818kg/ha), though approximately 8% higher than observations. The results of sensitivity analysis indicated that increases in precipitation or fertilization or decreases in soil clay fraction increased the N loading rate. In addition, the scenario simulation tests indicated that the application of urea at a rate of 170kgN/ha was the optimal fertilization method for the rice field, which maintained the optimal rice yields with a relatively low rate of N loading of 11.55kgN/ha. The revised DNDC model proved to be an effective tool for assessing best management practices that reduce N loading in rice paddy field in China.

Carbon sequestration in clay and silt fractions of Brazilian soils under conventional and no-tillage systems

The capacity of soils to sequestrate carbon (C) is mainly related to the formation of organo-mineral complexes. In this study, we investigated the influence of soil management systems on the C retention capacity of soil with an emphasis on the silt and clay fractions of two subtropical soils with different mineralogy and climate. Samples from a Humic Hapludox and a Rhodic Hapludox, clayey soils cultivated for approximately 30 years under no-tillage (NT) and conventional tillage (CT) were collected from six layers distributed within 100-cm soil depth from each site and from an adjacent native forest. After the removal of particulate organic matter (POM), the suspension (<53 µm) was sonicated, the silt and clay fractions were separated in accordance with Stokes' law and the carbon content of whole soil and physical fractions was determined. In the Humic Hapludox, the clay and silt fractions under NT showed a higher maximum C retention (72 and 52 g kg-1, respectively) in comparison to those under CT (54 and 38 g kg-1, respectively). Moreover, the C concentration increase in both fractions under NT occurred mainly in the topsoil (up to 5 cm). The C retention in physical fractions of Rhodic Hapludox varied from 25 to 32 g kg-1, and no difference was observed whether under an NT or a CT management system. The predominance of goethite and gibbsite in the Humic Hapludox, as well as its exposure to a colder climate, may have contributed to its greater C retention capacity. In addition to the organo-mineral interaction, a mechanism of organic matter self-assemblage, enhanced by longer periods of soil non-disturbance, seems to have contributed to the carbon stabilization in both soils.

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.