Function Of Soil Organic Matter Research Articles

Aggregation rate as a function of soil organic matter in heavy clayey Ferralsols

Heavy clayey Ferralsols are known for their high capacity to form water-stable macroaggregates. However, little is known about how different size classes of macroaggregates exchange mass with increasing organic matter content. This study aimed to characterize and detail aggregation dynamics with increasing organic matter content in these soils. Profiles of three heavy clayey Ferralsols were sampled to isolate the role of organic matter on the stability of aggregates. In this sampling scenario, a conceptual model was established to calculate [...]

315 Formation, persistence, and function of soil organic matter: how recent scientific advances apply in agroecosystems

Abstract Soil organic matter is fundamental to healthy and productive soils and building it is an effective means by which to draw down atmospheric greenhouse gas concentrations with added co-benefits. Scientific understanding of soil organic matter dynamics is constantly evolving, and the past decade has seen major advances and paradigm shifts. Soil organic matter creation from decaying plant litter is now thought to occur under two separate pathways, yielding two functionally different types: predominantly plant-derived, unprotected particulate organic matter (POM) and predominantly microbially-derived, mineral-associated organic matter (MAOM). The idea of naturally-occurring humic substances in soils has been largely abandoned, and long-term soil organic matter persistence is now understood to be driven mainly by mineral association. We will present the research behind these paradigm shifts, and show how considering POM and MAOM separately is key to understanding the mechanisms driving carbon accrual and persistence in soil, and therefore to guiding policy and management for soil carbon sequestration. We will present drivers of POM and MAOM contents, from individual fields to continents, including their capacity for sequestration and saturation in agricultural soils of the USA, and their responses to common management practices in agroecosystems.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Applications of stable carbon isotopes in soil science with special attention to natural 13C abundance approach

Since the invention of the isotope ratio mass spectrometer, isotope analysis has shed light on many key processes in the Earth’s ecosystems. Stable isotope analysis was first applied in the field of chemistry and geochemistry, while the use of isotopic fractionation for various biochemical reactions was elaborated later. The knowledge gained from isotope research led to a better understanding of the dynamics of the biosphere and to the more efficient study of interactions between the geosphere and biosphere. In soil research, stable isotopes are ideally suited to provide a wider insight into the element cycles in soil ecosystems. Stable carbon isotopes, in particular, have been in the focus of soil research, since soil organic matter (SOM) plays an important role not only in soil fertility, soil water management and many other physical, chemical and biological soil functions, but also in the global carbon cycle. If processes connected with these soil functions are isotopically labelled with stable carbon isotopes, the key reactions of C input, exchange and output in the soil and other soil organic matter functions can be studied accurately. Moreover, analysing the isotopic composition of CO2 exchange between the soil and the atmosphere helps to predict ecosystem responses to global changes.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Soil Organic Matter and Grain Yield of Rainfed Maize in Luvisols of Campeche, México

The organic matter is an important soil component, due its favorable effects on soil physical and chemical properties and, by consequence, on crop yields. So, the objective of this work was quantify the changes through the time of soil organic matter in rainfed maize (Zea mays L.) system, in a period of 30 years of continuous crop and in relation to system of natural vegetation system, and also the effects of these changes on the crop yield, in Luvisols soils of State of Campeche, México. In production cycle of 2004, 53 farmer plots in rainfed maize system of different land use time and five sites of forest natural vegetation system were sampled for soil, to quantify physical and chemical properties of soil, and it was taken data about climate, system management and crop yield. The data were analiced by regression analysis, considered the organic matter as function of land use time and factors of soil and management of system, and the crop yield as function of soil organic matter and factors of soil, climate and management of system. The soil organic matter diminished from 5.68% in forest natural vegetation system to 3.59% after 16 - 30 years of cultivation, and the soil annual incorporation of vegetative mulch (weeds and stubble of maize) increased the organic matter from 0.14% after 1 - 5 years to 0.46% after 16 - 30 years of cultivation. Without fertilization, the diminution of soil organic matter caused 907 kg⋅ha−1 less of maize yield, and the fertilization with 200 kg⋅ha−1 of diamonic phosphate (36 kg⋅N⋅ha−1 and 92 kg P2O5 ha−1) increased the maize yield 1224 kg⋅ha−1 after 1 - 5 years and 1421 kg⋅ha−1 after 16 - 30 years of cultivation, but not compensed less maize yield of 711 kg⋅ha−1 due the diminution of soil organic matter.

Composition-Dependent Sorptive Fractionation of Anthropogenic Dissolved Organic Matter by Fe(III)-Montmorillonite

Water transports organic matter through soils, where mineral-organic associations form to retain dissolved organic matter (“DOM”), influencing terrestrial carbon cycling, nutrient availability for plant growth, and other soil organic matter functions. We combined Fourier transform ion cyclotron resonance mass spectrometry with novel data analysis techniques to examine the role of sorptive fractionation in the associations between Fe(III)-montmorillonite and DOM from composted biosolids (“anthropogenic DOM”). To examine the influence of DOM composition on sorption and sorptive fractionation, we used resin-based separation to produce DOM subsamples with different molecular compositions and chemical properties. A large proportion (45 to 64%) of the initial carbon in every DOM solution sorbed to the Fe(III)-montmorillonite. However, when the compositions of the initial solutions were compared to the sorbed organic matter, the computed changes in composition were lower (10 to 32%). In fact, non-selective sorption was more important than selective sorption in every sample, except for the hydrophilic neutral (HiN) fraction, where high nitrogen content and acidic conditions appeared to enhance sorptive fractionation. The results from this study demonstrate that the importance of sorptive fractionation varies with DOM composition and other factors, and that non-selective sorption can contribute substantially to the formation of mineral-organic associations.

Leaching and sorption of neonicotinoid insecticides and fungicides from seed coatings

ABSTRACTSeed coatings are a treatment used on a variety of crops to improve production and offer protection against pests and fungal outbreaks. The leaching of the active ingredients associated with the seed coatings and the sorption to soil was evaluated under laboratory conditions using commercially available corn and soybean seeds to study the fate and transport of these pesticides under controlled conditions. The active ingredients (AI) included one neonicotinoid insecticide (thiamethoxam) and five fungicides (azoxystrobin, fludioxonil, metalaxyl, sedaxane thiabendazole). An aqueous leaching experiment was conducted with treated corn and soybean seeds. Leaching potential was a function of solubility and seed type. The leaching of fludioxonil, was dependent on seed type with a shorter time to equilibrium on the corn compared to the soybean seeds. Sorption experiments with the treated seeds and a solution of the AIs were conducted using three different soil types. Sorption behavior was a function of soil organic matter as well as seed type. For most AIs, a negative relationship was observed between the aqueous concentration and the log Koc. Sorption to all soils tested was limited for the hydrophilic pesticides thiamethoxam and metalaxyl. However, partitioning for the more hydrophobic fungicides was dependent on both seed type and soil properties. The mobility of fludioxonil in the sorption experiment varied by seed type indicating that the adjuvants associated with the seed coating could potentially play a role in the environmental fate of fludioxonil. This is the first study to assess, under laboratory conditions, the fate of pesticides associated with seed coatings using commercially available treated seeds. This information can be used to understand how alterations in agricultural practices (e.g., increasing use of seed treatments) can impact the exposure (concentration and duration) and potential effects of these chemicals to aquatic and terrestrial organisms.

Soil organic carbon fractionation for improving agricultural soil quality assessment – a case study in Southern Belgium (Wallonia)

Description of the subject. The paper presents and discusses a method for fractionating bulk soil organic carbon (SOC) in meaningful SOC fractions to better assess SOC status and its related soil ecosystem functions. Objectives. The objective is to perform an evaluation of ecosystem functions of soil organic matter at plot scale and compare it to the normal operative range of the local agro-ecological region. Method. By separating carbon associated with clay and fine silt particles (stable carbon with slow turnover rate, < 20 µm) and carbon non-associated with this fraction (labile and intermediate carbon with higher turnover rates, ≥ 20 µm), effects of management can be detected more efficiently at different scales. Conclusions. Soil organic carbon fractions, used as proxies for soil ecosystem functions, can be helpful because they represent SOC functional pools. This paper proposes to apply fractionation on samples taken at plot and regional scale. It is therefore possible to establish a normal operative range for a specific agro-region for comparison with the values in individual plots. This allows drawing a baseline for SOC fractions status in a specific agricultural unit. This approach provides valuable information to study and evaluate the impact of agricultural management in the context of enhancing soil quality and functions.

Organic Matter Stocks and the Interactions of Humic Substances with Metals in Araucaria Moist Forest Soil with Humic and Histic Horizons

ABSTRACT Soils with humic and histic horizons in tropical and subtropical ecosystems play an important role in determining the atmospheric C stock and its stabilization, climate regulation, water holding capacity, and environmental filtering, due to the different functions of soil organic matter (SOM). However, the processes and mechanisms that regulate SOM dynamics in these soils are not clear. The objectives of this study were: i) determine the C and N stocks and ii) investigate the SOM chemical fractions and their [...]

Molybdenum sources and isotopic composition during early stages of pedogenesis along a basaltic climate transect

Molybdenum (Mo) is an essential micronutrient and redox sensitive trace metal that has the potential to be a tracer of pedogenic processes. Globally, riverine δ98Mo values (based on 98Mo/95Mo relative to NIST SRM 3134) are heavier relative to bedrock, suggesting weathering processes preferentially retain light Mo isotopes, however, the mechanisms governing this process in soils are poorly understood. We investigated soil Mo abundance and isotopic composition as a function of climate along a well-constrained climosequence on a 10kyr lava flow in Hawaii receiving 660 to 2100mmyear−1 mean annual precipitation (MAP). We assessed Mo abundance and isotopic composition as a function of soil organic matter (OM) content, short-ranged-ordered (SRO) iron (Fe) (oxyhydr)oxide abundance, and Mo loss/gain. We find net accumulation of Mo across all soils (up to +139% gain) that is positively correlated with increasing MAP. The highest Mo gains are in surface soil horizons, and are correlated with high OM content. The isotopic composition of soil Mo, deviates from underlying basaltic bedrock, which ranges from δ98Mo values of −0.11‰ to −0.26‰. Soil Mo isotopic values are lightest at the dry sites (δ98Mo=−0.63‰) and become heavier with increasing precipitation (up to δ98Mo=+0.03‰). Samples with the heaviest δ98Mo values are from horizons with the largest net gains of Mo relative to bedrock and have higher OM content. In order to further constrain Mo fluxes into and out of the soil system, we measured Mo isotope ratios in local rainwater (average δ98Mo=+1.11‰), groundwater (average δ98Mo=+0.14‰), and vegetation (δ98Mo values between −0.18‰ and +0.64‰). Based on these data, we propose that Mo in these soils is substantially augmented by additions from precipitation, volcanic fog, and potentially anthropogenic inputs of Mo, and that retention of these inputs within soils is likely related to Mo–OM interactions. The large atmospheric inputs of Mo strongly modulate the δ98Mo values of dissolved Mo fluxes from soil to values only slightly heavier than bedrock. These patterns illustrate the potential for Mo as a tracer of atmospheric inputs and pedogenic processes in soils and help elucidate the mechanisms that drive heavy δ98Mo values in the global riverine Mo flux.

Key soil functional properties affected by soil organic matter - evidence from published literature

The effect of varying the amount of soil organic matter on a range of individual soil properties was investigated using a literature search of published information largely from Australia, but also included relevant information from overseas. Based on published pedotransfer functions, soil organic matter was shown to increase plant available water by 2 to 3 mm per 10 cm for each 1% increase in soil organic carbon, with the largest increases being associated with sandy soils. Aggregate stability increased with increasing soil organic carbon, with aggregate stability decreasing rapidly when soil organic carbon fell below 1.2 to 1.5 5%. Soil compactibility, friability and soil erodibility were favourably improved by increasing the levels of soil organic carbon. Nutrient cycling was a major function of soil organic matter. Substantial amounts of N, P and S become available to plants when the soil organic matter is mineralised. Soil organic matter also provides a food source for the microorganisms involved in the nutrient cycling of N, P, S and K. In soils with lower clay contents, and less active clays such as kaolinites, soil organic matter can supply a significant amount of the cation exchange capacity and buffering capacity against acidification. Soil organic matter can have a cation exchange capacity of 172 to 297 cmol(+)/kg. As the cation exchange capacity of soil organic matter varies with pH, the effectiveness of soil organic matter to contribute to cation exchange capacity below pH 5.5 is often minimal. Overall soil organic matter has the potential to affect a range of functional soil properties.

Function of soil organic matter and eff ective application of organic matter in arable land

Function of soil organic matter and eff ective application of organic matter in arable land

TRENDS IN RECOVERY OF MEDITERRANEAN SOIL CHEMICAL PROPERTIES AND MICROBIAL ACTIVITIES AFTER INFREQUENT AND FREQUENT WILDFIRES

ABSTRACTSince the 1970s, increase in fire frequency has been observed in all European Mediterranean regions. The objectives of this study were (1) to determine the effects of wildfire frequency on the recovery at short‐ and long‐term of soil chemical and microbial properties and (2) to identify the mechanisms underlying the recovery of these sites properties. Soils from 17 plots (Maures mountains range, Var, France) were classified into 5 wildfire regimes (i.e. not burned since at least 57 years ago, infrequently and frequently burned‐with time since fire between 4 and 17 years). Soil samples from these plots were analysed for their nutrient content, chemical functions of soil organic matter (SOM) using FT‐MIR spectroscopy and microbial mineralising activities. Our results showed that the frequent wildfire regime slowed down the recovery in the short term of SOM spectroscopic properties and nutrient availability. Both low quantity and low quality (i.e. high percentage of aromatic and phenolic organic forms) of soil organic matter were found to be related to soil microbial recovery at 4 years after frequent wildfires. The frequent wildfires improved the recovery in net nitrification and nitrate content, leading to an increase in catabolic evenness and a recovery in microbial C‐substrate utilisation profiles between 4 and 17 years. However, frequent wildfires slowed down the recovery of hydrolytic enzyme pool (i.e. FDA hydrolases) and phenol oxidase activity, both involved in soil C cycling. Overall, our observations suggest that 4 fires in 50 years is a threshold beyond which soil quality may be endangered. Copyright © 2011 John Wiley & Sons, Ltd.

Three-dimensional mapping of soil organic matter content using soil type–specific depth functions

This paper proposes a method for mapping depth functions of soil organic matter (SOM) that combines general pedological knowledge with geostatistical modeling. A pedometric soil map that represents soil type at any location with a probability distribution formed the starting-point. For each of the ten soil types depicted on this map a depth function structure was defined that describes the variation of SOM over depth based on pedological knowledge about soil profile morphology. To this end five depth function building blocks were defined, referred to as ‘model horizons’. For each soil type the depth function structure was obtained by stacking a subset of model horizons. The parameters of the ten soil type–specific depth functions were calibrated with data from soil profile descriptions and spatially interpolated using environmental covariates as predictors. The predicted parameters and the soil-type specific depth function structures then allowed us to construct the depth function of SOM for each soil type at each prediction site. By combining the soil type–specific depth functions with the probability distributions of the soil types, a probability distribution of depth functions was obtained at each location in the study area. The soil type–specific depth functions and their associated probabilities were used to map the SOM stock for depth intervals 0–30 cm, 30–60 cm, 60–90 cm, and 0–90 cm. The mapped SOM stocks were validated with an independent probability sample. R 2-values ranged between 0.09 and 0.75, with 0.46 for the 0–90 cm layer. The RSME was largest for the 30–60 cm layer (13.4 kg/m 2). This is typically the layer with the largest within-soil type and between-soil type variation of SOM and is therefore the most challenging to predict. Similar to other studies better predictions were found for the top layer than for subsurface layers, which illustrates a general challenge of capturing subsurface variation of soil properties by our pedometric models. The pedometric approach to three-dimensional mapping of SOM presented in this paper is closely related to the conventional approach that represents depth distribution of soil properties with representative profile descriptions that are associated to the map units of a soil type map. A comparison of our pedometric approach with the conventional approach showed that there was little difference in performance. However, our modeled depth functions might give a more realistic representation of the vertical variation than the discrete (stepped) functions based on representative soil profile descriptions and thus might provide better predictions of the SOM content at small depth intervals.

Biological, chemical and thermal indices of soil organic matter stability in four grassland soils

Abstract The various ecosystem functions of soil organic matter (SOM) depend on both its quantity and stability. Numerous fractionation techniques have been developed to characterize SOM stability, and thermal analysis techniques have shown promising results to describe the complete continuum of SOM in whole soil samples. However, the potential link between SOM thermal stability and biological or chemical stability has not yet been adequately explored. The objective of this study was to compare conventional chemical and biological methods used to characterize SOM stability with results obtained by thermal analysis techniques. Surface soil samples were collected from four North American grassland sites along a continental mean annual temperature gradient, each with a native and cultivated land use. Soil organic C concentrations ranged from 6.8 to 33 g C kg −1 soil. Soils were incubated for 588 days at 35 °C, and C mineralization rates were determined periodically throughout the incubation by measuring CO 2 concentration using an infrared gas analyzer (IRGA) to calculate biological indices of SOM stability. Hot-water extractable organic C (HWEOC) contents were determined before and after incubation as chemical indices. Finally, samples from before and after incubation were analyzed by simultaneous thermal analysis (i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)) to determine thermal indices of SOM stability. Long-term incubation resulted in the mineralization of up to 33% of initial soil C. The number of days required to respire 5% of initial soil organic carbon (SOC), ranged from 27 to 115 days, and is proposed as a standardized biological index of SOM stability. The number of days was greater for cultivated soils compared to soils under native vegetation, and generally decreased with increasing site mean annual temperature. HWEOC (as % of initial SOC) did not show consistent responses to land use, but was significantly lower after long-term incubation. Energy density (J mg −1 OM) was greater for soils under native vegetation compared to cultivated soils, and long-term incubation also decreased energy density. The temperatures at which half of the mass loss or energy release occurred typically showed larger responses to land use change than to incubation. Strong correlations demonstrated a link between the thermal and biogeochemical stability of SOM, but the interpretation of the thermal behavior of SOM in bulk soil samples remains equivocal because of the role the mineral component and organo-mineral interactions.

Photolysis Reaction of Linear Alkhylbenzene Sulphonate in Saturated Soil: Kinetics Parameters Evaluation

Fate of detergent of linear alkhylbenzene sulphonate (LAS) in the soil can be described if the values of kinetics parameters are known. The objectives of the study were to determine the kinetics parameters of photolysis reaction of LAS in saturated soil system, to identify variables affecting the parameters, and to set-up empirical equations correlating parameter to the pertinent variables. The experimental works were performed in batch reactor which was equipped with ultraviolet of 53 or 789 lumens. One hundred grams of soil was mixed with 1000 ml of water and then the mixture was left for 24 hour in the reactor. A certain amount of LAS was then added to the soil solution and stirrer was put on, so that the LAS concentration in the mixture became 20 ppm. Before the light put on, the soil solution was analyzed its biomass content using a plate count method and LAS concentration using Methylene Blue Active Substances (MBAS) method. Subsequently the slurry were stirred and lighted continuously. At a certain time interval, 5 cubic centimeters of solution was taken from the reactor then its biomass content and LAS concentration were analyzed. The variables studied were different soil type and the light intensity. When effects of one variable studied, the other variables were kept constant. Photolysis reaction of LAS in saturated soil took place both in soil solution and particle. The kinetics parameters are the function of soil organic matter (%OM) and clay mineral (%CM) content, as well as light intensity. The photolysis reaction rate constants of LAS in soil solution and particle for light intensity of 53 lumens are described as follows: k PS = 3.79 x 10 -4 (%OM) + 4.41 x 10 -3 (%CM) k PL = 7.73 x 10 -3 (%OM) + 1.18 x 10 -3 (%CM) For the light intensity of 789 lumens, the kinetics parameters are k PS = 16.3 x 10 -4 (%OM) + 0.075 x 10 -3 (%CM) 30 Photolysis Reaction of Linear Alkhylbenzene Sulphonate in Saturated Soil: Kinetics Parameters Evaluation k PL = 49.3 x 10 -3 (%OM) + 1.46 x 10 -3 (%CM) k d = 8.06 x 10 -4 (%OM) + 7.39 x 10 -5 (%CM)

Adsorption characteristics of tetracycline by two soils: assessing role of soil organic matter

The adsorption characteristics of tetracycline by 2 soils including a clay-rich soil (cinnamon soil) and an iron oxide-rich soil (red soil) were investigated as a function of soil organic matter (SOM). SOM is the main adsorbent for tetracycline in the soil environment. The results indicated that the adsorption kinetics of tetracycline by the different soils with or without organic matter was described by the Elovich equation and the exponent equation. The adsorption of tetracycline on red soil was quite rapid and equilibrium could be reached after 5 h. In contrast, the concentration of tetracycline in cinnamon soil reached equilibrium after 11 h. The difference in equilibrium time in cinnamon soil and red soil was caused by their dominant components for adsorption, including clays, organic matter, and Al/Fe hydrous oxides. It took longer for the penetration of tetracycline into the interlayers between clays and organic matter in cinnamon soil, but tetracycline needed less time for adsorption through surface complexation on oxide surfaces of red soil. Removing SOM from soil markedly shortened the equilibrium time (7 h) of adsorption and reduced the equilibrium concentration (Cs) in cinnamon soil, but not in red soil, because of different dominant components for adsorption in the 2 natural soils. In natural and SOM-free soils, >98% of tetracycline in solution could be sorbed. The adsorption of tetracycline on natural and SOM-free soils was well described by Freundlich adsorption isotherms. Batch adsorption experiments showed that the adsorption of tetracycline on natural red soil was stronger than that on natural cinnamon soil. Adsorption capacity (KF) decreased with an increase in SOM removed from soil, which is attributed to the effect of tetracycline sorbed by SOM in different soils. However, SOM affected the adsorption intensity (n) of different soils diversely; there was a decrease for red soil and an increase for cinnamon soil. In particular, red soil with high Al/Fe hydrous oxides had higher adsorption affinities than cinnamon soil.

Paddy Soil Stability and Mechanical Properties as Affected by Long-Term Application of Chemical Fertilizer and Animal Manure in Subtropical China

Wet stability, penetration resistance (PR), and tensile strength (TS) of paddy soils under a fertilization experiment for 22 years were determined to elucidate the function of soil organic matter in paddy soil stabilization. The treatments included no fertilization (CK), normal chemical fertilization (NPK), double the NPK application rates (2NPK), and NPK mixed with organic manure (NPK+OM).Compared with CK, fertilization increased soil organic carbon (SOC) and soil porosity. The results of soil aggregate fragmentation degree (SAFD) showed that fast wetting by water was the key fragmentation mechanism. Among the treatments, the NPK+OM treatment had the largest size of water-stable aggregates and greatest normal mean weight diameter (NMWD) ( P ≤ 0.05), but the lowest PR and TS in both cultivated horizon (Ap) and plow pan. The CK and 2NPK treatments were measured with PR > 2.0 MPa and friability index < 0.20, respectively, in the Ap horizon, suggesting that the soils was mechanically unfavourable to root growth and tillage. In the plow pan, the fertilization treatments had greater TS and PR than in CK. TS and PR of the tested soil aggregates were negatively correlated to SOC content and soil porosity. This study suggested that chemical fertilization could cause deterioration of mechanical properties while application of organic manure could improve soil stability and mechanical properties.

The maximum reservoir capacity of soils for persistent organic pollutants: implications for global cycling

The concept of maximum reservoir capacity (MRC), the ratio of the capacities of the surface soil and of the atmospheric mixed layer (AML) to hold chemical under equilibrium conditions, is applied to selected persistent organic pollutants (POPs) in the surface ‘skin’ (1 mm) of soils. MRC is calculated as a function of soil organic matter (SOM) content and temperature-dependent K OA and mapped globally for selected PCB congeners (PCB-28; -153; -180) and HCB, to identify regions with a higher tendency to retain POPs. It is shown to vary over many orders of magnitude, between compounds, locations and time (seasonally/diurnally). The MRC approach emphasises the very large capacity of soils as a storage compartment for POPs. The theoretical MRC concept is compared to reality and its implications for the global cycling of POPs are discussed. Sharp gradients in soil MRC can exist in mountainous areas and between the land and ocean. Exchanges between oceans and land masses via the atmosphere is likely to be an important driver to the global cycling of these compounds, and net ocean–land transfers could occur in some areas.