Soil Physical Fractions Research Articles

Carbon and nitrogen stocks in physical fractions of a subtropical Acrisol as influenced by long-term no-till cropping systems and N fertilisation

The long-term soil management effects on C and N stocks of soil physical fractions are still poorly understood for South American subtropical soils. This study aimed (i) to evaluate the influence of cereal- and legume-based cropping systems and N fertilisation on C and N stocks of the sand-, silt- and clay-size fractions of a no-tilled subtropical Acrisol in southern Brazil, (ii) to compute the Carbon Management Index (CMI) for those cropping systems using physical fractionation data, and (iii) to investigate the possible existence of finite capacity of those soil physical fractions to store C and N. Soil samples of a long-term experiment were collected from the 0–2.5 and 2.5–7.5 cm layers of three no-till cropping systems [fallow bare soil, oat/maize (O/M) and pigeon pea+maize (P+M)] under two N fertilisation levels (0 and 180 kg N ha−1). However, for fallow bare soil, only the non-fertilised sub-plot was sampled. An adjacent native grassland soil was sampled as a reference. The C and N stocks of the three soil physical fractions were higher in the legume-based cropping system (P+M) than in O/M and bare soil, because of the higher residue input in the former. The P+M cropping system restored the C and N stocks in sand- and silt-size fractions to the same levels found in grassland soil. Higher C and N stocks in all physical fractions were also obtained with N fertilisation. The C and N stocks and the C:N ratio were most affected by cropping systems in the sand- and least in the clay-size fraction. Particulate organic matter was found in the silt-size fraction, showing this fraction is not only constituted by mineral-associated organic mater, as commonly believed. Taking grassland soil as reference (CMI = 100), the CMI ranged from 46, in O/M no N, to 517, in P+M with N, pointing to a better soil management in the latter. The clay-size fraction tended to show a finite capacity to store C and N (48.8 g C kg−1 and 4.9 g N kg−1 of clay), which was not verified in sand- and silt-size fractions. The adoption of no-tillage and legume-based cropping systems with high residue input are adequate soil management strategies to improve soil quality and make the agricultural production systems more sustainable in subtropical regions.

Influence of compost on the physical properties and organic matter fractions of a fine sandy loam throughout the cycle of a potato rotation

Potato (Solanum tuberosum L.) rotations often require organic amendments to maintain or improve soil organic matter levels and soil physical properties. However, beneficial effects of compost can be modified by time of application and rotating tillage depth and intensity. This study was conducted to evaluate the effect of compost applied once at different phases in a 3-yr potato, barley (Hordeum vulgare L.), and red clover (Trifolium pretense L.) rotation on a range of soil physical properties and organic matter fractions for a Charlottetown fine sandy loam (Orthic Humo-Ferric Podzol) in Prince Edward Island. Soil samples (0–8 cm) were obtained during the second cycle of the rotation (after two compost applications) in the fifth year of the experiment. Soil properties were influenced by compost addition, time of compost addition, and crop phase. Compost-induced benefits in soil physical properties (bulk density, macro-porosity, oxygen diffusion rate, shear vane strength, water-filled pore space) were mainly expressed in the red clover phase of the rotation, where soil density was relatively high compared to the barley and potato phases, due to the absence of tillage. The soil physical parameters, however, were mainly within their established optimum ranges for this soil type. Soil water content at −0.033 MPa was increased by compost in the potato phase, compared to the control. Soil organic matter was influenced by both compost and crop C inputs. Compost increased soil particulate organic matter (POM) in the potato and barley phases. Due to differences in crop residue inputs, compost-related differences in organic matter were minimized in the red clover phase of the rotation. Compost addition increased potato tuber yield above the maximum yield obtained with nitrogen application. This “non-nitrogen” compost yield effect may be related to the slight, but significant, improvement in soil water-holding capacity. Overall , compost application in an intensive 3-yr potato rotation provided benefits for potato productivity and in both soil physical and biological properties. Key words: Soil organic carbon, particulate organic matter, soil physical properties, compost amendment, potato yield, eastern Canada

Physical fractionation of soil and structural and functional complexity in organic matter turnover

SummaryPhysical fractionation is used increasingly to study the turnover of organic matter in soil. This essay links the methods of fractionation to concepts of turnover by defining levels of structural and functional complexity that refer to experimentally verifiable pools of organic matter in the soil.Physical fractionation according to size and density of soil particles emphasizes the importance of interactions between organic and inorganic soil components in the turnover of organic matter. It allows the separation of free and occluded uncomplexed organic matter and of primary and secondary organomineral complexes. This methodological approach recognizes that the overall regulation of decomposer activity is through the structure of soil, which determines gas exchange, the availability of substrates and water, and the transport of solutes.Results from physical fractionations suggest three levels of structural and functional complexity in the turnover of organic matter in soil. Primary organomineral complexes isolated from fully dispersed soil account for the primary level of complexity. The clay‐, silt‐ and sand‐sized complexes are seen as the basic units in soil, surface reactions between substrates, organisms and minerals being the main regulatory mechanisms. Secondary complexes reflect the degree of aggregation of primary organomineral complexes and refer to the second level of complexity. Physical protection of uncomplexed organic matter and soil organisms and the creation of gas and moisture gradients are emergent features regulating the turnover at this level of complexity. The structurally intact soil (the soil in situ) constitutes the third level of complexity. This integrates the effects of primary and secondary complexes. Emergent structural features associated with this level are resource islands, macropores, roots, mesofauna, tillage and soil compaction, the corresponding functional features being related to the transport and exchange of solutes and gases, and the spatial distribution and comminution of litter and uncomplexed organic matter. Thus, a thorough understanding of the turnover and storage of organic matter in soil can be acquired only by considering all levels of complexity in the decomposition subsystem.

Particulate Organic Matter Composition and Pythium Damping‐Off of Cucumber

Organic matter–mediated root rot suppression is unpredictable in field soils. This study was conducted to determine whether particulate organic matter (POM) composition and content were related to Pythium damping‐off (DO) incidence in a sand amended with sawdust‐bedded dairy manure compost (15% compost:85% sand, v/v) incubated in pots for 506 d. Suppressive and conducive POM composition was then related to literature values for agricultural and forest soil POM fractions. The suppressive potential of the substrate was determined with a Cucumis sativus L. (cucumber)/Pythium ultimum DO bioassay. Particulate organic matter composition was determined spectroscopically. The compost‐amended sand supported suppression of DO for a period of ∼1 yr. Suppression was sustained by the degradation of the less decomposed coarse and mid‐sized POM fractions. After these fractions stabilized in mass, suppression was lost. Plant constituents were highly degraded during composting before amendment to sand. Compost‐derived POM composition changed little as suppression was supported for 1 yr. In contrast, aromatic and aliphatic contents and alkyl‐ and O‐alkyl C declined as suppression was lost. Suppressive POM was similar in composition to forest soil organic horizons and soil unprotected light fraction (ULF), suggesting that the least‐decomposed soil physical fractions may be the only fractions compositionally capable of supporting suppression of DO in field soils.

QUARTZITE-COBBLE WEATHERING IN ALLUVIAL-FAN SOILS OF THE VIRGINIA BLUE RIDGE

Extremely weathered quartzite cobbles are embedded in Paleudults, Hapludults, and Fragiudults developed on alluvial fans along the western footslopes of the Blue Ridge, central Virginia. The cobbles are intriguing because their weathering patterns and physical condition imply long-term in situ weathering. This study was conducted to explore the feasibility of using the cobbles to correlate the soils by determining if there is a soil-cobble weathering relationship. Ninty-three cobbles were collected from 10 locations. Cobble bulk density (ρ cb ) was compared using the Kruskal-Wallis nonparametric procedure. Free Fe from the cobbles (Fe cb ) was compared with a two-level nested analysis of variance for unequal sample sizes. Pearson's correlation coefficients were calculated for ρ cb versus several soil and cobble properties. Among five sites on gently sloping landscapes, weathered ρ cb ranged from 2.17 to 2.32 Mg m -3 and was not significantly different. At the other sites ρ cb was significantly different and ranged from 2.39 to 2.52 Mg m -3 . Variation of Fe cb among cobbles within sites was 72.8%, thus making it difficult to suggest a within-site weathering scenario. Variation among sites of the Fe cb was only 13.1% suggesting similar weathering processes controlled cobble alteration across the study area. No significant correlation was found between ρ cb and soil physical properties or cobble sand fractions. Correlation coefficients for ρ cb versus Fe cb (r = -0.73 ** ), hydroxy interlayered vermiculite (r = -0.76 ** ), and slope (r = 0.69 * ) indicate that pedoenvironment is a primary factor for cobble weathering. ρ cb , Fe cb , and hydroxy-interlayered vermiculite seem to be promising parameters for correlating Blue Ridge alluvial-fan soils when sites are within narrowly defined slope classes.

Carbon allocation in calcareous grassland under elevated CO2: a combined 13C pulse‐labelling/soil physical fractionation study

Summary To test whether plant–soil C fluxes in natural grassland increase under elevated atmospheric CO2 concentration, intact calcareous grassland monoliths exposed to ambient or elevated CO2 were pulse‐labelled and the dynamics of the 13C label followed throughout the rest of the growing season. The experiment revealed no increased fluxes of C to soils at elevated CO2. The only changes found were relatively small shifts towards increased C allocation to roots by the end of the growing season. This effect was probably because wetter soil under elevated CO2 prolonged the growing period. At elevated CO2, plant C pools increased below ground (+28%) at the end of the season, resulting in slightly increased root : shoot ratios. Plant 13C pools increased significantly below ground. There were no effects of CO2 enrichment on 13C in soil microbes, fine roots or earthworms. Elevated CO2 caused a shift in soil particle size distribution towards smaller aggregate sizes, but had no effect on the total C and 13C content of low‐ and high‐density soil fractions. The absence of effects of CO2 on the labelling of soil microbial biomass, and of C and 13C accumulation in low‐density macro‐organic fractions, suggest that there is no significant effect of elevated CO2 on root exudation or turnover, agreeing with published labelling studies, but conflicting with CO2‐exchange budgets.

Techniques for the differentiation of carbon types present in lignite-rich mine soils

The objective of this study was to assess high energy UV photo-oxidation, 13C CP/MAS NMR spectroscopy and 14C activity measurements for their suitability in differentiating carbon derived from plant material and that derived from lignite. The conceptual approach included analysis of bulk soil samples and physical fractions of lignite-rich mine soils. Additionally, the <53 μm fraction of selected samples was subjected to 2 hours of high-energy UV photo-oxidation. Quantification of the lignite carbon content by 14C activity measurements showed that a mixture of lignite and recently formed organic matter occurs in the bulk surface soil horizon as well as a number of particle size and density fractions. Using 13C CP/MAS NMR spectroscopy, carbon species, characteristic for lignite- and plant-derived organic matter can be identified. Both methods yield corresponding results. It was also shown that lignite-derived carbon is more resistant to the high-energy UV photo-oxidation treatment than carbon derived from recent plant material. This method provides qualitative indications of lignite contribution to soils.

Effect of no-till cropping systems on soil organic matter in a sandy clay loam Acrisol from Southern Brazil monitored by electron spin resonance and nuclear magnetic resonance

In weathered tropical and subtropical soils organic matter is crucial for soil productivity and its quantity depends heavily on soil management systems. This study evaluated the effect of no-till cropping systems on organic matter content and quality in a sandy clay loam Acrisol soil (Paleudult in US taxonomy) from Southern Brazil. Ten cropping systems with varying additions of C and N were conducted for 12 years (from 1983 to 1994). The addition of crop residues increased total organic carbon (TOC) and total nitrogen (TN) in the soil at 0–17.5 cm depth, and this increase was directly related with C and N added or recycled by the systems. The crop residues added to the soil were associated with reduced semiquinone free radical concentration, detected by electron spin resonance (ESR), in the organo-mineral aggregates <53 μm and humic acid (HA) samples, in the soil at 0–2.5 cm depth. This showed that stable organic matter originating from crop residues was less humidified than the original soil organic matter. Results obtained from organo-mineral aggregates showed a higher amplitude (highest and lowest values were 5.47 and 2.09 × 10 17 spins g −1 of TOC, respectively) of semiquinone free radical concentration than HA samples (highest and lowest values were 2.68 and 1.77 × 10 17 spins g −1 of HA, respectively). These data showed that alterations due to tillage in soil organic matter characteristics, e.g., humification degree can be better identified through a combination of soil physical fractionation and spectroscopic analysis. Semiquinone content in the HA samples, detected by ESR, related significantly to aromaticity, as measured by nuclear magnetic resonance (NMR) of 13C. Management systems including no-till and cropping systems with high C and N additions to the soil improved its quality in Southern Brazil.

Soil organic matter composition under primary forest, pasture, and secondary forest succession, Región Huetar Norte, Costa Rica

Secondary forests are increasingly wide-spread on neotropical soils. In this study, we investigated if, and how, the establishment of a secondary forest on abandoned pasture affect the quality of soil organic matter (SOM). We approached this by a combination of physical fractionation of soil, where particulate SOM (light fraction and sand-associated SOM) is separated from mineral-bound SOM (silt- and clay-associated SOM), and structural chemical analyses, including measurements of well-decomposable carbohydrates and the more refractory lignin. Particle-size separation revealed that agricultural use of a soil being formerly under primary forest resulted in a depletion of the particulate SOM pool, whereas clay- and silt-bound SOM was less affected. Abandonment of the pasture and growth of a secondary forest raised the C content in all separates to a pre-cultivation level within 18 years, and sand-associated C was even higher as compared to the primary forest. The lignin and carbohydrate signature showed that the land use rarely affected the chemical composition of SOM within the different fractions. This was corroborated by solution 13C NMR spectroscopy of the NaOH-soluble SOM. The results suggested that land use primarily influences the C balance across the light fraction and the size separates, with the particulate SOM pool being the most significant SOM component in the context of management impacts on these soils. While the gross chemical composition of SOM within the fractions remained unaffected, some molecular differences indicated a shift in the microbial community and/or activity at transformation of primary forest into pasture and after abandonment of the pasture with growth of secondary forest.

A single procedure to recover DNA from the surface or inside aggregates and in various size fractions of soil suitable for PCR-based assays of bacterial communities

A single DNA procedure to recover bacterial DNA from various soil microenvironments which differ in their physical, chemical and structural properties was developed. These microenvironments, obtained by a combination of soil washes and physical fractionation, were the outer part or macroporosity (outside and surface of aggregates), the inner part or microporosity (inside of aggregates) and various size and stability classes of soil aggregates and particles. The DNA extraction method involved sample homogenization and cell disruption by grinding in liquid nitrogen, followed by enzymatic lysis with lysozyme and proteinase K. High yields of high molecular weight DNA (≥ 23 kb) were obtained for all microenvironments. Crude DNA yields for the various soil microenvironments were between 0.7 and 51.4 μg DNA·g −1 soil sample and were positively correlated with bacterial cell abundance ( r = 0.91). Further purification steps allowed to recover at least 60 % of the DNA extracted from the various microenvironments. The suitability of the extracted DNA to undergo enzymatic amplification reactions and the effectiveness of the extraction procedure in recovering DNA from various native bacterial groups was tested using primers for archaebacterial 16S rDNAs, universal and group-specific eubacterial 16S rDNAs primers (β- and γ-proteobacteria, High G+C Gram-positive bacteria, and Bacillus species and relatives). Successful amplification of less ubiquitous genes was also obtained with primers targeting nitrogen fixation ( nifH) and mercury resistance ( merRTΔP) genes.

Estimating soil texture from sample density

Abstract A number of laboratory methods for determining soil physical fractions have been proposed. The methods being cumbersome and time consuming cannot be adopted for routine analysis in soil testing laboratories. Therefore, a simple method which could provide quantitative estimates of the soil size fractions is required. This study used soil sample density (ratio of mass of soil in a standard scoop to its volume) to estimate soil separates. Regression models were developed for predicting soil separates from sample density. Sand, silt, and silt+clay fractions were best described by second degree polynomials with coefficients of determination ranging from 0.75–0.80. Although the exponential function was statistically equivalent to the second degree polynomial for estimating clay, it was preferable as it yielded a better shape of the fitted curve at sample densities <1.2 g cm‐3. Validation of the models with independent data showed that model‐predicted values of various size fractions matched well with l...

Incorporating Nonextractable Atrazine Residues into Soil Size Fractions as a Function of Time

AbstractSoil organic matter is directly implicated in pesticide nonextractable (bound) residues formation, but the role of different soil organic constituents is unknown. Physical soil fractionation allows separation, with minimal chemical modification, of organic constituents whose nature and properties are dependent on particle size. Physical fractionation was used to determine the role of soil organic constituents in atrazine (6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine) bound residues formation. 14C‐ring‐labeled atrazine was applied to the top of soil lysimeters (0.3 by 1.2 m) in the field. The lysimeters were periodically removed during 16 mo, and the soil was physically fractionated into seven size fractions by sieving, sedimentation, and centrifugation: &gt;200, 50 to 200, 20 to 50, 5 to 20, 2 to 5, 0.2 to 2, and &lt;0.2 µm. The largest proportion of total bound residues in the whole soil was in the clay size (0.2–2 µm) fraction, which also contained 50% of the total soil organic C (OC). The ratio of bound residues to OC content decreased with the particle size, and it was highest in fractions &gt;50 µm, those rich in nonhumified organic matter. The low capacity of humified organic matter in the finest size fractions to form bound residues is presumably due to the chemical nature of the humified organic matter; however, accessibility of organic compounds stabilized on the fine mineral surfaces or included in aggregates structures is also a possibility. The most stable bound residues are associated with humified organic matter, especially in clay size fraction (0.2–2 µm). Physical soil fractionation as a function of incubation time allows definition of the different capacity of soil organic constituents to form bound residues in relation to their localization in the size fractions.

Carbon Turnover in Soil Physical Fractions

AbstractSoil organic matter (SOM) as represented in mathematical simulation models involves several hypothetical pools of differing resistance to decay. These conceptual pools satisfy requirements of modeling, but usually have little in common with existing information on physical and chemical properties of SOM. Using our data on turnover times for soil C in fractions of natural aggregates and primary particles, we attempted to relate age of C in physical fractions with that in widely accepted theoretical pools. Soil from a field experiment with 14C‐labeled soybean residues was sampled periodically and separated into physical fractions. The amounts of 14C associated with these fractions at different times provided data for calculation of decay rates and turnover times. The most labile fraction of SOM was plant fragments with turnover time ranging from 1 to 3 yr, which was inversely related to fragment size. Soil aggregates were found to be enriched in C compared with whole soil. This was most pronounced for coarser aggregates whose construction apparently involved the relatively labile plant fragments in some progressive state of decay. The macroaggregates with partially processed C showing turnover from 1 to 3 yr contrasted with microaggregates that included more highly humified C having a longer residence time of ≈7 yr. Various soil fractions differing in residence time of associated C were assembled into several groups that demonstrated consistency with conceptual pools of two widely accepted simulation models. Data from 13C natural abundance studies of soil and of primary fractions were in harmony with models requiring at least two pools of stable SOM.