Variation In Aggregate Stability Research Articles

Variation of root traits and its influences on soil organic carbon stability in response to altered precipitation in an alpine meadow

Soil organic carbon (SOC) dynamics are strongly controlled by plant roots. Yet, how variation of root traits under precipitation change influences SOC stability remains unclear. As part of a 5-year field experiment manipulating precipitation including 90 % (0.1P), 50 % (0.5P), 30 % (0.7P) decrease, and 50 % increase (1.5P), this study was designed to assess the effects of changing precipitation on root traits and production dynamics by minirhizotron and examine how such influences regulate SOC stability in an alpine meadow on the Qinghai-Tibetan Plateau. We found that root length density (RLD), specific root length (SRL), root branching intensity (RBI), and root residue carbon input (RC input) exhibited no significant response, whereas root turnover (RT), root carbon (C), nitrogen (N) concentrations and C/N ratio were altered by precipitation change with nonlinear trends. Absorptive root RT positively correlated to manipulated precipitation within the interannual precipitation range in topsoil, but it showed no significant change under extreme drought treatment. Alpine meadows can maintain the SOC content and density under varied precipitation. However, it showed significant variation in aggregate stability and organic carbon (OC) distribution in aggregates in topsoil, which were mainly due to the strong direct effects of soil moisture and partly related to RLD and RC input of transport roots. Although subsurface soil aggregate stability and OC associated with aggregates were not modified, our results indicated a risk of SOC stability variation in subsurface soil if absorptive root RT and SRL changed. These findings provide vital information to predict responses of SOC dynamics of alpine meadow to future climate change.

Variations in Aggregate Stability and selected Soil Chemical Properties Under Different Land Use Systems in Ikpe Ikot Nkon, Southeastern, Nigeria

Variations in Aggregate Stability and selected Soil Chemical Properties Under Different Land Use Systems in Ikpe Ikot Nkon, Southeastern, Nigeria

Aggregation and fractal dimension of aggregates formed in sand dunes stabilized byPistachioPAMandPistachioPVAc mulches

Wind erosion is a serious environmental problem that has received increased attention by attracting the interest of academics, policy makers and the public. In this study, the effects of combined mulches (denoted asPistachioPAMandPistachioPVAc, used for stabilizing the sand dunes in theDavaran plain in southeasternIran) on aggregate formation and stability indices were investigated by the theory of fractal geometry. In addition, the temporal changes in soil organic carbon (SOC) content and microbial respiration rate (MRR) in the soils treated with the mulches were compared with the control samples during a 5‐month experiment. The results showed significant (P < 0.01) increases in theSOCcontent andMRRfollowing the application (1.5 l m−2) ofPistachioPVAcandPistachioPAMmulches. The rate of release ofCO2was measured in the soils treated with the mulches studied. The largest rate ofCO2release from the three samples taken in weeks 2, 5 and 19 from the beginning of the experiment was about 23.0 µg‐CO2day−1 g−1soil. The smallest and largest mean weight diameters (MWD) of the aggregates formed were observed in the control (0.06 mm) and thePistachioPVAc(1.38 mm) treatments, respectively. The use of mulches had significant (P < 0.01) effects on the fractal dimension of aggregates. The more stable and coarser aggregates formed in the presence ofPistachioPVAchad the smallest fractal dimension. The largest negative correlation between the properties investigated and the fractal dimension was forSOCandMWD. Therefore, it appears that the theory of fractal dimensions is useful for explaining the temporal variation of aggregate stability in soil stabilized by combined mulches.HighlightsEffects of combined mulches on aggregate formation were investigated with fractal geometry theory (FGT).The more stable and coarser aggregates formed in the presence ofPistachioPVAmulch had the smallest fractal dimension.Mulches increased soil organic carbon and microbial respiration significantly (P < 0.01).TheFGTmight be useful for explaining the temporal variation in aggregate stability of stabilized sand dunes.

Short-Term Dynamics of Soil Aggregate Stability in the Field

Aggregate stability is a key property affecting the movement and storage of water, seedling emergence, and soil sensitivity to erosion. Many studies have shown that aggregate stability changes through time. Field monitoring studies performed with a relatively large (monthly) time step showed the seasonal trend of aggregate stability. However, shorter time step monitoring is required to explore dynamics of aggregate stability at short term. For now, biological activity was recognized to be the main factor of aggregate stability dynamics, but previous studies were currently based on the external stimulation of aggregate stability. The objectives of this study were to assess variations in aggregate stability at short time steps in the field and to identify the factors controlling these variations of stability. A 6-mo field monitoring was performed at short time step (2–5 d) on a bare field of Luvisol without organic amendment. Aggregate stability was measured for both on surface and subsurface materials by the ISO 10930 (2012) method. Rain amount and intensity, air temperature and humidity, soil temperature, water content and hydric history, and soil water repellency were measured as explanatory factors. The results showed that aggregate stability varied greatly (up to 40%) over a few days for both surface and subsurface. Short term dynamics of aggregate stability were already shown by laboratory experiments, but such dynamics was never observed in the field for a bare soil without external stimulation of biological activity. For the surface, short time step variations of surface aggregate stability were primarily controlled by soil water content (WC0 and WC1/2), hydric history (ΔWC4 and API), and rain intensity. While large changes in aggregate stability were found for the subsurface, explanatory factors remain to be found.

Extensive diversity of prion strains is defined by differential chaperone interactions and distinct amyloidogenic regions.

Amyloidogenic proteins associated with a variety of unrelated diseases are typically capable of forming several distinct self-templating conformers. In prion diseases, these different structures, called prion strains (or variants), confer dramatic variation in disease pathology and transmission. Aggregate stability has been found to be a key determinant of the diverse pathological consequences of different prion strains. Yet, it remains largely unclear what other factors might account for the widespread phenotypic variation seen with aggregation-prone proteins. Here, we examined a set of yeast prion variants of the [RNQ+] prion that differ in their ability to induce the formation of another yeast prion called [PSI+]. Remarkably, we found that the [RNQ+] variants require different, non-contiguous regions of the Rnq1 protein for both prion propagation and [PSI+] induction. This included regions outside of the canonical prion-forming domain of Rnq1. Remarkably, such differences did not result in variation in aggregate stability. Our analysis also revealed a striking difference in the ability of these [RNQ+] variants to interact with the chaperone Sis1. Thus, our work shows that the differential influence of various amyloidogenic regions and interactions with host cofactors are critical determinants of the phenotypic consequences of distinct aggregate structures. This helps reveal the complex interdependent factors that influence how a particular amyloid structure may dictate disease pathology and progression.

Biosolids Increase Soil Aggregation and Protection of Soil Carbon Five Years after Application on a Crested Wheatgrass Pasture

Biosolids application to rangelands and pastures recycles nutrients and organic matter back to soils. The effects of biosolids (20 and 60 dry Mg ha(-)(1)) and N+P fertilizer on soil aggregate stability, bulk density, aeration porosity, and total C and N of stable aggregates were evaluated 4 and 5 yr after surface application to a crested wheatgrass [Agropyron cristatum (L.) Gaertn.] pasture in the southern interior of British Columbia (BC). The experiment was established in 2001 in a randomized complete block design with four replications. The 60 Mg ha(-1) biosolids treatment (Bio 60) had a greater aggregate mean weight diameter (MWD) and proportion of water-stable soil aggregates > 1 mm relative to the control and fertilizer treatments. Temporal variation in aggregate stability was attributed to seasonal variations in soil water content. Surface application of 60 Mg ha(-1) of biosolids increased C concentrations within water-stable aggregates relative to the control from 29 to 104, 24 to 79, and 12 to 38 g kg(-1) for the 2 to 6, 1 to 2, and 0.25 to 1 mm size fractions, respectively. The concentration of N within aggregates increased in similar proportions to C. Neither soil bulk density, nor aeration porosity were affected by biosolids application. Increased aggregation and the accumulation of soil C within aggregates following biosolids application creates a potential for better soil C storage, soil water retention, nutrient availability, and ultimately the overall health of semiarid perennial pastures.

Extent of soil water repellency under long-term no-till soils

Water repellency (WR), the ability of a soil to slow the water entry (contact angle < 90°), can be indispensable to stabilize soil aggregates and promote long-term soil organic carbon (SOC) sequestration. Data on WR for agricultural soils are, however, extremely limited to ascertain the extent of this property. Thus, we assessed the WR in long-term no-till (NT) fields adjacent to plow tillage (PT) and woodlot (WL) and its statistical relationships with SOC concentration, soil particle-size distribution, and aggregate stability across 11 soils distributed in Major Land Resources Areas (MLRAs: 121, 122, and 125 in Kentucky, 99, 124, and 139A in Ohio, and 139B, 139C, 140, 147, and 148 in Pennsylvania) in the eastern US. The WR tended to increase with increasing soil water potential, and management impacts on WR depended on soil water potential. Mean water drop penetration test (WDPT) ranged from 0.5 to 12 s while water repellency index ( R index) ranged from 1.5 to 5 for air-dry aggregates in the surface 0- to 5-cm soil depth. At the same depth, WR in NT was significantly higher than in PT by 1.5 to 6.0 times in 8 out of 11 soils. These soils had weak and very weak water repellency. In MLRA 124, NT had WR 30% higher than PT soil in the 0- to 5-cm depth but had lower WR in lower depths. The SOC concentration explained 28% ( P < 0.001) of the variations in LogWDPT, which, in turn, explained 45% ( P < 0.001) of the variations in aggregate stability. The LogWDPT increased with an increase in sand concentration ( r = 0.44; P < 0.001) and decrease in clay concentration ( r = − 0.41; P < 0.001). The WDPT was moderately correlated ( r = 0.39; P < 0.05) with R index. The WDPT was more strongly correlated with SOC concentration, aggregate stability, and soil texture than R index. Overall, NT farming induced a slight increase in water repellency in most soils, attributed to increases in SOC concentration.

Caracterização de propriedades edáficas em áreas sob manejo orgânico e natural na região serrana do Estado do Rio de Janeiro

This work was accomplished in Nova Friburgo, highland region of Rio de Janeiro State. It was studied two rural properties whose producers use organic and natural management of the soil. The natural management differs from the organic for not using crop rotation and organic fertilization with animal dejections. The two rural properties present distinct soil classes; therefore, a comparison among the two soil management systems was not possible. The study had as main objective to evaluate the degree of modification of soil properties in areas submitted to organic and natural management. In each area soil samples were collected under crop areas and control. In natural management, the control was a fallow area. In organic management, the control was a secondary forest area. The results showed that in the area under natural crop management levels of Ca, Ca+Mg, P and pH in water were statistically higher, when compared to the control. For H+Al, the highest values occurred in the reference area, when compared to cultivated areas. In the organic management, higher values of Ca, Ca + Mg, Na, K and P were observed in the crop area in all depths. However, for H + Al and NO3 - , the higher values were found in the control area. A degradation of the soil physical properties in the area under natural management was indicated by variation in aggregate stability and soil bulk density, but the organic management has benefited those properties. The selected and evaluated variables were capable to discriminate the different management systems studied by the main components analysis.

A Model to Predict Soil Aggregate Stability Dynamics following Organic Residue Incorporation under Field Conditions

Our ability to predict the effects of various organic amendments on soil aggregate stability is limited due to the complexity of the biological, chemical, and physical mechanisms involved. Based on previous experimental results, this study developed a model (Pouloud) to predict the dynamics of aggregate stability following the incorporation of various organic residues under field conditions. Following Monnier's conceptual model and previously published data, a lognormal function is first used to describe changes in aggregate stability after organic inputs under laboratory conditions. Using principal component analysis, the parameters of the lognormal function are associated with the biochemical characteristics of the organic products such as water‐extractable polysaccharide, cellulose and hemicellulose, and lignin contents. To simulate aggregate stability dynamics under field conditions, the effects of soil moisture, soil temperature, and N availability are taken into account by specific functions obtained from the literature. When model simulations were compared with experimental results under field conditions, variations in aggregate stability were generally well reproduced. The sensitivity of the model to climate variations and organic residue characteristics was tested. Soil N availability and the substrate lignin content are major factors that influence the prediction of aggregate stability dynamics. Our results suggest that prediction of aggregate stability dynamics under field conditions using organic substrate characteristics and simple climatic data is possible. More work is required to test the model and broaden its applicability to other soil and climatic conditions.

Aggregation of a degraded lowland soil during restoration with different cropping and drainage regimes

Rate of change of surface soil aggregation under different cropping and subsurface drainage regimes was studied on a badly degraded lowland soil in the Lower Fraser Valley, British Columbia, Canada. The soil was the silty clay loam Humic Luvic Gleysol. Two cropping practices — continued spring-sown barley underseeded with clover for winter cover cropping and a 3 year grass ley — were established in a subsurface drained site and a poorly drained (no subsurface drainage) site. Grass ley consistently improved surface aggregate stability of drained and undrained soils when compared to cash-winter cover cropping integration. Improved aggregate stability with grass was significantly correlated with increasing soil organic carbon content. Aggregate stability and its correlation with organic carbon varied with time of sampling, being lower in the early spring and higher in the fall. Seasonal variation in aggregate stability was attributed to soil water content at sampling.

Seasonal variation in the aggregate stability of downland soils

Abstract. Seasonal variation in the aggregate stability of chalk downland soils on the South Downs, East Sussex, UK, was studied using two measures of aggregate stability: water stable aggregation by wet sieving and dispersibility by a turbidimetric determination. Aggregate stability and organic carbon content were assessed on a monthly basis at 20 sites over a 19‐month period.Results indicated considerable variation in water stable aggregation over the time period studied but little variation in dispersibility. There were differences between sites mainly reflecting differences in organic carbon content. Soils with more organic carbon showed less seasonal variation in aggregate stability than soils with small amounts of organic carbon. This suggests that in less organic soils organic materials, mainly microbial in origin, play an important role in forming stable aggregates, though their effect is transient.

Temporal Variation in Aggregate Stability on Conventional and Alternative Farms

Low-input alternative farm management practices have been shown to decrease erosion rates and increase soil organic matter contents relative to conventional management practices in eastern Washington state. Little is known about the effect of such alternative practices on aggregate stability. Temporal variations in aggregate stability were measured at three slope positions on soils from adjacent farms managed for many years using conventional vs. alternative tillage and crop rotations. The conventional farm was first cultivated in 1908, and uses a winter wheat (Triticum aestivum L.)-spring pea (Pisum sativum L.) rotation with summer fallow every sixth year. The alternative farm was first cultivated in 1909, and uses a winter wheat-spring pea-Austrain winter peas [P. sativa ssp. arvense (L.) Poiret]-spring wheat-spring pea-summer fallow rotation with an alfalfa (Medicago sativa L.)-wheatgrass [Elymus trachycaulus (Link) Gould ex Shinn.] mixture every 19th and 20th year. Surface soils (0–15 cm) were collected at foot-, back-, and topslope positions from both farms during October 1987, March 1988, and June 1988. The 0.5- to 1.0-mm sieved aggregate-size fraction from each farm, landscape position, and sampling time was analyzed for aggregate stability using the high-energy moisture characteristic method. Aggregate stability on both farms decreased significantly from October to March in response to precipitation and cycles of winter freezing and thawing. Significant increases in stability occurred from March to June on both farms. These increases were interpreted in terms of recovery of cohesion by aggregates following their disruption during the winter. Differences in stability resulting from management practices on the two farms were not significant, even though organic C contents on the alternative farm were significantly higher than those on the conventional farm.

Structural degradation of two Vertisols under continuous cultivation

Changes in soil physico-chemical properties as a result of continuous cereal cropping for up to 64 years of two southern Queensland Vertisols were investigated. The soils were the Waco black earth and the Langlands grey clay. For both soils, cultivation increased bulk densities relative to those of respective virgin soils, but soil density varied little among cultivated sites. The size and strength of dry seedbed aggregates decreased owing to cultivation, but remained suitable for wheat establishement. The stability of wet aggregates decreased with cultivation as indicated by dispersion and slaking measurements. The increased dispersion was associated with decreases in hydraulic conductivity which may limit infiltration and storage of soil water during the summer fallow, thereby reducing crop yields. Organic carbon contents decreased with cultivation while exchangeable sodium percentage (ESP) and pH values increased. Increased dispersibility was correlated with increased sodicity, although ESP values remained below accepted critical values. The relationship was strongest when both dispersibility and exchangeable sodium were expressed on an oven-dry soil basis rather than as the relative parameters, dispersion ratio and ESP. Organic carbon content accounted for less than 40% of the temporal variation in aggregate stability. The changes in physical and chemical properties with cultivation were consistent with exposure of subsurface material following soil erosion at rates typical for the region. We concluded that erosion control would limit further changes in levels of exchangeable sodium and thus soil dispersibility and hydraulic conductivities. The rate of loss of organic carbon would also be reduced and soil physical properties and chemical fertility would be maintained.

Temporal Changes in Wet Aggregate Stability

Aggregate stability, a property that influences a soils erodibility and hydraulic characteristics, has been shown in previous investigations (e.g., Bullock et al., 1988) to vary over time for some northwestern U.S. soils. The objectives of this study were to evaluate three procedures for measuring aggregate stability and quantify variation in aggregate stability over time (that is, within a growing season) for selected soils across the United States. In 1988 and 1989, soils from 11 states were sampled monthly from April to July and in September. The stability of 1- to 4-mm aggregates from each sample was measured by 1) wetting air-dry (A-D) aggregates in a vaporizer followed by wet-sieving; 2) immersing A-D aggregates immediately prior to wet sieving; and 3) vapor-wetting field-moist aggregates followed by wet sieving. The sieving of vapor-wetted field-moist aggregates best revealed temporal variation. In general, the aggregate stability of northern soils varied more over time than did the aggregate stability of southern soils, probably due to differences in freezing and thawing. Biological activity likely accounted in part for temporal changes in the stability of soils from the upper Midwest. Trends in stability change over time in soils from one region of the United States to another were seldom similar.

Aggregate Stability in the Palouse Region of Washington: Effect of Landscape Position

AbstractAn extensive study was conducted to measure spatial patterns in aggregate stability in the steep, hilly, and highly erodible loessial soils of the Palouse region in southeastern Washington. Soil samples were collected at 20‐m spacings along four 800‐m‐long transects within a winter wheat field and analyzed for aggregate stability at a slow and a fast rate of wetting, organic C, amorphous Fe, soil water content, and particle size distribution. Aggregate stability under slow wetting, organic C, and clay content were significantly different at summit, shoulder, toeslope, and footslope positions on the landscape. More specifically, aggregate stability and organic C content were highest in footslope and toeslope positions, and lowest at the summit. Clay content was highest at the summit and lowest in footslope positions. Aggregate stability at a fast rate of wetting had a coefficient of variation (40%) that was nearly double that for slow wetting, and was not well correlated to either landscape position or measured soil properties. The variation in aggregate stability under slow wetting could be modeled in terms of organic C content and elevation. Other properties such as amorphous Fe and water or clay content were only weakly correlated to aggregate stability. Thus, although the spatial patterns in aggregate stability exhibited moderate variability (CV = 22%), the patterns were closely related to changes in a few key properties, namely, organic C content and landscape position. A likely explanation for these findings is that soil erosion removes the topsoil and organic matter from the ridgetops, thus exposing subsoil horizons, which are higher in clay content and lower in aggregate stability.

EFFECTS OF CROPPING ON MACRO-AGGREGATION OF A MARINE CLAY SOIL

Field and growth chamber experiments were conducted to evaluate the short-term effects of selected crops on macro-aggregation of a Kamouraska clay soil. Under field conditions, the growth of barley (Hordeum vulgare L.) and alfalfa (Medicago sativa L.) for up to 2 yr resulted in increased macro-aggregate size and stability compared to a fallow control and to initial conditions. Under these two crops, the proportion of water-stable aggregates of the 2- to 6-mm fraction increased from 25% in May 1986 to 40% in September 1987 at the expense of the 0.25- to 1.0-mm fraction which decreased from 37% to 19% over the same period. Macro-aggregation after 2 yr was not different in corn (Zea mays L.) and fallow control. Seasonal variations in aggregate stability were significant but small and less important than the effects of cropping treatments. Results of the growth chamber experiment agreed with those obtained under field conditions suggesting that controlled environment experiments can be used to model cropping effects on the aggregation of this soil. Key words: Aggregation, soil structure, clay soil, corn, barley, alfalfa