Differences In Aggregate Stability Research Articles

Effects of cyanobacteria- and moss-biocrusts on soil aggregate stability and splash erosion in croplands of the China Mollisols area.

To investigate the effects of biocrusts development on aggregate stability and splash erosion of Mollisols and to understand its function in soil and water conservation, we collected biocrusts (cyano crust and moss crust) samples in croplands during the growing season and measured the differences in aggregate stability between biocrusts and uncrusted soil. The effects of biocrusts on reduction of raindrop kinetic energy were determined and splash erosion amounts were obtained with single raindrop and simulated rainfall experiments. The correlations among soil aggregate stability, splash erosion characteristics, and fundamental properties of biocrusts were analyzed. The results showed that compared to uncrusted soil, the cyano crust and the moss crust decreased the proportion of soil water-stable aggregates <0.25 mm by 10.5% and 21.8%, respectively, while their soil water-stable aggregates 5-10 mm were 4.0 and 8.8 times as that of uncrusted soil. In contrast to uncrusted soil, the macroaggregate content (R0.25), mean weight diameter (MWD), and geometric mean diameter (GMD) of biocrusts were 31.5%, 76.2%, and 33.5% higher, respectively. In addition, biocrusts reduced raindrop kinetic energy by an average of 0.48 J compared to uncrusted soil. The breakthrough raindrop kinetic energy of cyano crust and moss crust were 2.9 and 26.2 times as that of uncrusted soil, while the reduction of raindrop kinetic energy by cyano crust with high biomass was 1.3 and 6.6 times as that of medium and low biomass, respectively. Under the single raindrop and simulated rainfall conditions, biocrusts reduced splash erosion amounts by 47.5% and 79.4%, respectively. The proportion of aggregates >0.25 mm in the splash soil particles of biocrusts (37.9%) was 40.3% lower than that of uncrusted soil, while the proportion of aggregates >0.25 mm decreased as biocrust biomass increased. Moreover, the aggregate stability, splash erosion amount, and fundamental properties of biocrusts were significantly correlated. The MWD of aggregates was significantly and negatively correlated with the splash erosion amount under single raindrop and simulated rainfall conditions, indicating that the improved aggregate stability of surface soil caused by biocrusts accounted for reducing splash erosion. The biomass, thickness, water content, and organic matter content of biocrusts had significant effects on aggregate stability and splash characteristics. In conclusion, biocrusts significantly promoted soil aggregate stability and reduced splash erosion, which had great significance to soil erosion prevention and the conservation and sustainable utilization of Mollisols.

Specific ion effects on soil aggregate stability and rainfall splash erosion

Soil interparticle forces can pose important effects on soil aggregate stability and rainfall splash erosion. Meanwhile, these interparticle forces are strongly influenced by specific ion effects. In this study, we applied three monovalent cations (Li+, Na+, and K+) with various concentrations to investigate the influence of specific ion effects on aggregate stability and splash erosion via pipette and rainfall simulation methods. The specific ion effects on soil interparticle forces were quantitatively evaluated by introducing cationic non-classical polarization. The results showed that aggregate stability and splash erosion had strong ion specificity. Aggregate breaking strength and splash erosion rate at the same salt concentration followed the sequence as Li+ > Na+ > K+. With decreasing salt concentration, the difference in aggregate breaking strength or splash erosion rate between different cation systems increased initially (1–10–2 mol L–1) and later was nearly invariable (10–2–10–4 mol L–1). The experimental results were well quantitatively explained by soil interparticle forces considering cationic non-classical polarization. Furthermore, both aggregate breaking strength and splash erosion rate of three cations revealed a strong positive linear relation with net force subjected to cationic non-classical polarization (R2 = 0.81, R2 = 0.81). These results demonstrated that different non-classical polarization of cations resulted in different soil interparticle forces, and thus led to differences in aggregate stability and splash erosion. Our study provides valuable information to deeply understand the mechanisms of rainfall splash erosion.

The effect of four calcium‐based amendments on soil aggregate stability of two sandy topsoils

AbstractThe structural stability of soil is a physical characteristic that affects soil degradation processes. Calcium‐based amendments, such as calcium carbonate, calcium sulfate, and calcium oxide/hydroxide, have been shown to improve the stability of soil aggregates. This study seeks to determine which calcium‐based soil amendments, and at what concentration, are the most efficient in improving aggregate stability of sandy topsoils derived from granitic and metamorphic parent materials, and to analyze the mechanisms involved. In the pot experiment, soils amended with CaCO3, CaCl2, and CaSO4 did not present significant differences in aggregate stability compared to the control or among each other. In contrast, Ca(OH)2 soil amendment brought the greatest stability to the soil aggregates. A dose of 1% Ca(OH)2 significantly increased the stability of soil aggregates. This effect is due to the reaction of Ca(OH)2 with atmospheric CO2 which leads to the formation of CaCO3, a delayed reaction not showed by the other soil amendments tested. Likewise, the greater solubility of Ca(OH)2 compared to CaCO3 exerts a greater aggregation effect on soil. Thus, the mechanism of action of Ca(OH)2 is related to cementation, rather than flocculation. Future studies should be carried out to demonstrate the effectiveness of Ca(OH)2 under field conditions.

125 Years of Soil and Crop Management on Sanborn Field

Long-term management systems cause changes to soil physical properties that may affect soil erosion and erodibility. A study was conducted to evaluate the effects of 125 years of continuous crop management on Sanborn Field for selected soil physical properties. Intact soil cores were collected from continuous corn (Zea mays L.), continuous wheat (Triticum aestivum L.), continuous timothy (Phleum pratense L.), and a rotation of corn–wheat–red clover (Trifolium pratense L.). Soil samples were collected from the surface horizon throughout 1 year (April, July, and November 2014 sampling dates). Aggregate stability, soil splash detachment, bulk density, and soil strength were measured. Significant differences in aggregate stability (P < 0.01), splash detachment (P < 0.01), soil shear strength (P < 0.05), and bulk density (P < 0.05) were found among the treatments. Continuous timothy had three to four times better aggregate stability and 50% to 75% less splash detachment compared with continuous wheat and corn, respectively. Lowest aggregate stability, lowest soil strength, highest bulk density, and highest soil splash detachment were found under continuous corn. Highest aggregate stability was found during July. Annual crops with tillage have a negative effect on soil quality and erodibility. Comparing results after 125 years with data collected after 105 years illustrates that properties have not changed dramatically during the past 20 years. Assessing the effects of long-term soil management on soil quality and erodibility is critical for society to determine the amount of soil erosion associated with selected soil management and to develop appropriate conservation practices to minimize this challenge and promote long-term sustainability.

Conversion of forest into irrigated pasture II. Changes in the physical properties of the soil

The replacement of natural vegetation (NV) by agricultural species may cause important changes in the ecosystem. Land use and management result in modifications of soil properties and functions, especially with the application of irrigation. The aim here therefore, was to assess the physical properties that are indicative of soil quality in an area under irrigated pasture (PAST), and to compare the results with those obtained in an area under natural vegetation. In order to identify changes in the soil, some physical attributes {aggregate stability, soil water retention curve, Dexter soil physical quality index (S), and least limiting water range} were determined, together with the total amount of lipids and glomalin, which can act as structure-stabilizing agents for different classes of aggregate. Rhizodeposition, a characteristic of grasses, and associated with the supply of plant biomass from the management practices used in PAST, may explain the differences in aggregate stability in the studied areas. The results showed that, despite the highest levels of lipids being found in PAST, it can be inferred that, due to the sandy texture and trampling by animals, there has been no effective structuring of the soil on the surface. In contrast to the trend observed for the lipid content of the soil, glomalin levels were greater in NV, and it was demonstrated that the glomalin content reacts sensitively to changes in land use. The least limiting water range (LLWR) is narrower under PAST when compared to NV, and reflects the loss of the physical quality of the soil due to the management of the pasture. Under such conditions therefore, plants may be susceptible to water stress when there is no irrigation. It can be concluded that the management of animals and pasture adopted in this study negatively affected the physical properties under evaluation, despite the better chemical and organic conditions of the soil.

Influence of tall fescue endophyte infection on structural stability as quantified by high energy moisture characteristic in a range of soils

Plant microbial symbionts could play a significant role on aggregate stability, an important index for soil physical quality. So far there is little information available linking the presence of endophyte in the aboveground parts of plants, with aggregate stability of rhizosphere soil. We hypothesized that the presence of Epichloë coenophiala in the aboveground parts of tall fescue (Festuca arundinacea Schreb.=Schedonorus arundinaceus and Lolium arundinaceum) may alter the structural properties of rhizospheric soil. A greenhouse pot experiment was performed to evaluate the evidence for a relationship between endophyte–tall fescue symbiosis and rhizosphere aggregate stability for a range of soils. Traditional methods for measuring aggregate stability would not be able to distinguish the impacts of these biotic factors because they usually rely on rapid immersion of aggregates in water which is very disruptive. In this study, the aggregate stability was quantified by high energy moisture characteristic (HEMC) because HEMC is capable to accurately characterize small differences in aggregate stability among the treatments and soils. Clonally propagated tillers of tall fescue (genotype 75C) naturally infected with E. coenophiala (E+) and endophyte-free ones (E−) were used. Plants were grown under optimal water, light and temperature conditions. After eight months, when plants reached the maximum vegetative growth, intact soil aggregates adherent to plant roots (i.e. rhizosphere soil) and those free of root (i.e. bulk soil) were collected for HEMC determination. The HEMCs of slow and fast wetted aggregates were determined at high matric suctions of 0 to 50hPa. HEMC data was modeled using a modified van Genuchten function. Then, various indices including volume of drainable pores (VDP), relative VDP (RVDP), VDP ratio (VDPR), modal suction (hmodal), relative hmodal (Rhmodal), stability index (SI), relative SI (RSI), stability ratio (SR), relative SR (RSR), slope at the inflection point (Si), and inflection point slope ratio (SiR) were derived to characterize soil structural stability. Endophyte presence was correlated with increased soil organic carbon (SOC) pools especially for the fine-textured soils. Changes in the SOC storage of rhizosphere soil altered the structural stability and increased the SR. The effect of endophyte infection on SR of the rhizosphere soil was also more evident in medium- and fine-textured soils due to physical protection of SOC. Lower values of hmodal and higher values of VDP of fast wetted aggregates revealed more stable macropores and greater aggregate stability in the E+ rhizosphere soil vs. E− one. VDPR, SR and SiR values of rhizosphere soil were greater in E+ plants associated soils. Fine-textured soils had lower values of SiR because of their greater values of Si in the slow wetting treatment. Plant root presence could improve the soil structure compared to the bulk soil due to exudation of cementing materials; values greater than 1 for RVDP, RSI and RSR indicate that plant root enhanced the structural stability of rhizosphere soil. In contrast to stability ratio, the effects of Epichloë endophyte–tall fescue symbiosis on relative structural stability indices such as RVDP, Rhmodal, RSI, and RSR were more pronounced in coarse- and medium-textured soils. Overall, endophyte–tall fescue symbiosis could increase aggregate stability of rhizosphere soil and thus its physical quality.

Effect of humic acids from Leonardite on the stability of soil aggregates and melon roots under greenhouse conditions

Leonardite is an oxidized form of lignite carbon, which is obtained from fossilized organic materials. Such materials are used for the extraction of humic acids (HA). The result of the addition of HA of organic origin on soil structure is known; however, the effects of adding HA of Leonardite on soil structure have been scarcelyinvestigated. The objectives of this research were (1) to determinethe influence of humic acids derived from Leonardite in increasing the aggregate stability of an Aridisol under greenhouse conditions, and (2) evaluate the morphology of the root xylem during the phenological development of melon plants (Cucumis melo L.). Three treatments of HA solution application to the soil were used: soil without solution application (HA0), and application of HA solutionto the soil with pH 6 (HA6) or (HA7). Aggregate stability (As) and bulk density (Da) were evaluated as soil variables. Development and quantification of xylem area were studied on plants. There were significant differences in aggregate stability. Also, there was an increase in the root xylem area, and the best treatment was when AH7 solution was applied. Humic acids derived from Leonardite increased the stability of soil aggregates when plants grew under greenhouse conditions, and fostered the development of xylem conduits during the fruiting stage.

Biophysical Underpinnings of the Repeat Length Dependence of Polyglutamine Amyloid Formation

There are now 10 expanded CAG repeat diseases in which both disease risk and age of onset are strongly dependent on the repeat length of the polyglutamine (polyQ) sequence in the disease protein. Large, polyQ-rich inclusions in patient brains and in cell and animal models are consistent with the involvement of polyQ aggregation in the disease mechanism. This possibility is reinforced by studies showing strong repeat length dependence to the aggregation process, qualitatively mirroring the repeat length dependence of disease risk. Our understanding of the underlying biophysical principles that mediate the repeat length dependence of aggregation, however, is far from complete. A previous study of simple polyQ peptides showed that N*, the size of the critical nucleus that controls onset of aggregation, decreases from unfavorable tetramer to favorable monomer over the range Q23 to Q26. These data, however, do not explain why, for all peptides exhibiting N* ∼ 1, spontaneous aggregation rates continue to increase with increasing repeat length. Here we describe a novel kinetics analyses that maps out the nonlinear dependence with repeat length of a nucleation efficiency term that is likely related to aspects of nucleus structure. This trend accounts for why nucleus size increases to tetrameric at repeat lengths of Q23 or below. Intriguingly, both aggregation and age of onset trend with repeat length in similar ways, exhibiting large changes per added Gln at low repeat lengths and small changes per added Gln at relatively long repeat lengths. Fibril stability also increases with repeat length in a nonlinear fashion.

Estabilidade de agregados em argissolo sob diferentes usos, comparado com mata

A estrutura exerce grande importância no comportamento agrícola do solo, pela sua influência em propriedades do solo. Assim, objetivou-se avaliar a estabilidade de agregados e relacioná-la com diferentes frações orgânicas de um ARGISSOLO Vermelho-amarelo distrófico sob mata, seringueira, laranjeira e pastagem degradada. A amostragem foi realizada em trincheiras, de 0 a 10, 10 a 20 e 20 a 30 cm. Foram analisadas textura, argila dispersa em água, grau de floculação, densidade do solo, carbono orgânico total (COT), nitrogênio total (NT), ácidos fúlvicos (AF), húmicos (AH) e humina (HN), carbono da biomassa microbiana (C-Micr) e matéria orgânica leve (MOL). Foi determinada a estabilidade de agregados em água e calculados índices de estabilidade. Os resultados mostram diferenças de densidade do solo, seguindo a ordem mata &lt; seringueira &lt; laranjeira &lt; pastagem. A estabilidade de agregados não se altera entre os usos nas profundidades de 0 a 10 e 10 a 20 cm, enquanto de 20 a 30 cm nota-se a ordem mata &gt; seringueira &gt; laranjeira &gt; pastagem. No solo sob mata, seringueira e laranjeria a estabilidade de agregados está relacionada aos maiores teores de COT, NT,AF, AH, HN, C-Micr e MOL, enquanto na pastagem está relacionada principalmente a ciclos de umedecimento e secagem.

Wildfire effects on soil erodibility of woodlands in NW Spain

AbstractKnowledge of soil erodibility following wildfire is of crucial importance for prioritisation of post‐fire restoration practices for soil erosion mitigation. The present work therefore aims to determine the effect of wildfire on soil erodibility for common woodlands in Galicia, NW Spain. This is done by comparing selected topsoil properties of 28 pairs of recently wildfire‐burned and neighbouring unburned sites on different geologic substrates. The soil properties were selected for their supposed importance in erodibility, and include aggregate size distribution and water aggregate stability. Comparison of the burned and unburned sites suggested that wildfire had a noticeable negative effect on aggregate size distribution but not on particle size distribution. The effect on aggregate stability was highly variable. Aggregate stability was clearly lower at the burned than unburned site in about a third of the cases and in the remaining cases either basically the same at both sites or higher at the unburned site. The differences in aggregate stability, like those in aggregate size distribution, appear to be associated with changes in organic carbon content. The impact of wildfire on soil erodibility is supposed to operate through its effect on soil organic matter and, thus, to depend strongly on fire severity. Soil erodibility is then little affected by low‐severity wildfires but markedly diminished following high‐severity wildfires. All burned topsoils were strongly to very strongly water repellent. Fire‐enhaced repellency is therefore viewed as a key factor in the post‐fire runoff and erosion processes repeatedly observed in recently burned areas in NW Spain. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Differences in aggregate stability due to various sewage sludge treatments on a Mediterranean calcareous soil

Three types of sewage sludge from the same wastewater treatment plant but processed in different ways (composted, fresh and thermally dried) were applied in doses equivalent to approximately 10 Mg of dry matter per hectare to the surface of a loamy soil in order to improve its physical condition. The effect of this organic amendment on the structural stability of the soil was measured using a procedure which explores three types of disaggregation mechanisms: slaking, mechanical breakdown by raindrop impact and disaggregation induced by differential swelling. Two years after the surface application of the sludge, the treated soils have a higher organic carbon and hydrolyzable carbohydrate content and greater structural stability than the control treatment. It was observed that while all three types of sludge reduced soil disaggregation due to differential swelling and splash, only the treatments with composted and thermally dried sludge were effective in reducing slaking.

Efeito do manejo do solo e da matéria orgânica solúvel em água quente na estabilidade de agregados de um latossolo argiloso

Estudou-se a estabilidade de agregados de um latossolo vermelho distroférrico submetido às seguintes situações: vegetação nativa de floresta; culturas anuais por 20 anos; pomar cítrico manejado com cobertura verde permanente com a leguminosa Arachis prostrata Bong. ex Benth.; pomar com cobertura de vegetação espontânea (predomínio de gramíneas) controlada com roçadora (3-4 vezes no período de chuvas) e uma gradagem a disco ao ano (no período seco); pomar sem vegetação por meio de capina manual. Os tratamentos no pomar foram mantidos durante 9 anos. A estabilidade de agregados foi determinada em amostras submetidas ou não a tratamento para retirada da matéria orgânica solúvel em água quente. O solo sob vegetação de floresta teve maior quantidade de agregados estáveis, seguido do solo sob pomar com cobertura permanente de leguminosas ou gramíneas. A estabilidade dos agregados não foi afetada pela extração do carbono solúvel em água quente, embora tenha havido correlação positiva entre o teor de carbono solúvel em água quente e a agregação do solo.

Pore structure changes during decomposition of fresh residue: X-ray tomography analyses

The morphology of soil pores is a crucial factor in the understanding of the ecology of soil microorganisms at a small scale. X-ray computer tomography (CT) enables to visualize the soil pore space in three dimensions. We aimed at exploring possible changes in porosity and pore morphology during the incubation of soil to which fresh residue was added. These changes were compared to changes in porosity and pore morphology in field aggregates. A set of CT images is presented from newly formed, incubated and field aggregates from both a sandy loam and a silt loam soil. These aggregates are about 6–8 mm diameter. After incubation of the artificial aggregates, we observed the formation of micro-cracks. At the same time, the total void porosity > 27 μm increased from 3.5% to 7%. The pore size distribution indicated that this extra porosity was present in the 27–67 μm range. Furthermore, it was found that this new pore space was (partly) located near decomposing residue. To quantify the observed changes in pore morphology, we explored the use of the mass fractal dimension and variograms. After correction for differences in porosity, we found that incubation decreased the mass fractal dimension to the level of field aggregates in the sandy loam soil, while no differences were observed in the silt loam soil. In contrast, variogram parameters of the different treatments did not change consistently with the observations in both soils. In conclusion, the data suggest that microbial activity changes the morphology of the pore structure. However, the observed changes in the pore architecture were not sufficient to explain the large differences in aggregate stability among the analyzed samples.

Dynamics of Carbon Sequestered in Concentric Layers of Soil Macroaggregates

Methods used to study carbon sequestration by soil aggregates have often excluded the concentric spatial variability and other dynamic processes that contribute to resource accessibility and solute transport within aggregates. We investigated the spatial gradients of carbon (C) and nitrogen (N) from the exterior to interior layers within macroaggregates, <TEX>$6.3\sim9.5$</TEX> mm, sampled from conventional tillage (CT) and no tillage (NT) sites of a Hoytville silt clay loam. Spatial gradients in C accumulation within macroaggregates were related to the differences in C dynamics by determining the sizes and the turnover rates of fast C and slow C pools in the concentric layers of aggregates. Aggregate exteriors contained more labile C and were characterized by greater C mineralization rates than their interiors in both management systems. In contrast, C in the interior layers of aggregates was more resistant in both systems. These results indicated the spatial differentiation of C dynamics within macroaggregates, i.e., exterior layers as a reactive site and interior layers as a protective site. Greater total C distribution in the exterior layers of NT aggregates indicated more influx of C from the macropores in interaggregate space than C. mineralization (net gain of C), whereas lower C distribution within the exterior layers of CT aggregates indicated net loss of C by greater C mineralization than C influx. We found total C increased approximately 1.6-fold by the conversion of CT soils to NT management systems for a period of 36 years. Differences in total accumulation and the spatial distribution of C within aggregates affected by management were attributed to the differences in aggregate stability and pore networks controlling the spatial heterogeneities of resource availability and microbial activity within aggregates.

Organic matter addition, N, and residue burning effects on infiltration, biological, and physical properties of an intensively tilled silt-loam soil

Seventy years of different management treatments have produced significant differences in runoff, erosion, and ponded infiltration rate in a winter wheat ( Triticum aestivum L.)–summer fallow experiment in OR, USA. We tested the hypothesis that differences in infiltration are due to changes in soil structure related to treatment-induced biological changes. All plots received the same tillage (plow and summer rod-weeding). Manure (containing 111 kg N ha −1), pea ( Pisum sativum L.), vine (containing 34 kg N ha −1), or N additions of 0, 45 and 90 kg ha −1 were treatment variables with burning of residue as an additional factor within N-treatments. We measured soil organic C and N, water stability of whole soil, water stable aggregates, percolation through soil columns, glomalin, soil-aggregating basidiomycetes, earthworm populations, and dry sieve aggregate fractions. Infiltration was correlated ( r = 0.67–0.95) to C, N, stability of whole soil, percolation, and glomalin. Basidiomycete extracellular carbohydrate assay values and earthworm populations did not follow soil C concentration, but appeared to be more sensitive to residue burning and to the addition of pea vine residue and manure. Dry sieve fractions were not well correlated to the other variables. Burning reduced ( p < 0.05) water stability of whole soil, total glomalin, basidiomycetes, and earthworm counts. It also reduced dry aggregates of 0.5–2.0 mm size, but neither burning nor N fertilizer affected total C or total N or ponded infiltration rate. Water stability of whole soil and of 1–2-mm aggregates was greater at 45 kg N ha −1 than in the 0 and 90 kg N ha −1 treatments. Zero N fertilizer produced significantly greater 0.5–2.0 mm dry aggregate fractions. We conclude that differences in infiltration measured in the field are related to relatively small differences in aggregate stability, but not closely related to N or residue burning treatments. The lack of an effect of N fertilizer or residue burning on total C and N, along with the excellent correlation between glomalin and total C ( r = 0.99) and total N ( r = 0.98), indicates that the major pool of soil carbon may be dependent on arbuscular mycorrhizal fungi.

Effects of liming on void space and aggregation in an upland grassland soil

The aim of this paper is to study changes in water-stable aggregation and void space in an upland grassland soil following a surface application of calcium carbonate intended to increase the diversity of soil fauna. Samples of the H and Ah horizons were collected at the time of the first lime application (June 1999) and 1 and 2 years thereafter. Void space in both horizons was quantified by image analysis of thin sections and the size distribution of water-stable aggregates in the Ah horizon was determined by laser granulometry. The effect of liming was largely seen in the H horizon where pH in CaCl 2 increased from 3.4 to 4.1. Percentage void space and % voids >1000 μm 2 increased significantly in the H horizon between 1999 and 2001, and there was a significant liming effect. In the mineral Ah horizon, only the change in void space over time was statistically significant. There were no significant differences in aggregate stability following liming or over time, despite a significant reduction in organic carbon concentration between 1999 and 2001. The change in void space with liming was consistent with reported changes in the abundance of larger enchytraeid genera such as Fridericia and with changes in earthworm abundance. Changes in void space with time may be linked to the cessation of grazing at the experimental site.

The influence of plant species, fertilization and elevated CO2 on soil aggregate stability

We tested the effects of plant species, fertilization and elevated CO2 on water-stable soil aggregation. Five annual grassland species and a plant community were grown in outdoor mesocosms for 4 years, with and without NPK fertilization, at ambient or elevated atmospheric CO2 concentrations. Aggregate stability (resistance of aggregates to slaking) in the top 0.15 m of soil differed among plant species. However, the more diverse plant community did not enhance aggregate stability relative to most monocultures. Species differences in aggregate stability were positively correlated with soil active bacterial biomass, but did not correlate with root biomass or fungal length. Plant species did not affect aggregate stability lower in the soil profile (0.15–0.45 m), where soil biological activity is generally decreased. Elevated CO2 and NPK fertilization altered many of the factors known to influence aggregation, but did not affect water-stable aggregation at either depth, in any of the plant treatments. These results suggest that global changes will alter soil structure primarily due to shifts in vegetation composition.

Aggregate stability kit for soil quality assessments

Aggregate stability affects soil strength and, therefore, the soil's ability to transmit liquids and gases, which are important functions for crop production and ecosystem health. Because aggregate stability is an indicator of vital soil functions, it can be used to assess soil quality. For soil quality assessments, there is a need for a quantitative field method for measuring aggregate stability that is simple to perform, low cost, and available for routine assessments by land managers. A method is presented that follows the commonly used or standard single-sieve wet-sieving method for aggregate stability. A combination of manual sieving and drying apparatus is constructed from a trunk-style tackle box. Sieves are constructed from a 60-mesh screen and PVC reducing adapters. The method requires manual sieving, 30 cycles per minute (0.5 cycles/s) for 3 min. Percent water aggregate stabilities from the proposed manual sieving technique were not significantly different from aggregate stabilities obtained from the mechanical wet sieving technique. The proposed low cost method was able to distinguish differences in aggregate stability caused by differences in soil type and land use. This low cost method provides a tool that can be used in conjunction with other measurements to assess relative differences in soil quality.

Microbial-induced soil aggregate stability under different crop rotations

Changes in the quantity and quality of soil organic carbon, and their effect on soil aggregate stability as a result of growing different crops in rotation with wheat, were investigated on a red earth (Oxic Paleustalf) in Wagga Wagga, New South Wales, Australia. After two cycles of the 1 : 1 rotation, while the total organic carbon in the 0–5 cm soil depth was similar (15.1 g/kg), significant differences in water stable aggregation were observed in the order: wheat/lupin=wheat/barley >wheat/canola>wheat/field pea. Using a selective extraction technique, significant differences in the quality (composition) of the soil organic carbon were detected in the soils from the different rotations. Soil from the lupin rotation had the highest salt- and acid-extractable carbon whereas that from the barley rotation had the highest level of hot-water-extractable carbon and microbial biomass carbon. Rather than total carbon or other extractable fractions, the observed differences in aggregate stability were only significantly (P<0.05) related to microbial biomass carbon, which made up only 1.3–1.7% of the total carbon pool. Multiple linear regression analysis indicated that with the exception of salt-extractable carbon, inclusion of any other of the less labile fractions failed to improve the correlation relationship. The labile nature of the microbial biomass carbon therefore accounted for the transient existence of the differences in aggregate stability under different rotation crops. The latter was found to be transient and disappeared at the end of the subsequent wheat crop.

Correlation of physicochemical properties and sub-erosional landforms with aggregate stability variations in a tropical Ultisol disturbed by forestry operations

The stability of soil aggregates against water erosion is a largely unstudied factor in research on the spatially-distributed impacts of tropical forestry. Soil strata with particularly weak aggregates are likely to be the focus for erosional activity whether buried or exposed by either natural processes or forestry activities. Understanding the location and likely cause of such intrinsic instability would, therefore, allow better spatially-distributed parameterisation of erosion models. This study has aimed to identify the range in aggregate stability within a region of Bornean Ultisol disturbed by forestry operations, and to identify the bulk soil properties associated with such stability variations. The sites sampled were subject to a range of denudational processes including piping, rilling and landslide-triggered erosion. Soil profiles with rates of erosion in excess of 10 mm a −1 were shown to have less aggregate stability (as characterised by the rainfall simulation survival index (RSSI)) than those with no visible signs of erosion. Further, large differences in aggregate stability between soil horizons of the same profile were observed and seen to be statistically correlated with the bulk soil properties of organic carbon and clay content (i.e. stabilising agents) and the dispersing agent of exchangeable sodium percentage (ESP) at sites undergoing erosion. Organic carbon appeared to be the most important governing factor, accounting for 56% of the variance in the aggregate stability. The presence of strongly and weakly expanding 2 : 1 clays within the soils may have been an additional de-stabilising factor. Reduced porosity was also observed at eroding sites with weak soil aggregates. The properties of EC 25 and sodium adsorption ratio (SAR) were not correlated with the RSSI or the presence of soil erosion. Identification of the role of organic carbon, clay and ESP in the stability of these tropical forest aggregates is important in focusing future, more intensive research on the spatial parameterisation of models to simulate forestry impacts on erosion.