Changes In Aggregate Stability Research Articles

Do temperature and moisture conditions impact soil microbiology and aggregate stability?

PurposeStudies predicting the impacts of climate change on erosion have considered numerous variables, such as rainfall erosivity and vegetation cover, but have not considered potential changes in soil erodibility. Erodibility is an intrinsic property of the soil, strongly correlated with the stability of soil aggregates. It is influenced by soil physico-chemical attributes, including the microbiological community. The study aim was to determine how shifts in temperature and moisture conditions, which other studies have shown affect microbiological communities, might affect aggregate stability.MethodsUsing an experimental approach with laboratory microcosms, aggregates from a sandy loam soil and a clay soil were incubated at three temperatures and three moisture conditions in a factorial experimental design. Aggregate stability was quantified using rainfall simulation. Microbiological indicator metrics were measured to evaluate treatment microbiological impacts, including community composition (PLFA), biomass carbon, and respiration.ResultsTemperature and moisture content affected aggregate stability significantly, but differently for the two soil types tested. For the sandy loam soil, aggregate stability decreased significantly with increasing moisture content. For the clay soil, aggregate stability increased significantly with increasing temperature. For both soil textures, temperature and moisture content affected microbiological community composition and respiration. Regression analysis indicated that microbiological properties were significant predictors of aggregate stability.ConclusionOur results emphasise the dynamic nature of soil aggregate stability. Changes in microbiological metrics suggest possible biological mechanisms for aggregate stability changes, which should be investigated further to better understand the potential impacts of climate change on soil erodibility and erosion.

Warming-dominated climate change impacts on soil organic carbon fractions and aggregate stability in Mollisols

Mollisols contain high amounts of soil organic carbon (SOC), which is highly susceptible to climate change; thus, climate change could indirectly influence soil aggregate stability, but the dominant factor affecting aggregate stability remains controversial. Here, a soil transplanting test from high-latitude to low-latitude locations was initiated in 2004 to investigate the influences of warming-dominated climate change (approximately 3–4.7 ℃) on the quantity and molecular composition of OC fractions in surface (0–20 cm) soils, aggregate stability changes and underlying mechanisms. Different initial soil organic matter (SOM) contents of 50.6 g kg−1 (SOM5), 58.8 g kg−1 (SOM6), and 108.9 g kg−1 (SOM11) were established in situ soils and in transplanted soils to simulate warming. The 15-year warming-dominated climate change presented no noticeable change in the SOC content in the lower SOM Mollisols (SOM5 and SOM6) but increased the SOC content by 13.3% in the higher SOM Mollisol (SOM11). In terms of labile OC fractions, warming-dominated climate change significantly increased the dissolved organic carbon (DOC) content by 20.1%–47.7% but reduced the easily oxidizable organic carbon (EOC) and microbial biomass carbon (MBC) contents by 22.1%–33.6%. Irrespective of any treatment, warming-dominated climate change decreased soil aggregate stability, as evidenced by the reduction in mean weight diameter (MWD) and geometric mean diameter (GMD) of 41.7%–79.3% and an increase in fractal dimension (D) of 28.6%–58.5%. For hierarchically organized soil aggregates, warming-dominated climate change increased the proportion and OC content of particulate organic matter inside free microaggregates (Fm-POM) as well as nonaggregated silt + clay-sized organic matter (nA-MOM). However, climate change decreased the proportion and OC content of silt + clay-sized fractions inside microaggregates within macroaggregates (mM-MOM). Of importance, warming-dominated climate change increased the amount of carbohydrates and decreased the amount of lignin in the mM-MOM, Fm-POM, and nA-MOM fractions. Therefore, we speculated that chemical protection by mineral association within macroaggregates and nA-MOM fractions and physical protection by the occlusion of POM within free microaggregates might be the primary mechanisms for SOC stabilization under long-term climate change in Mollisols. The long-term warming-dominated climate change results demonstrated a positive effect on SOC content in higher SOM Mollisols, DOC, carbohydrate C, free microaggregate-associated C and nonaggregated silt + clay-sized associated C but exhibited a negative effect on EOC, MBC, lignin C, silt + clay-sized inside microaggregates within macroaggregates C and aggregate stability. These variables all contributed to the reduction in soil aggregate stability and might act as sensitive indicators of warming-dominated climate change in Mollisols, which in turn affect farmland ecosystem C fluxes in response to further climate change.

Site characteristics determine the duration of structure liming effects on clay soil

Adding carbonated or non-carbonated lime to clay soils can lead to changes in aggregate stability. In Sweden, ‘structure liming’ with a mixed product (normally 80–85% calcium carbonate and 15–20% calcium hydroxide) is subsidised through environmental schemes to increase aggregate stability, thereby mitigating losses of particulate phosphorus (PP). This study assessed the effects of structure liming on aggregate stability in eight clay soils in southern Sweden, using turbidity as a proxy for aggregate stability. Turbidity in leachate from simulated rain events performed on aggregates (2–5 mm) in the laboratory was measured one and six years after application of four treatments 0, 4, 8 and 16 t ha-1 of a mixed structure liming product. The effect on turbidity was analysed for all application rates, but also as the contrast between the unlimed control and the mean of the limed treatments, to identify the general effect. A significant effect of structure liming on turbidity was found after one year. The effect decreased over time, but was still detectable after six years. However, there was a significant interaction between trial and treatment, indicating different reactions on different soils and suggesting that not all clay soils are suitable for structure liming if the desired objective is to lower the risk of PP losses. Clay content, initial pH and mineralogy may explain the different responses to structure liming. These findings show a need for a site-specific structure liming strategy. As a tentative recommendation, soils with a minimum clay content of approximately 25–30% and pH <7 should be preferred for structure liming.

Impacts induced by the combination of earthworms, residue and tillage on soil organic carbon dynamics using 13C labelling technique and X-ray computed tomography

Soil organic carbon (SOC) plays a key role in soil fertility and atmospheric CO2 concentration. Our previous work found that tillage induced changes in aggregate stability and pore structure were major factors contributing to the effect of tillage system on SOC. Effects of earthworms and residue on SOC vary among tillage practices, but we don't clearly know how earthworms, residue and tillage interact to affect SOC. A further study was carried out by adding 13C labeled maize residue and earthworms to undisturbed soil mesocosms from conventional tillage (CT) and no tillage (NT) plots and then incubating for 180 days at 18 °C. SOC, 13C, and soil aggregate were measured prior to and after incubation and pore structure was measured after incubation. Newly added residue had the same decomposition rate in CT and NT. Earthworms reduced the SOC content in CT, and the newly added residue reduced the SOC in NT. The earthworms increased average pore diameter of >500 μm, and decreased the pore number of 100−500 μm in NT. The newly added residue improved the SOC by increasing the MWD in CT. The earthworms could change the SOC by increasing the volume fraction in NT. This result is helpful in understanding how the earthworms and residue affect the SOC under different tillage practices and provides direction for developing strategies to increase soil fertility and mitigate emission of CO2 to the atmosphere.

Effectiveness of wheat straw mulch and Polyacrylamide on shallow stability of roadside slopes

Soil aggregate instability on unprotected roadside slopes can cause landslide, soil erosion and sedimentation. Different biological and chemical soil stabilizers are used to reinforce the instable slopes. In the present study, straw mulch and Polyacrylamide (PAM) combinations were investigated on a clay soil of road cutslope in campus of Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran. The selected cover treatments were Polyacrylamide (2, 3 and 4 g·m–2) with wheat straw mulch (50, 150 and 250 g·m–2) which was spread by hand to attain 75% groundcover on a 1:1 slope. After the three months, soil sampling was done to determine the changes in aggregate stability of soil. Results showed that the most efficient treatment with respect to mean weight diameter of soil aggregates in dry and wet sieving (MWDdry and MWDwet), aggregate stability index (AS) and aggregate destruction index (DI) was treatment of B (150 g·m–2 wheat straw mulch and 3 g·m–2 Polyacrylamide tackifier) with 34%, 68% and 47% increment in MWDdry, MWDwet and AS, respectively as well as 37% reduction in DI as compared to the bare soil control.<br /><br />

Tillage-induced effects on SOC through changes in aggregate stability and soil pore structure

Soil structure plays a key role in soil organic carbon (SOC) dynamics. To determine how soil structure and aggregate affects SOC, we collected undisturbed soil cores of 0–5 cm layer (Typic Hapludoll) at an experimental site in Northeast China. The site had been under continuous tillage treatments of conventional tillage (CT) and no tillage (NT) for 17 years. We measured SOC by elemental analysis, aggregate size distribution by wet sieving, and soil pore parameters of pore size distribution, pore average diameter, pore numbers, pore connectivity, pore anisotropy, and pore fractal dimension by X-ray computer tomography. SOC content was significantly correlated with aggregate-associated SOC and soil water-stable aggregate content. CT with residue removal and annual plowing and cultivation increased <53 μm and 53–250 μm aggregates. CT decreased total SOC of 0–5 cm soil layer but increased aggregate-associated SOC of <53 μm. NT with greater residue input increased total SOC of 0–5 cm soil layer by 26.0% and aggregate mean weight diameter by 111.8% and increased aggregates of 250–1000 μm and >1000 μm. Soil under NT had a greater total number of micropores and greater connectivity whereas CT had a greater total number of macropores, average macropore diameter, anisotropy, and fractal dimension. Structural equation modeling showed that CT can decrease SOC of 0–5 cm soil layer by different paths, including increased anisotropy and macropore porosity, and NT can increase SOC of 0–5 cm soil layer by different paths, including increased mean weight diameter and connectivity. These results enhance our understanding of the relationship between soil structure and SOC, and could guide tillage management decisions to increase SOC.

Impact of drying-wetting cycles on the soil aggregate stability of Alfisols in southwestern China

Drying-wetting cycles are important environmental factors controlling potential changes in aggregate stability. However, the relationships between soil aggregate stability and drying-wetting cycles under different breakdown mechanisms are not clear. We conducted a simulation study to investigate the effects of drying-wetting cycles on aggregate stability under the three main breakdown mechanisms (slaking, microcracking, and mechanical breakdown) without soil organic matter inputs. Four initial aggregate size classes (i.e., 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm) within a clay-loam soil, which undergoes intense drying-wetting cycles, were selected and subjected to eight levels of drying-wetting cycles (i.e., 0, 1, 2, 3, 5, 7, 10, and 15 cycles). Aggregate stability was measured with three treatment methods involving fast wetting (FW), slow wetting (SW), and shaking after prewetting treatments (ST). Our study showed higher resistance to mechanical breakdown and lower resistance to slaking. Compared with the mean weight diameter (MWD), the normalized mean weight diameter (NMWD) was a better indicator of comparing the susceptibility of different initial size aggregates to breaking up into smaller aggregates. The values of NMWD for 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm aggregates were 0.1, 0.1, 0.09, and 0.09, respectively, for FW; 0.35, 0.34, 0.33, and 0.32, respectively, for SW; and 0.46, 0.45, 0.44, and 0.42, respectively, for ST. This indicated that as initial aggregate size increased, aggregates were more prone to breaking down. The effects of drying-wetting cycles on aggregate stability varied greatly among different aggregate breakdown mechanisms. For 1 to 2, 2 to 3, 3 to 5, and 5 to 7 mm size aggregates, drying-wetting cycles decreased MWD by 61%, 67%, 70%, and 53%, respectively, for SW, and 69%, 72%, 80%, and 78%, respectively, for ST. For FW, MWD increased by 5%, 7%, 8%, and 36% after the first two cycles, respectively, while it decreased by 36%, 36%, 44%, and 15% after 15 cycles. The results showed that different aggregate breakdown mechanisms influence the effects of drying-wetting cycles on aggregate stability. This study provides an important basis for better soil management practices and yields insights for an improved understanding of the role of drying-wetting cycles on aggregate stability.

Tropical plant communities modify soil aggregate stability along a successional vegetation gradient on a Ferralsol

Soil aggregate stability is viewed as a promising indicator of the restoration status in eroded ecosystems, where the change in plant community composition through successional dynamics is a key driver of ecosystem restoration. In many tropical regions, ecological restoration is an important issue, but the relationship between different types of plant communities and soil aggregate stability is poorly understood. We examined how tropical plant communities modified soil aggregate stability along a successional vegetation gradient on a Ferralsol in New Caledonia. We identified five plant communities, ranging from an early (sedge dominated ecosystems) to a late successional stage (well-established, dense, mixed rainforest). Aggregate stability, total soil organic carbon (SOC) and iron and aluminium sesquioxides were measured in soil originating from each community. Results showed that aggregate stability of the Ferralsol was very high, even on eroded sites, likely due to the high levels of iron and aluminium sesquioxides. The levels of iron sesquioxides were particularly high (>10%), due partly to the frequency of wildfires in the region. Total SOC increased from sedge-dominated communities (<1%) to mixed rainforest (>3.5%). Aggregate stability was modified by plant cover and community composition and increased from sparse, early successional vegetation to late successional dense, mixed rainforest. Our study showed that certain plant species have a positive impact on soil aggregate stability and should be considered for ecological restoration on Ferralsols. e.g., Costularia arundinacea, Garcinia amplexicaulis and Myodocarpus fraxinifolius. In conclusion, we suggest that vegetation dynamics should be taken into account when investigating changes in aggregate stability in a context of ecosystem restoration.

Immediate and long-term effect of tannins on the stabilization of soil aggregates

Soil aggregates are organomineral associations with a fundamental importance for soil structure and function. Litter from vegetation alters aggregate formation and stability, and polyphenols such as tannins found in leaves, roots and wood play an important role in soil biogeochemical and biological processes. However, the effect of tannins on soil physical properties remains largely unexplored. We hypothesized that tannins influence aggregate stability through their ability to (i) complex proteins in the soil and (ii) perturb the gelling property of root mucilage. Therefore, Mediterranean soil aggregates were incubated with condensed tannins, either as a pure substrate or in combination with a standard protein (bovine serum albumin, BSA) and with a model root mucilage polysaccharide (polygalacturonic acid, PGA) able to form gel-like structures with divalent cations (notably Ca2+) widely present in Mediterranean calcareous soils. The changes in aggregate stability were monitored under controlled conditions, immediately after the addition of tannins, after 2 weeks, 3 and 6 months of incubation. Tannins added alone did not yield a significant effect on aggregate stability. However, modulatory effects were found when combinations of treatments occurred. Tannins positively modulated the stabilizing effect of the BSA, giving credit to our hypothesis on the stabilizing role of the tannin-protein complex forming macromolecules, thus enforcing soil particle cohesion within aggregates. However, tannins negatively altered the stabilizing effect of PGA, suggesting that the expected perturbation of the PGA gelation occurred, with detrimental consequences for aggregate stability. Over time, tannins maintained the effect of BSA, suggesting a protective effect of tannins, possibly linked to their ability to slow down the degradation of nitrogen compounds through protein binding. Overall, we showed that tannins reacted with other organic compounds resulting in specific effects on physical soil properties, thus demonstrating that tannins in soils play a role beyond their effects on biogeochemical aspects.

Soil structure breakdown following land use change from forest to maize in Northwest Vietnam

Conversion of forest to agricultural land for maize cultivation is known to negatively affect soil fertility. However, limited knowledge is available of the impact on aggregate stability and interconnected soil properties. The aims of the present study were to (1) quantify soil aggregate stability, (2) assess aggregate stability changes after land use change, and (3) determine the interactions of aggregate stability with clay, soil organic carbon (SOC), exchangeable Ca2+ and Mg2+, and soil erosion rates. The topsoils of three soil types in Northwest Vietnam were analyzed in chronosequences (0–18 y after land use change from forest to continuous maize) by two methods: Wet sieving and sonication. By differentiation of these aggregate stability measurements, the impact of both methods on aggregate size distribution could be quantified separately and compared. The sonication method indicated a more homogeneous disaggregation whereas the wet sieving method was more suitable to detect low aggregate stabilities. Soil aggregate stability declined simultaneous with a decrease of SOC and exchangeable Ca2+ and Mg2+, which both declined with increasing time since land use change. The Alisol and Luvisol chronosequences were 1.9 times more stable under primary forest than under maize (shown by sonication) whereas the Vertisol chronosequence was 2.5 times more stable under primary forest (shown by wet sieving). Over the 18 y chronosequence the topsoils had 1.6kgm−2 lower SOC and 3.2gkg−1 lower Ca2+ contents. This study highlights the destabilization of soil in interaction with a degradation of relevant chemical soil properties with differentiated aggregate stability methods.

Long-term manure amendments reduced soil aggregate stability via redistribution of the glomalin-related soil protein in macroaggregates.

Glomalin-related soil protein (GRSP) contributes to the formation and maintenance of soil aggregates, it is however remains unclear whether long-term intensive manure amendments alter soil aggregates stability and whether GRSP regulates these changes. Based on a three-decade long fertilization experiment in northeast China, this study examined the impact of long-term manure input on soil organic carbon (SOC), total and easily extractable GRSP (GRSPt and GRSPe) and their respective allocations in four soil aggregates (>2000 μm; 2000–250 μm; 250–53 μm; and <53 μm). The treatments include no fertilization (CK), low and high manure amendment (M1, M2), chemical nitrogen, phosphorus and potassium fertilizers (NPK), and combined manure and chemical fertilizers (NPKM1, NPKM2). Though SOC, GRSPe and GRSPt in soil and SOC in each aggregate generally increased with increasing manure input, GRSPt and GRSPe in each aggregate showed varying changes with manure input. Both GRSP in macroaggregates (2000–250 μm) were significantly higher under low manure input, a pattern consistent with changes in soil aggregate stability. Constituting 38~49% of soil mass, macroaggregates likely contributed to the nonlinear changes of aggregate stability under manure amendments. The regulatory process of GRSP allocations in soil aggregates has important implications for manure management under intensive agriculture.

Effects of forest expansion on mountain grassland: changes within soil organic carbon fractions

Background and aims Grassland abandonment followed by forest succession is the dominant land-use change in the European Alps. We studied the impact of current forest expansion on mountain grassland on changes in physical soil organic carbon (SOC) fractions along a land-use and management gradient, focusing on changes in aggregate stability and particulate organic matter (POM).

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.

Comparison of some quality properties of soils around land-mined areas and adjacent agricultural fields

When agricultural lands are no longer used for agriculture and allowed to recover its natural vegetation, soil organic carbon can accumulate in the soil. Measurements of soil organic carbon and aggregate stability changes under various forms of land use are needed for the development of sustainable systems. Therefore, comparison of soil samples taken from both agricultural and nearby area close to land-mined fields where no agricultural practices have been done since 1956 can be a good approach to evaluate the effects of tillage and agriculture on soil quality. The objective of this study was to compare tillage, cropping and no tillage effects on some soil-quality parameters. Four different locations along the Turkey-Syria border were selected to determine effects of tillage and cropping on soil quality. Each location was evaluated separately because of different soil type and treatments. Comparisons were made between non-tilled and non-cropped fallow since 1956 and adjacent restricted lands that were tilled about every 2 years but not planted (T) or adjacent lands tilled and planted with wheat and lentil (P). Three samples were taken from the depths of 0-20 and 20-40 cm each site. Soil organic carbon (SOC), pH ,electrical conductivity, water soluble Ca(++), Mg(++), CO₃⁻² and HCO₃⁻, extractable potassium (K(+)) and sodium (Na(+)), soil texture, ammonium (NH₄⁺-N) and nitrate (NO(3)-N), extractable phosphorous and soil aggregate stability were determined. While the SOC contents of continuous tillage without cropping and continuous tillage and cropping were 2.2 and 11.6 g kg(-1), respectively, it was 30 g kg(-1) in non-tilled and non-planted site. Tillage of soil without the input of any plant material resulted in loss of carbon from the soil in all sites. Soil extractable NO(3)-N contents of non-tilled and non-cropped sites were greatest among all treatments. Agricultural practices increased phosphorus and potassium contents in the soil profile. P(2)O(5) contents of planted soils were approximately 20 to 39 times greater than those of non-tilled and non-cropped soils at different sites. FTIR spectra showed that never tilled sites had greater phenol, carboxylic acid, amide, aromatic compounds, polysaccharide and carbohydrates than other treatments.

Soil fertility management: Impacts on soil macrofauna, soil aggregation and soil organic matter allocation

Maintenance of soil organic matter through integrated soil fertility management is important for soil quality and agricultural productivity, and for the persistence of soil faunal diversity and biomass. Little is known about the interactive effects of soil fertility management and soil macrofauna diversity on soil aggregation and SOM dynamics in tropical arable cropping systems. A study was conducted in a long-term trial at Kabete, Central Kenya, to investigate the effects of organic inputs (maize stover or manure) and inorganic fertilizers on soil macrofauna abundance, biomass and taxonomic diversity, water stable aggregation, whole soil and aggregate-associated organic C and N, as well as the relations between these variables. Differently managed arable systems were compared to a long-term green fallow system representing a relatively undisturbed reference. Fallowing, and application of farm yard manure (FYM) in combination with fertilizer, significantly enhanced earthworm diversity and biomass as well as aggregate stability and C and N pools in the top 15 cm of the soil. Earthworm abundance significantly negatively correlated with the percentage of total macroaggregates and microaggregates within macroaggregates, but all earthworm parameters positively correlated with whole soil and aggregate associated C and N, unlike termite parameters. Factor analysis showed that 35.3% of the total sample variation in aggregation and C and N in total soil and aggregate fractions was explained by earthworm parameters, and 25.5% by termite parameters. Multiple regression analysis confirmed this outcome. The negative correlation between earthworm abundance and total macroaggregates and microaggregates within macroaggregate could be linked to the presence of high numbers of Nematogenia lacuum in the arable treatments without organic amendments, an endogeic species that feeds on excrements of other larger epigeic worms and produces small excrements. Under the conditions studied, differences in earthworm abundance, biomass and diversity were more important drivers of management-induced changes in aggregate stability and soil C and N pools than differences in termite populations.

Effects of artificial soil surface management on changes of aggregate stability and other soil properties

Studies of size distribution, stability of the aggregates, and other soil properties are very important due to their influence on tilth, water infiltration, and nutrient dynamics and more importantly on accelerated erosion but are affected by soil surface management. Both chemical e.g. pH, organic carbon, (OC), exchangeable cations e.g. calcium (Ca), magnesium (Mg) and physical properties such as aggregate size and stability are dynamic properties, which vary in response to forces affecting soil environment. This study was carried out to have a long and detailed understanding of how artificial surface soil covering materials can protect the soil physical and chemical properties from the ravages of soil erosion. Such endeavor would allow us to make suitable modification to soil conservation practices to enhance soil stability and importantly soil productivity. Data measured for eight years on induced erosion experiments on a Ferralsol covered by artificial soil netting locally called sombrite at Campinas, Brazil, were used to examine the effects of accelerated soil erosion on soil chemical and physical properties. Each erosion plot had an area of 25 x 4 m. Four soil treatments were chosen: bare (control) and three artificially covered soils. Changes of soil properties observed in relation to aggregation were bulk density, total and aeration porosity, available water capacity and changes in soil chemical properties. From pooled data stepwise multiple regressions were done to find out the relationship of aggregate stability with pH, calcium, and organic matter. There were no significant differences (P=0.05) in mean weight diameters (MWD) and size distribution irrespective of whether samples were collected from the upper middle or lower parts of the erosion plots. There were very gradual decline in mean weight diameters (MWD) and size distribution of water-stable aggregates during the study period. Bulk density increased appreciably on bare soils but remained almost constant in artificially covered soils. However, using Duncan tests, the treatment differences were not significant (P=0.05). Non-significant correlation (P=0.05) between MWD and the soil chemical properties were observed. Coefficients of determination for pH, OC, MWD and Ca could not sufficiently assist in explaining changes in aggregate stability as a result of surface cover. Key words: artificial soil covers, soil surface management, soil properties, accelerated erosion, aggregate stability.

Aggregate stability and related properties in NW Patagonian Andisols

Resilience is an ecological concept explaining responses to disturbance or stress. For the soil system, it is defined as the ability of the soil to recover its condition of origin. Not much is known about the factors and processes that determine soil resilience and this research project was conducted to study soil resilience through analysis of changes in aggregate stability of Andisols supporting Pinus ponderosa Dougl. that replaced former Nothofagus sp. forest. Five sites with three treatments each were selected. Treatments included native forest, thinned exotic plantation forest and non-thinned exotic plantation forest. Bulk density and aggregate stability of the first 5 cm of the A-horizon were determined in undisturbed samples at field moisture, and the aggregate size distribution was determined involving the fractions of 8–2; 2–0.25; 0.25–0.053 and < 0.053 mm (diameter). The results indicated that the percentage of aggregates between 2 and 0.25 mm was significantly higher under Nothofagus (39.7%) than under pine without thinning (28.8%), with soil under thinned pine (32.0%) in between. The decrease in macroaggregates under pine without thinning was accompanied by a decrease in organic carbon content (from 70.2 g kg - 1 to 42.4 g kg - 1 ). Other results of importance were i) the high water stable aggregates under the different types of vegetation (> 80% of > 0.25 mm aggregates), indicating the relatively high stability of such Andisols; ii) highest organic matter contents found in the smallest sized aggregates, and iii) formation of stable soil aggregates to be significantly correlated with organic matter, Al activity, base content and soil reaction.

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.

The causes of piping in a set of abandoned agricultural terraces in southeast Spain

This study aims to identify some of the factors contributing to the piping process in abandoned terraces in seven sites located in the Murcia Region (southeast Spain). The local landscape has the appearance of badland geomorphology, possessing deep valleys which until the 1970s were terraced and cultivated. The terraces have now been abandoned to the piping process. The research focuses on the relationship between the size and position of pipes and the physical and geochemistry properties of the local highly dispersive marl lithology; and the extent to which the reworking of the materials for terrace cultivation has exacerbated piping erosion. Morphologically, the terracing in the stream beds has resulted in a series of gentle benches separated by steeper risers with variable ‘drop’, (riser height). A series of flat infiltration surfaces result, and because the resculpting produces a reduced bulk density infill with enhanced throughflow, infiltrating water is forced along steep hydraulic gradients to a low outfall at the base of the riser. Paralleling studies of land levelling and terrace abandonment in similar materials elsewhere, piping in the study area is shown to be closely related to the physical and chemical characteristics of the materials its sodicity and dispersive character. Considerable differences in structure, texture, mineralogical composition and material's surface crust and subsoil geochemical properties emerged from ‘site signature’ plots, and these results suggest that agricultural processes affect the dispersive character of the crust materials, presumably because agricultural activity increases the buffering effect of organic acids on the dispersed clay. This is supported by the observation that on the one site that was levelled but not used for agriculture, a stable organically-distinct crust like the abandoned agricultural sites has not formed. The changes to the material's aggregate stability and bulk density properties when terracing occurs is examined, but insufficient evidence is available from the data collected to unpick the causes of aggregate stability changes in the study plots. The way in which differences between crust and subsoil enhance infiltration is discussed, and the role of the terrace ‘drop’ in influencing hydraulic gradients and pipe development is illustrated. A possible index to describe the relationship between evacuated material volumes and the controlling parameters discussed above is tested. It is concluded that terracing which is intended as a conservation practice has, in dispersive materials, actually enhanced piping, leading to the abandonment of farming.

Effects of potassium humate on aggregate stability of two soils from Victoria, Australia

The potential for using commercially available potassium humate (K-humate) derived from Victorian Brown Coal to improve soil structure and aggregate stability was assessed in this study. Laboratory experiments were conducted on two agricultural soils from the southeastern and northern parts of Victoria, Australia. Bulk surface soil (0–15 cm) samples of an acidic soil (pH 4.6) from Dixons Creek in the Yarra Valley (DeBortoli Vineyards) and an alkaline sodic soil (pH 8–9) from North Mooroopna (waste water irrigated site under grazed pastures) in the Goulburn Valley were evaluated for any changes in aggregate stability after treatment with various application rates of K-humate and subjection to a series of wetting and drying (W/D) cycles. On each soil, six rates of K-humate (0, 0.05, 0.10, 0.50, 1.00, and 10.00 g/kg of soil) were applied, and four sequences of wetting and drying cycles (0, 3, 7, and 10 cycles) were carried out using the 2–5-mm aggregate size fractions. Mean-weight diameter (MWD) of the treated samples was determined after wet-sieving and drying. Compared with the controls, K-humate-treated samples showed significant improvement in MWD for both soils, although treatment did not prevent some decline in MWD after seven wetting/drying cycles. Low treatment rates gave significant improvement for the acidic soil; however, rates of 1.0 g/kg and above were required for significant improvement for the sodic soil. This work demonstrates that potassium humate is potentially effective as a soil conditioner in improving aggregate stability of acidic and sodic soils against adverse effects of cyclic seasonal wetting and drying conditions.