Microaggregate Stability Research Articles

Exploring the West African forest island phenomenon: scientific insights gained, successes achieved and capacities strengthened.

Anthropogenic activities around local villages in mesic savanna landscapes of West Africa have resulted in soil improvement and forest establishment outside their climatic zones. Such unique 'forest islands' have been reported to provide ecosystem services including biodiversity conservation. However, the science underpinning their formations is limitedly studied. In 2015 and with funding support from the Royal Society-DFID (now FCDO), we set out to investigate the biogeochemistry of the forest islands in comparison with adjacent natural savanna and farmlands across 11 locations in Burkina Faso, Ghana and Nigeria. Our results showed that the forest islands do not differ significantly from the adjoining ecosystems in soil mineralogy implying that their formation was anthropogenically driven. We observed greater soil organic carbon and nutrient distributions in the forest islands, which also had more stable macro (>500 μm) and meso-aggregates (500-250 μm) than the adjoining agricultural lands. We found that soil micro-aggregate (250-53 μm) stability was climate (precipitation) driven in the West African ecosystems while meso- and macro-aggregate stability was land-use driven. In one of the unique forest islands we studied in the Mole National Park of Ghana, we found its mineral-associated organic carbon over 40% greater than the adjoining natural savanna with potential implications for the achievement of the global initiative of the '4p1000' in West Africa. We conclude that the North-South-South research collaboration has established clearly, the science underlying the age-long West African forest island phenomenon and has, among many successes, led to capacity building of young scientists driving cutting-edge research in climate change adaptation and food systems transformation in the sub-region.

Water dispersible clay and micro-structure of soils from coastal plain sands, shale and false-bedded sandstones

Information regarding water-dispersible clays in soils of contrasting textures is needed to evaluate soil qualities that influence susceptibility of soils to slaking in water. Water-dispersible clays (WDCs), plastic limits, and basic properties of soils on Coastal Plain Sands (CPS), Shale (Shale), and False-Bedded Sandstones in southern Nigeria were quantified for use in evaluating management options of some tropical soils. Results revealed that WDCs, micro-aggregate indices, and plastic limits differed significantly amongst the parent materials. At 0–15 cm depth, clay content was 29.7%, 11.2 and 3.8%, for Shale, CPS and False-Bedded sandstones, respectively, indicating significant high clay content in Shale. The highest SOC of 29.8 g kg−1., and very slow saturated hydraulic conductivity (Ksat) of 6.33 cm h−1 were found in Shale. False-Bedded Sandstone had the lowest SOC content of 7.7 g kg−1 and highest Ksat value of 31.10 cm h−1 at 0–15 cm topsoil. Micro-aggregate stability indices measured by aggregated silt+clay (ASC) and clay flocculating index (CFI) were significant in CPS at 0–15 cm, whereas water-dispersible clay (WDC) was significantly higher (p < 0.05) in Shale at 8.32%, compared to very low value of 0.76% in False-Bedded Sandstone. Plastic limit (PL) of 16.5%, was significantly higher (p < 0.05) in Shale, indicating its high sensitivity to mechanical deformation when wet. There was a significant positive correlation between clay content and WDC (r = 0.815, p < 0.01), indicating the positive influence of clay content on WDCs and micro-structural indices, and the tendencies for quick degeneration of the soils.

Variables related to the stability of soil macro and microaggregates of a toposequence in the Brazil south region

Soil aggregates are the basic components of soil structure, being important to its environmental properties and functions. The objective of this study was to investigate the variables related to the aggregates stability, measured by the method of dry and wet sieving and by applying ultrasound energy, in three different soils of a toposequence in southern Brazil. The study was performed in three soils sampled in the city of Pinhais / PR: i) Oxisol (top slope); Ii) Inceptisol (medium third) and; Iii) Gleisol (floodplain). The aggregates stability was determined measured by the method of dry and wet sieving (macroaggregates), as well as by applying ultrasound energy (microaggregate). The results were correlated with soil chemical, physical and mineralogical attributes. The organic matter, the low crystallinity Fe oxides and kaolinite were the most important variables for the profile structural stability. In turn, the soil texture did not show a direct relation with soil aggregation. The Gleisol show highest structural stability, followed by Oxisol and Inceptisol.

Long-term application of agronomic management strategies effects on soil organic carbon, energy budgeting, and carbon footprint under rice–wheat cropping system

In the plains of western North India, traditional rice and wheat cropping systems (RWCS) consume a significant amount of energy and carbon. In order to assess the long-term energy budgets, ecological footprint, and greenhouse gas (GHG) pollutants from RWCS with residual management techniques, field research was conducted which consisted of fourteen treatments that combined various tillage techniques, fertilization methods, and whether or not straw return was present in randomized block design. By altering the formation of aggregates and the distribution of carbon within them, tillage techniques can affect the dynamics of organic carbon in soil and soil microbial activity. The stability of large macro-aggregates (> 2 mm), small macro-aggregates (2.0–2.25 mm), and micro-aggregates in the topsoil were improved by 35.18%, 33.52%, and 25.10%, respectively, over conventional tillage (0–20 cm) using tillage strategies for conservation methods (no-till in conjunction with straw return and organic fertilizers). The subsoil (20–40 cm) displayed the same pattern. In contrast to conventional tilling with no straw returns, macro-aggregates of all sizes and micro-aggregates increased by 24.52%, 28.48%, and 18.12%, respectively, when conservation tillage with organic and chemical fertilizers was used. The straw return (aggregate-associated C) also resulted in a significant increase in aggregate-associated carbon. When zero tillage was paired with straw return, chemical, and organic fertilizers, the topsoil's overall aggregate-associated C across all aggregate proportions increased. Conversely, conventional tillage, in contrast to conservation tillage, included straw return as well as chemical and organic fertilizers and had high aggregate-associated C in the subsurface. This study finds that tillage techniques could change the dynamics of microbial biomass in soils and organic soil carbon by altering the aggregate and distribution of C therein.

Effects of different amendments on aggregate stability and microbial communities of coastal saline–alkali soil in the Yellow River Delta

AbstractOrganic amendments have been widely used in coastal saline–alkali soil remediation; however, the mechanisms involved and the interactions between organic and inorganic amendments are still unclear. In this work, furfural residue (particulate; C/N ratio: 51.87; O‐alkyl C + di‐O‐alkyl C: 42.35%, aromatic C: 40.89%) and black liquor (dissolved; C/N ratio: 3.11; O‐alkyl C + di‐O‐alkyl C: 32.20%, aromatic C: 28.32%) were tested to examine their effects on chemical properties, water‐stable aggregate fractions, chemical compositions of solid‐state soil organic matter (SOM), gloaming‐related soil protein contents, and microbial communities of coastal saline–alkali soil under a 400‐day incubation experiment. Furthermore, organic amendments mixed with mineral amendment (4:1) were employed to explore the interactions between organic and inorganic amendments. Furfural residue had stronger and longer effects on soil macroaggregate stability (~240 days, intense) than black liquor (~15 days, weak), and mineral amendment addition had a positive effect on the stability of microaggregates. Our results revealed that qualities (primary form, C/N ratio, and chemical composition) of organic amendment which can change microbial communities by increasing soil C/N ratio and effective chemical compositions of solid‐state SOM, are the key factors in promoting the rapid formation and longer stability of coastal saline–alkali soil aggregates. Moreover, inorganic amendment addition can further improve the formation and stability of microaggregates rather than those of macroaggregates. This study provided a much‐needed technical basis for remediation of coastal saline–alkali soil.

Functional stability: From soil aggregates to landscape scale in a region severely affected by gully erosion in semi-arid central Mexico

Soil aggregate stability is a vital indicator of soil structure and potential degradation and is essential to understand gully erosion in arid and semi-arid regions. Stability of soil aggregates depends on soil mass properties, local conditions (within the gully), and hillslope conditions. The objectives of this study were 1) to analyze soil aggregate stability at three gully wall positions: headscarps, sidewalls, and gully bottoms, and 2) to explore the relationships between soil aggregate stability and local and landscape conditions to know how stability at different scales is interrelated. Soil macro aggregate stability was estimated from soil samples using the soil aggregate stability kit. Conversely, Cornell infiltrometers were used to evaluate soil microaggregate stability by applying 5-min rainfall events with an intensity of 150 mm−1. Local (within gully) conditions were registered in the field using indicators from the ephemeral drainage line assessment method. Landscape conditions (outside gully), such as terrain attributes, anthropogenic features, and vegetation cover, were computed from topographic maps, digital elevation models, and satellite imagery. Relationships between soil aggregate stability and local and landscape conditions were explored using classification and regression trees. The highest soil macro and micro aggregate stability were found at the gully headscarps, while gully bottoms had the lowest soil aggregate stability. Soil macroaggregate stability was related to terrain attributes, ground cover, and gully dimensions. In contrast, microaggregate stability was associated with ground cover, terrain attributes, gully dimensions, and distance to roads and farm dams. Our study showed that soil aggregate stability differs along gully walls and that soil aggregate stability can be related to different variables at the local and hillslope scales. This knowledge is a valuable indicator for understanding gully erosion and the foundation for restoration strategies in semi-arid regions.

Effect of slope curvature and gradient on soil properties affecting erodibility of coastal plain sands in Akwa Ibom State, Nigeria

A study was conducted to assess the effect of slope curvature and gradient on soil properties affecting erodibility of coastal plain sands in Akwa Ibom State. Twelve locations comprising six each of concave (CC) and convex (CV) slopes, and three slope gradients, namely, gentle (GS), moderate (MS) and steep slopes (SS) in CC and CV were identified for the study. Bulk and core soil samples were collected from the top 30 cm soil depth at three slope positions of slope curvature and gradient categories for physical and chemical characterization, and derivation of microaggregate stability indices and erodibility factor, K. Results showed that coarse sand (CS), fine sand (FS), bulk density (Bd), total porosity (TP) and saturated hydraulic conductivity (Ksat) were significantly (p ≤ 0.05) higher on CC than CV, and among the slope gradients, GS &gt; MS &gt; SS. The interaction of slope curvature and gradients was significantly higher on CC than CV. A similar pattern of differences was also observed inorganic matter (SOM), calcium (Ca), magnesium (Mg), and other exchangeable cations, but the effect of slope curvature and gradients interaction was not significant. The microaggregate stability indices, water dispersible clay (WDC), water dispersible silt (WDS), dispersion ratio (DR), clay dispersion ratio (CDR), modified clay ratio (MCR), clay flocculation index (CFI), aggregated silt + clay (ASC) and aggregated clay (AC) were significantly higher on CV than CC, and differences among slope gradients and interaction of slope curvature and gradients were not significant. The erodibility factor, K, was significantly higher on CV than CC, while the slope gradients were similar in their effects on K. Linear regression analysis showed that K-factor was highly significantly related to CS, FS, Si, Cl, Bd, TP and Ksat, as well as WDC, WDS, CDR, DR, CR, MCR, CFI, ASC and AC. However, CS, Si, Ksat, Cl, ASC and WDS which explained &gt; 40% of the variability in Kfactor could be relied upon as indices of soil erodibility in the coastal plain sands in Akwa Ibom State.

A contribution of molecular modeling to supramolecular structures in soil organic matter#

AbstractBackgroundKnowledge of the stabilizing mechanisms of soil organic matter (SOM) is extremely important for numerous soil functions. For this, insight into the nature of organic matter through appropriate model concepts are crucial.AimsFor several years, a heated debate has emerged on the transformation and stabilization of SOM. In the present work, we try to contribute to this debate using molecular modeling and providing a comprehensive overview of the history of application of molecular modeling tools and developing structural concepts of SOM.MethodsMolecular modeling methods based on quantum and/or classical mechanics were used to model SOM and related properties including interactions with reactive surfaces of soil minerals.ResultsModeling of SOM aggregates revealed that hydrogen bonds and cation bridges are the main stabilizing factors in soil solution, whereas pH modifies the stability. The modeled supramolecular SOM aggregates exhibit physicochemical properties, similar to those of humic substances (HS) described in literature. The interactions of the HS models with surfaces in kaolinite nanopores led to a partial disintegration of the aggregates into individual molecules and/or smaller subaggregates.ConclusionsFrom the molecular modeling point of view, supramolecular microaggregate models that exhibit the properties of HS are stable in the soil solution. However, their binding to reactive mineral soil constituents can be also in the form of individual molecules or subaggregates. Thus, HS microaggregate stability is relative, depending on the interacting environment. This reconciles two points of view of HS: either as small molecules and/or supramolecular structures.

Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and non-paddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-microaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

On the Adsorption Mechanism of Humic Substances on Kaolinite and Their Microscopic Structure

Soil organic matter (SOM) and various inorganic minerals represent key components of soils. During pedogenesis and due to biological activity these species interact, having a crucial impact on the formation of an aggregated soil structure with a hierarchical arrangement from nano to macro scale. In this process, the formation of organo–mineral microaggregates represents a dominant factor affecting soil functions and properties. This study focuses on the interactions between humic substances (HSs) and the mineral kaolinite as typical representatives of SOM and soil minerals. By performing classical molecular dynamics (MD) simulations on models of HSs and kaolinite, we demonstrate how two dominant but chemically different kaolinite surfaces affect the stability of HSs microaggregates. By analyzing volumetric, structural, and energetic properties of SOM–kaolinite models, we explain possible mechanisms of the formation of stable SOM–clay aggregates and show how a polarized environment affects the electrostatic interactions, stabilizing the microscopic structure of SOM–mineral aggregates. Our results showed that when stable aggregates of HSs are confined in kaolinite nanopores, their interactions with kaolinite surfaces disintegrate them into smaller subaggregates. These subaggregates are adsorbed more strongly on the polar aluminol surface of kaolinite compared to less the active hydrophobic siloxane surface.

Variations in soil aggregation, microbial community structure and soil organic matter cycling associated to long-term afforestation and woody encroachment in a Mediterranean alpine ecotone

Afforestation and subsequent expansion of trees on former grasslands may significantly impact the structure and activity of the soil microbial community, altering soil aggregation and affect its potential to store and cycle organic matter (OM). We investigated OM dynamics in aggregate-size topsoil samples collected along a Mediterranean alpine ecotone consisting of three vegetation types (grassland/shrubland, mixed shrubland-pine, and pine forest) in central Spain. Analytical determinations of soil organic carbon (SOC), total nitrogen (TN), particulate OM (POM), mineral-associated OM (MaOM), and the stable isotopic composition of carbon were conducted in each of the four aggregate-size fractions considered. Additionally, the structure of the microbial community (assessed as PLFA abundance), and the β-glucosidase and β-glucosaminidase activities were determined in bulk soil samples. More than half of the soil mass was contained within small macroaggregates regardless of vegetation type. SOC and TN values increased with decreasing aggregate-size classes across all vegetation types. The stability of microaggregates was negatively affected by the expansion of woody vegetation, which resulted in tree-dominated stands showing comparatively lower SOC and TN values in the smaller aggregate-size classes. On the other hand, these vegetation dynamics promoted soil macro-aggregation. While SOC contents did not show significant differences between land covers, vegetation shifts induced changes in the soil microbial community. Soil δ13C values, the abundance of gram-positive bacteria and β-glucosidase activity were significantly higher in grasslands/shrublands than in forests, while significantly higher fungi/bacteria ratio was observed in forests. Small macroaggregates appear to play a key role in the stabilisation of relatively unprocessed OM across all vegetation types, as suggested by their significantly higher concentrations of POM. However, this fraction represents the most labile pool of OM, and as such, it is the most exposed to mineralisation. We conclude that the afforestation and potential vegetation shifts experienced in Mediterranean alpine grasslands lead to distinct changes in soil microbial communities, aggregation and soil OM dynamics, which given the strong temperature sensitivity to decomposition commonly reported in cold environments, suggests that soil OM in these high-elevation ecosystems may become highly vulnerable to environmental change.

Micro-Aggregate Stability in Greenhouse Management System in Northeast China

Micro-Aggregate Stability in Greenhouse Management System in Northeast China

Rheological evaluation of soil aggregate microstructure and stability across a forested catena

Rheological characteristics of soils, including their deformation and flow behaviors when subjected to external stress, can provide important information on microstructural stability. In this study we used rheological measurements to examine the soil aggregate microstructure and stability of four different soil orders – Alfisol, Mollisol, Inceptisol, and Entisol – along a forested catena in Mazandaran Province, northern Iran. Amplitude sweep tests were used to quantify the initial values of the storage and loss moduli, deformation limit (when the material begins to transition from reversible to irreversible deformation), deformation at flow point (when the material becomes viscous), and integral z (which summarizes the overall visco-elasticity of the material). The deformation limit was significantly higher in subsoil layers than topsoil layers, and was also higher in the Mollisol than the other pedons. The flow point and integral z values, which relate to the structural stiffness of soil matrices, were largest in the Btg horizons of the Alfisol and Mollisol, implying that these soils had more rigid microstructures. In contrast, the Entisol Ckg horizon, which had high sand content and little soil development, had the lowest values for all properties, thus indicating a lack of micro-aggregate stability. Regression analyses revealed that integral z was influenced by soil physicochemical properties, and was higher in soils whose clay fraction was dominated by expansive clay minerals and pedogenic iron and aluminum sesquioxides. Altogether, the rheological parameters indicated that older, more developed soils had greater microstructural stability than their less developed counterparts. As a result, rheological measurements may be useful for identifying the major factors that affect soil aggregation, and can indicate the relative amount of soil development along gradients such as the studied forest catena.

The rise of artificial soil carbon inputs: Reviewing microplastic pollution effects in the soil environment.

The surge in the use of plastic materials, its poor handling and disposal have led to an increase in microplastic pollution in terrestrial environments. Microplastic pollution in soils is of concern due to potential influences on soil properties which play a critical role in plant growth and soil fertility. Moreover, the soil environment is a key nexus linking the atmosphere, hydrosphere, biosphere and lithosphere, and thus represents a crucial conduit for pollutant migration from the anthroposphere. In this review we evaluate the effects of microplastics in the soil environment with a specific focus on physical properties and biological function in the rhizosphere. Our review reveals that agricultural sources, particularly plastic mulches and waste applications, represent the main source of soil microplastic inputs. Once in the soil environment, microplastic effects on soil properties are highly variable depending mainly on soil type and microplastic characteristics. Soil properties relating to erosion-risk (i.e., bulk density), structural integrity (i.e., aggregate stability, particularly micro-aggregate stability), and water-storage capacity (i.e., evaporation rate, desiccation) are generally adversely impacted by soil microplastic inputs. Soil microplastic effects on rhizosphere function (i.e., plant health and microbial activity) are remarkably varied with some studies revealing positive impacts, such as enhanced plant-symbiotic fungi associations, from soil plastic additions. However, all identified publications reported at least one detrimental MP-induced impact on plant responses. Finally, our review revealed associations between microplastic properties and soil functional parameters - in particular, polymer size and morphology control soil water-holding properties whereas polymer type influences plant response. These associations will be helpful in targeting future research directions on this important topic that intersects all of the Earth's spheres.

Initial soil aggregate formation and stabilisation in soils developed from calcareous loess

In soil, carbonates are important inorganic binding agents, controlling aggregate formation and soil structural stability. Until today, a clear understanding of the aggregate forming mechanisms in calcareous soil is missing. The objective of this study was to elucidate the initial aggregate formation in calcareous loess from the point when soil organic matter content was low and soil properties were mostly controlled by the parent material. We used a space-for-time chronosequence approach on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler, Germany. We selected six time points from 0 to 24 years after reclamation in order to investigate the early processes of soil aggregate formation. Samples from two sampling depths (1–5 cm and 16–20 cm) were wet sieved into four size classes: silt- and clay-sized fraction (<63 µm), large microaggregates (63–200 µm), small macroaggregates (200–630 µm), and large macroaggregates (>630 µm). Each aggregate size class was characterized for soil organic carbon, total nitrogen and CaCO3 content. Organic matter amendment induced the formation of large macroaggregates (>630 µm), but we did not detect an accumulation of soil organic carbon neither in microaggregates nor in the silt- and clay-sized fraction. The silt- and clay-sized fraction contained large amounts of CaCO3 throughout the whole space-for-time chronosequence. This finding shows that in undeveloped loess material, microaggregation is controlled by cementation by carbonates inherited from the loess. These carbonates were likely easily dissolved during contact with water, therefore, aggregates in the early stage of soil development were not water-stable. Soil aggregate formation on young calcareous soils consists of two main mechanisms, 1. fresh soil organic matter predominantly contributes to macroaggregate formation, and 2. intrinsic cementing of loess through carbonates affects microaggregate stability. Therefore, the early stages of soil development in loess material do not show an aggregate hierarchy as we would expect in soils with high silt and clay content, such as Cambisols and Luvisols.

Assisted phytoremediation of lead contaminated soil using Atriplex halimus and its effect on some soil physical properties

Lead is one of the most harmful trace elements found in soils. Despite reductions in environmental lead exposure, an historical lead pollution continues affecting soils in many areas. To mitigate the impact of lead, phytoremediation assisted with addition of chelating agents can be a suitable option. In order to evaluate the phytostabilization and/or phytoextraction capacity of Atriplex halimus, a pot assay with six phytoremediation treatments was established using different combination of two chelating agents (citric acid and fulvic acids), a composted substrate as organic amendment and a soil highly contaminated with lead (2952 ± 280 mg Pb kg−1, pH 6.30 ± 0.13) collected in the Valparaiso Region, Central Chile. Chelating agents were applied to irrigation water considering a concentration of 4 g L−1, while the organic amendment was applied at the beginning of the assay (dose of 5% v/v). After 180 days, the addition of the chelating agents significantly increased the Pb concentrations in tissues, an effect that was enhanced by application of the organic amendment, reaching concentrations of 32, 42, and 360 mg Pb kg−1 in leaves, stems, and roots, respectively. The treatments considered the addition of fulvic acids showed higher total plant dry weight levels (16.4 and 18.2 g, respectively) compared to the rest, which ranged from 8.3 to 10.0 g. With respect to soil physical properties, chelating agents increased microaggregate stability, but fulvic acids addition significantly decreased the soil available water. In general, Atriplex halimus demonstrated good potential for the phytostabilization of lead contaminated soils and a limited phytoextraction capacity.

Soil organic carbon fractions and aggregation of a tropical Alfisol as affected by plant residues

Incorporation of plant residues into soil plays vital role in enhancing quality of fragile tropical soils. This study evaluated short-term effect of plant residues of different quality on organic carbon (OC) fractions, carbon management indices and stability of a sandy loam soil with low (&lt; 2%) initial OC content. The residues are Banana clippings (BC), Chromolaena odorata (CO), Leucaena leucocephala (LL), Maize Stover (MS), Neem clippings (NC), and Panicum maximum (PM) applied at 0 t ha-1 (control), 10 t ha-1 and 20 t ha-1 . These treatments were arranged in a randomized complete block design and replicated three times. Undisturbed soil samples were collected at 0-20 cm depth for determination of percent water stable aggregates (%WSA), aggregate ratio (AR) and Mean Weight Diameter (MWD). Pre-sieved (2 mm) soil samples from same depth were collected to determine structural index (SI), microaggregate stability, total OC (CT), labile OC (CL) and non-labile OC (CNL). Carbon indices, i.e. lability index (LI), carbon management index (CMI) and carbon pool index (CPI) were computed from the OC fractions to evaluate the best management practice. The OC fractions were significantly (p≤ 0.05) higher with LL applied at 20 t ha-1 (19.29 g kg-1 for CT, 1.20 g kg-1 for CL and 18.09 g kg-1 for CNL) than other treatments. This was followed by PM 10 t ha-1 , with 17.16 g kg-1 for CT and 16.44 g kg-1 for CNL. The LI (1.29) was highest with MS 10 t ha-1 , while CMI (166.6) and CPI (1.85) were highest with LL 10 t ha-1 and 20 t ha-1 respectively. Significant increase in %WSA of &gt; 0.25 mm size over &lt; 0.25 mm was observed with addition of residues which indicates binding of smaller aggregates into large fractions; while AR (11.97) was highest with CO 10 t ha-1 . Aggregate stability showed MWD (2.02 mm) was highest in soils treated with CO followed by PM (MWD = 1.86 mm) both at 20 t ha-1 ; but SI trend was higher with addition of LL and PM. Computed micro-aggregate stability indices were generally similar for all treatments. High positive correlations were obtained between MWD and CL (r = 0.634), LI (r = 0.686) and CMI (r = 0.641) with addition of Leucaena leucocephala. In conclusion, L. leucocephala, followed by P. maximum and C. odorata provided shortterm improvement in physical quality of this soil over other plant residues

Sesquioxides and Their Contribution to Soil Aggregation under Long-term Greenhouse Soil Management

ABSTRACTThe effect of greenhouse soil management on soil aggregation and sesquioxides cycling should be evaluated to maximize the benefits of solar greenhouses rationally. The aggregation process of long-term greenhouse soil management and the influence of aggregate-associated sesquioxides on this process were investigated. Wet-sieving method was used to determine aggregate size distribution. Sesquioxide forms were also determined. The aggregates stability decreased and a similar trend for the stability of microaggregates was observed. The macroaggregate stability in 5 years of greenhouse age was statistically significantly lower than those in other ages. Compared to free and complex oxides, amorphous oxides had the closest relationships with mineral-associated organic carbon and soil aggregation. These results indicated that amorphous oxides were effective in binding aggregates and contributed significantly to macro- and micro- aggregate stability.

Microaggregate and Macroaggregate of Andisol Affected by Arbuscular Mycorrhizal Fungi and Rhizobacteria

.The formation of soil microaggregate and macroaggregate involves different microbes. A Screenhouse experiment to determine the effect of Arbuscular Mycorrhizal Fungi (AMF) and rhizobacteria on soil microaggregate stability and soil macroaggregate stability (soil microaggregate and macroaggregate stability) had been carried out. The treatments included AMF (control and Glomus sp.) and rhizobacteria (Pseudomonas diminuta, P. diminuta + Bacillus alvei, P diminuta + P malei, P malei). The results showed that inoculation Glomus sp. and rhizobacteria didn’t increase the stability of microaggregates (0.2 mm – 0.3 mm and < 0.2 mm). Inoculation Glomus sp and rhizobacteria increased water stable aggregate (2,83 mm – 4,8 mm, 0,50 mm – 1,00 mm, and 0,30 mm - 0,50 mm) and inoculation Glomus sp increased water stable aggregate (2,00 mm – 2,83 mm dan 1,00 mm – 2,00 mm). The experiment indicated that rhizobacteria didn’t effect soil microaggregate due to silt fraction domination and Glomus sp was involved in the formation of macroaggregate

Initial microaggregate formation: Association of microorganisms to montmorillonite-goethite aggregates under wetting and drying cycles

There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events.