Microaggregate Stability Research Articles

Physico‐chemical changes in bauxite residue following application of spent mushroom compost and gypsum

AbstractPhysical properties exhibited by unvegetated mine wastes pose limitations to vegetation establishment and growth. In an attempt to promote vegetation cover on bauxite residue, a field trial was established to determine the effect of spent mushroom compost (SMC) and gypsum amendment on enhancing the physical properties of the residue. SMC was incorporated at rates of 0, 60, 80 and 120 t ha−1 with gypsum at 0, 40 and 90 t ha−1 and Holcus lanatus sown at a rate of 80 kg ha−1.The addition of SMC and gypsum was beneficial in improving the physical properties of the residue and promoting growth. Principally increasing organic content of the residue decreased bulk density and particle density whilst improving substrate porosity. Residue pH, EC and sodicity of the residue were also affected by the amendments, which positively impacted on microaggregate stability and preventing clay dispersion. Pearson correlations demonstrate that the most significant parameters in determining clay dispersion potential are the pH and the exchangeable sodium percentage (ESP) of the residue. Copyright © 2009 John Wiley & Sons, Ltd.

Stimulation of microbial extracellular enzyme activities by elevated CO2 depends on soil aggregate size

AbstractIncreased belowground carbon (C) transfer by plant roots at elevated CO2 may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO2 (550 ppm) on four extracellular enzymes: β‐glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (Cmic by SIR) and enzyme activities in three aggregate‐size classes: large macro‐ (> 2 mm), small macro‐ (0.25–2 mm), and microaggregates (< 0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although Ctotal and Cmic as well as the activities of β‐glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate‐size classes by elevated CO2, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO2 were 1.2–1.9‐fold higher than under ambient CO2. This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO2. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO2 revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO2 affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.

Colloidal stability in some tropical soils of southeastern Nigeria as affected by iron and aluminium oxides

Abstract The stability of microaggregates in soils as opposed to its dispersion is a very important soil phenomenon that checks degradation arising from unguided tillage and soil erosion. Ten soils from southeastern Nigeria were sampled from their typical A and B horizons for the study. The aim was to identify the extent of colloidal stability of the soils and the forms of Fe and Al oxides in the soils contributing to their stability. The soils are mostly Ultisols and Inceptisols formed on sandstones and shale parent materials. The soils are low in soil basic cations including the soil organic carbon (SOC). The major clay mineral is kaolinite while the soil is acid in reaction. The various forms of soil Fe and Al oxides are high with the total forms of Fe and Al being most dominant and > dithionite extracted Fe and Al > oxalate extracted Fe and Al > pyrophosphate extracted Fe and Al. The water-dispersible clay and silt (WDC) and (WDSi) which are index of dispersion in most soils are low to medium thus reflecting in the low to medium dispersion ratio (DR). The clay flocculation index (CFI) and aggregated silt + clay (ASC) were moderate to high implying the high potential stability of the soils. Soil organic carbon did not seem to be contributing much to the stability of the microaggregates while oxalate and pyrophosphate extractable Fe (Feox, Fep) and to some extent total Al (Alt) were among the different forms of oxides that act as aggregating agents. We propose here that rather than SOC acting as a disaggregating agent in the soils, it might have acted in association with these oxides in a linkage or bridge such as C–P–OM–C to ensure stability of the soils.

Effect of Organic Residue Management on Soil Hydro-Physical Characteristics and Rice Yield in Eastern Himalayan Region, India

Monocropping of lowland rice is the predominant system in valley land and high-rainfall regions of northeastern India. Recently, the unsustainability of this system has owing to soil structural deterioration and nutrient depletion, become evident, resulting from continuous puddling on the same piece of land even if recommended inorganic fertilizers have been applying regularly. To regenerate and maintain soil structure in rice fields, farmers in this region frequently use farm yard manure (FYM), but low availability of FYM because of its other uses indicates that it could not meet the organic matter requirements of this system. We evaluated the long-term effects of different locally available jungle grasses and weeds on soil hydro-physical properties and rice yield through a 5-year field experiment (2000 to 2005) at the Indian Council of Agricultural Research (ICAR) Complex for North Eastern Hilly (NEH) Region, Umiam, Meghalaya, India and also compared their negative or positive effects with FYM in acidic, Typic Hapludalf soil. The objective was to determine whether locally available grasses (Jungle grass) and weeds (Ambrosia sp.) could be used as alternative sources of organic materials for lowland rice in scarcity of FYM. Results showed that incorporation of FYM or jungle grass or Ambrosia sp., continuously for 5 years in puddled rice soil improved soil organic carbon (SOC) by 21.1%; the stability of micro-aggregates, moisture retention capacity, and infiltration rate of the soil by 82.5, 10, and 31.3%, respectively; and soil bulk density decreased by 12.6%. Application of FYM produced better soil physical environment than that under jungle grass and Ambrosia sp. However, as these grasses and weeds contained higher nutrient levels than FYM and increased the SOC slowly, these organics may serve as alternative to FYM and may have a dramatic effect on long-term productivity of rice. The gradual improvement in soil physical properties led to a significantly higher yield of rice. We conclude that applying organic manure or organic residue annually may mitigate the negative effect of puddling and related properties and therefore could contribute to the sustainability of the wetland rice ecosystem.

Platelet function and the von Willebrand factor antigen in the hepatosplenic form of schistosomiasis mansoni

Forty-five individuals with hepatosplenic schistosomiasis mansoni were studied with the aim of measuring levels of von Willebrand factor antigen (vWF:Ag), detecting abnormalities in platelet morphology and aggregation, and identifying changes to surface antigens. Haemograms, platelet aggregation tests, flow cytometry investigations of CD41/CD42b antibody and vWF:Ag assays were performed. Mean platelet counts were low (77,522/mm3) and 82.2% of patients presented thrombocytopenia. An inverse relationship between spleen size and platelet count was seen. Macroplatelets were found in 57.1% of patients, indicating good bone-marrow response, but were insufficient to compensate for the decrease in platelets due to splenomegaly. Decreased or absent platelet aggregation was seen in 50% of patients, probably due to low platelet counts. Markers for GPIIb/IIIa were normal in more than 90% of patients, not supporting the increased capture and destruction of platelets in the spleen that is hypothesized to occur with cirrhosis. Similar to cirrhosis, vWF:Ag levels were high or very high in 70.5% of patients. High levels of vWF:Ag were associated with platelet counts <100,000/mm3, larger spleen diameter and oesophageal varices. In conclusion, hepatosplenic schistosomiasis leads to a lower platelet count due to pooling in the spleen and, consequently, impaired aggregation, but not to increased capture and destruction of platelets in the spleen. High vWF:Ag levels probably promote stabilization of platelet microaggregates and prevent minor manifestations of thrombocytopenia such as petechiae, ecchymosis and gingival bleeding.

Effects of ploughing frequency and compost on soil aggregate stability in a cotton–maize (Gossypium hirsutum‐Zea mays) rotation in Burkina Faso

AbstractCropping systems have a strong influence on soil structural characteristics, including aggregate stability. An experiment combining compost (Co) and mineral fertilizer inputs with different tillage frequencies was conducted in a cotton–maize rotation system on two soil types (Lixisol and Luvisol) in Burkina Faso. The objective was to investigate an alternative soil fertility management regime that protects soil structure for cotton and maize production. We tested the hypothesis that organic fertilizer applications and reduced ploughing frequency can improve aggregate stability. The effects of reduced tillage (RT; ox‐ploughing/hand hoe scarifying the next year) and annual ox‐ploughing (AP) combined with Co and no compost (nCo) applications on the stability of soil aggregates subjected to wet sieving, were assessed. In the second year of the experiment (maize), the macroaggregate stability in the RT plots was 87% higher than in the AP plots on the Lixisol, 26% higher on the Luvisol. In the third year (cotton) the difference in treatment effects was less. The treatments influenced macroaggregate stability in the year of scarification, and the microaggregate stability in the year they were ploughed. We concluded that a soil management regime, with ploughing only every second year and with compost as well as mineral fertilizer inputs, is appropriate to replace the common practice of annual ploughing with application of mineral fertilizers in this cotton–maize cropping system.

Nutrient Element Contents and Cation Exchange Capacity in Fine Fractions of Southeastern Nigerian Soils in Relation to Their Stability

Soils collected from 15 locations from SE Nigeria at the 0‐ to 20‐cm depth were studied for the nutrient elements of fine fractions and their role in the stability of the soils. The objective was to understand the role of these elements in the stability of the aggregates. The fine fractions were clay and silt, and elements measured in the fine fractions were exchangeable sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), exchangeable acidity (EA), cation exchange capacity (CEC), and available phosphorus (P). The aggregate stability was measured at the microlevel with clay dispersible indices and water‐stable aggregate (WSA) <0.25 mm, and at macrolevel with other WSA indices and mean‐weight diameter (MWD). Soils varied from loamy sand to sandy clay. There were more exchangeable cations, CEC, EA, and available P in clay than in the silt fraction. Whereas EA values ranged from 2.8 to 10.4 cmol kg−1, they were between 1.6 and 9.2 cmol kg−1 in silt. The CEC in the clay fraction was from 7.4 to 70 cmol kg−1 and between 4.0 and 32.8 cmol kg−1 in the silt fraction. The WDC were from 50 to 310 g kg−1 while the average dispersion ratio (DR) was generally higher than the corresponding clay‐dispersion ratio (CDR), and the MWD ranged from 0.45 to 2.68 mm. Soils with WSA skewed mostly to higher WSA (>2–1.00 mm) had a higher MWD. Exchangeable Ca2+ in clay correlated significantly with CDR and WSA sizes 1.0–0.5 mm and 0.5–0.25 mm (r=0.45,* 0.51,* and 0.60*), respectively, but negatively correlated with clay flocculation index (CFI) (r=−0.45*). Also, available P in clay correlated respectively with CDR and CFI (r=0.45*, −0.45*), whereas K+ in silt correlated significantly with WDSi (r=0.64*), CFI (r=0.62*), and CDR (r=−0.65*). Principal component analysis revealed that elemental contents in the silt fraction can play very significant roles in the microaggregate stability.

Lignin from rice straw Kraft pulping: Effects on soil aggregation and chemical properties

Lignin contained in pulping liquor that is generated during the pulping process for papermaking is a disposal problem for the pulp and paper industry. Separating lignin and other organic components from pulping liquor with inorganic acids may improve its applicability to fields as a beneficial soil amendment while offering a potential disposal alternative. Sulfuric acid-precipitated lignin from rice straw pulping liquor applied at rates of 1.67 and 3.34 g C kg −1 soil was incubated to evaluate its effects on soil properties over 8 weeks of incubation. Addition of this acid-precipitated lignin at these rates decreased soil pH by 0.24–0.53 units over 8 weeks of incubation, suggesting that this sulfuric acid-precipitated lignin from pulping liquor may have potential as a soil acidifying agent. Soil electrical conductivity (EC) only increased by up to 0.36 dS m −1, but highest EC levels were less than 4 dS m −1, indicating that lignin applied at both rates would not cause salinity problems. Application of this lignin increased soil organic C by 1.46 and 3.13 g C kg −1, and total soil N by 0.07 and 0.17 g N kg −1 over the incubation period. Lignin improved the macroaggregation of >2 mm size fraction, and increased wet microaggregate stability of >2 mm and 0.5–0.25 mm aggregates compared to a nonamended control. The results of this study suggest that this acid-precipitated lignin from pulping liquor may have potential as a beneficial soil amendment.

Physicochemical properties of earthworm casts and uningested parent soil from selected sites in southwestern Nigeria

Earthworms modify both the physical and chemical properties of soils. In a study on the possible modification of soil properties by earthworms, earthworm casts and uningested A and B soil horizons from three sites in southwestern Nigeria were analysed for selected physical and chemical properties. The casts were derived from the earthworm species Hyperiodrilus africanus. Results were analysed by a combination of analysis of variance with comparison of means and correlation analysis. Earthworm casts were significantly enriched in exchangeable bases, organic matter, base saturation and cation exchange capacity (CEC). The light fraction organic matter (LFOM) (density < 1.75 g cm −3) was more concentrated in the A horizon while the polysaccharides attached to heavy fraction (HFPS) (density > 1.75 g cm −3), presumably composed mainly of the exudates of soil microorganisms, were significantly highest in the casts. The clay and sesquioxide contents differed significantly among the groups analysed in the order B horizon > casts > A horizon, while the sand content ranged in the order A horizon > casts > B horizon. However, the fine sand and silt soil fractions were significantly highest in the casts. The crystalline sesquioxide contents were highest in the B horizon, while the amorphous Fe and the sesquioxides associated with organic complexes were higher in casts. There was no significant difference in the soils’ content of dispersible clay, while the wet stability of >250 μm aggregates from the B horizon was less than those from casts and the A horizon. Macroaggregate stability in casts was positively correlated with LFOM, amorphous Fe and Al, while microaggregate stability was positively correlated with HFPS. Meanwhile, microaggregate stability in the soil B horizon was mainly due to the crystalline forms of Al and Fe.

Microaggregation and stability of organic matter in the forest-steppe chernozems of the Volga region

Based on the regularities of changes in the accumulation, composition, and properties of the microstructural components separated from virgin chernozems by granulodensimetric fractionation, an hypothesis relating the mechanisms of the microaggregate formation and stability with the spatial and temporal distribution of the soil organic matter was confirmed. Schemes were composed for describing the relationships of the soil microstructure with the stability of the organic matter and the dynamics of its renewal in chernozems. Parameters were proposed for characterizing the equilibrium status of soil organic matter.

Minimum quantity of urban refuse compost affecting physical and chemical soil properties

The increasing production of urban waste requires urgent responses because of various environmental problems that arise when urban refuse is stored in landfills or incinerated. Recycling of domestic waste and composting of its organic fraction has been indicated as a possible disposal solution. A three-year experiment was conducted to quantify the minimum rate of urban refuse compost (URC) addition able to improve some physical and chemical soil properties at the lowest cost and environmental impact. URC was added to a silty clay soil and to a sandy loam soil 0%, 3%, 6%, 9% rate (w/w). Samplings were made 12, 24 and 36 months after URC application. To study the only effect of compost on soil due to its interaction with the soil matrix, each soil-compost mixture was divided into three boxes and kept outdoors weed free. After 12 months, 3% URC resulted the minimum quantity able to ameliorate several soil properties. In silty clay soil this rate significantly ameliorated microaggregate stability and hydraulic conductivity, but negative effects were observed on electrical conductivity. After 24 months, 3% rate significantly increased soil organic matter content. In the sandy loam soil, after 12 months, 3% rate of URC determined a positive effect on organic matter and cone resistance in dry soil condition. Electrical conductivity increased at 3% URC addition. The minimum URC quantity affecting hydraulic conductivity and plastic limit was 6%, and 9% for the liquid limit. Under these experimental conditions, the lowest rate (3%) of URC incorporation to soils appears to be the minimum quantity able to improve most of the soil properties influencing fertility. What the results show is that, to achieve sustainability of urban refuse compost application to agricultural soil, further research is needed to investigate soil property changes in the range between 0% and 3%.

Quantitative Rheological Indicators for Soil Physical Degradation

A lot of attention is being paid to the understanding of the influence of soil degradation on human life at the beginning of the 21 st century. Among the many types of degradation processes, structural degradation is widespread on huge areas in Europe. For better control, it is needed to get familiar with all the driving forces, the main reasons that lead to soil degradation (Várallyay, 2003; ESB, 2002). In addition to unfavourable natural conditions, inappropriate land use has an important impact on micro-aggregate stability and the rate of tolerance to deformation forces, such as cultivation and erosion. Rheological measurements provide new quantitative information on particle-particle interaction, the colloidal stability and structure of concentrated suspensions in general. Field samples from loess derived agricultural soils were investigated. In addition to general laboratory analyses (soil organic matter content, CaCO 3 content, CEC), conventional, simple aggregate stability, and water retention measurements and rheology were applied for investigating the micro-aggregate stability of the samples. The evaluation of pseudoplastic flow curves indicated close relationships between the strength and stability of the physical network and the composition of the suspensions. These soil properties have strong, well-defined connection with tolerating cultivation and capability for erosion.

Soil organic matter fractions and microaggregation in a Ultisol under cultivation and secondary forest in south-eastern Nigeria

The study of the role of soil organic carbon (SOC) in restoration of soil fertility and stability of soil microaggregates is of importance in soils that degrade rapidly. We studied 4 profiles in a Ultisol under secondary forest and cultivation to identify the SOC microaggregate-associated fractions and their roles in microaggregate stability. The soils are coarse-textured, deep, and low in soil nutrients and SOC, probably due to high rates of mineralisation. Microaggregate-associated SOC was also low with most of the SOC protected by the &lt;63 µm fractions. Clay content was negatively correlated with &lt;2, 63–2, and &lt;63 µm associated SOC (r = –0.45*, –0.42*, –0.40*, respectively). Clay flocculation index and clay dispersion ratio were significantly correlated with &lt;2 and 63–2 µm associated SOC, while water-stable aggregates &lt;0.25 mm were negatively correlated with all SOC fractions determined. Principal component analysis revealed that SOC fractions associated with 2000–63 µm aggregate sizes were the SOC fractions that best explained the variance in aggregated silt + clay, indicating their contribution to microaggregate stability. This was attributed perhaps to the production of polysaccharides and materials released by microbial activities from this recently deposited or incompletely decomposed SOC. The other soil properties that in addition to SOC contributed to either dispersion or microaggregate stability of these soils were exchangeable Na+, Mg2+, and CEC.

Wet Aggregate Stability of Some Botswana Soil Profiles

ABSTRACT During intense and short rainfall events, important losses of fertility are expected as a result of erosion in drylands because the nutrient pools are concentrated in the topsoil. Therefore, we evaluated the kinetics of aggregate stability in some Haplargids and Torripsamments in Botswana as it represents a resistance factor against losses of materials, measuring also the release of organic matter and phosphorus during wet sieving. We found very low contents of 1–2 mm aggregates in both soil types—although the Haplargids had higher levels of this size aggregate (19%) compared with the Torripsamments (10%)—and no differences between the topsoil and the deeper horizons. The total losses of aggregates were similar in topsoils and deep soil horizons and independent from soil order, but the Entisols had a greater proportion of aggregate breakdown, 38% on the average, when the sample was water-saturated. Only clay and silt content, and cation exchange capacity showed good correlations with aggregate stability. Organic matter or iron oxides had no effect, suggesting that aggregation is the result of cation bridging between mineral particles. Up to 97% of the organic matter released into water was lost at the beginning of the wet sieving, but the amounts were not related to aggregate breakdown, indicating that organic matter, besides not influencing the stability of macroaggregates, had little effect on the stabilization of microaggregates. Phosphorus was also released during the wet sieving, and was related to soil available P (r = 0.713 after 5 min). With no effect of organic matter on aggregate stability, and nutrients concentrated in the topsoil, the vulnerability of these soils to fertility losses is therefore extremely high.

Effect of Antecedent Moisture Content and Organic Waste Amendment on the Stability of Microaggregates of Some Kenyan Soils

(2004). Effect of Antecedent Moisture Content and Organic Waste Amendment on the Stability of Microaggregates of Some Kenyan Soils. East African Agricultural and Forestry Journal: Vol. 70, No. 1, pp. 23-27.

Groundwater irrigation and phosphogypsum application: their influence on soil properties and pasture production in northern Victoria

Saline–sodic irrigation water [ECiw = 0.1, 0.8, 2.5, 4.5, 7.5 dS/m, SARiw = 3, 5, 11, 17, 29 (mmolc/L)0.5] was applied to lucerne plots over 1991–95 followed by further irrigation [ECiw = 0.1, 0.8, 4.5 dS/m, SARiw = 3, 5, 17 (mmolc/L)0.5] of perennial pasture in 1997–99 with and without added phosphogypsum (PG = 0, 5 t/ha; May 1998). The later irrigation treatments and PG did not decrease exchangeable sodium percentage (ESP) in the B horizon nor alter bulk soil physical properties. Microaggregate stability to wetting decreased with increasing�residual sodicity and decreasing soil electrolyte concentration. The stability of topsoil aggregates was explained by higher organic carbon content while subsoil aggregate instability was related to higher clay content and higher ESP. Pasture yield decreased by 2 t/ha with an associated increase in soil ESP from 9 to 15%. Pasture yield was highest for ECiw = 0.8 dS/m and this treatment marginally increased N uptake in leaves. Higher yields for this treatment could not be readily explained by soil physico–chemical measurements.

Macro- and micro-aggregate stability of soils determined by a combination of wet-sieving and laser-ray diffraction

Soil structural stability affects the profitability and sustainability of agricultural systems. Different-sized structural units have different stability mechanisms and respond differently to such external factors as rain, wind, irrigation and management. A comprehensive analysis of the soils structural stability requires its characterization at the macroand micro-aggregate scales. We determined the aggregate stability of 36 soils at the macro-aggregate scale using wetsieving methods and of 20 soils at the micro-aggregate scale using laser-ray diffraction techniques. All the tests gave consistent estimates of aggregate stability. Most soils were homogeneous and quite stable at the macro-aggregate level as determined by the "water stable aggregate" parameter, but differed significantly among them and were quite unstable at the micro-aggregate level as determined by the "mean weight diameter of micro-aggregates" parameter. Slaking induced by the fast wetting of aggregates was the main destabilizing mechanism in these soils (88% of the soils had slaking stability index values &lt; 0.5), whereas most soils were quite tolerant to the mechanical shaking of aggregates (89% of the soils had stirring stability index values &gt; 0.5). The combination of the macro- and micro-aggregate stability tests is a consistent way for describing the structural stability of the studied soils.

Influence of soil properties on the aggregation of some Mediterranean soils and the use of aggregate size and stability as land degradation indicators

Soil aggregation in relation to other soil properties was studied along a climatological transect in the Southeast of Spain. Three sites were selected along this transect ranging from semiarid to subhumid climatological conditions. The aggregate size distribution, the macro and microaggregate stability of the superficial soil horizon and their relations with other soil properties were analysed. Large aggregates (>10, 10–5, 5–2 mm) were present in highest proportions in the most arid of the studied areas. These large aggregates were associated with high values of water-stable microaggregates; however, they did not improve soil structure and are related to high bulk densities and low water retention. Aggregates 1–0.105 mm were positively correlated to medium, fine, very fine sand and silt fractions and to organic matter. Aggregates <0.105 mm were positively correlated to organic matter and clay content. Small aggregate sizes (1–0.105 and <0.105 mm) had a positive influence on soil water retention and they seem a good indicator of soil degradation. Water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6%. Below that threshold the carbonate content was strongly correlated with aggregate stability. A general conceptual scheme of associations between aggregate size distribution, water-stable microaggregation and textural characteristics for the area was developed. The land use history affecting soil overlaps the pattern of climatological situations and has to be taken into account. Aggregate size distribution and stability can be used as indicator of soil degradation, but not as a unique parameter.

Laminin and α-Dystroglycan Mediate Acetylcholine Receptor Aggregation via a MuSK-Independent Pathway

Specific isoforms of laminin (LN) are concentrated at neuromuscular junctions (NMJs) where they may participate in synaptic organization or function. In myotubes from C2 cells, LN is concentrated within the majority of spontaneous acetylcholine receptor (AChR) aggregates. Neural agrin substantially increases this colocalization, suggesting that agrin can recruit LN into AChR aggregates. Addition of LN to C2 myotubes induces a more than twofold increase in the number of AChR aggregates. These aggregates have a larger size and are more dense than are those induced by agrin, suggesting that LN is involved in the growth and/or stabilization of AChR aggregates. Consistent with this hypothesis, an antiserum to LN reduces the size of individual AChR aggregates but increases their number. In C2 myotubes, extracellular matrix receptors containing the integrin beta1 subunit are poorly colocalized with AChR aggregates, suggesting that integrins may not be involved in LN-induced aggregation. In contrast, almost all AChR aggregates are associated with dystroglycan immunoreactivity, and monoclonal antibody (mAb) IIH6 against alpha-dystroglycan (alpha-DG), a LN and agrin receptor, causes a concentration-dependent inhibition of LN-induced aggregation. Moreover, S27 cells, which lack a functional alpha-DG, and two C2-derived cell lines expressing antisense DG mRNA fail to aggregate AChRs in response to LN. Finally, LN-induced AChR aggregation does not involve the phosphorylation of the muscle-specific tyrosine kinase receptor (MuSK) or the AChR beta subunit. We hypothesize that the interaction of LN with alpha-DG contributes to the growth and/or stabilization of AChR microaggregates into macroaggregates at the developing NMJ via a MuSK-independent mechanism.

The dynamics of carbon in particle-size fractions of soil in a forest-cultivation sequence

Cultivation of forest and grassland soils induces heavy changes in soil organic matter (SOM) dynamics. To better predict the effect of cultivation, there is a need to describe which organic pools are affected and to which extent. We used a chronosequence of thick humic forest soils converted to maize cultivation for 40 yr in southwest France. The dynamics of soil carbon was investigated through particle-size fractionation and the use of 13C allowed to distinguish forest-derived organic matter and new crop-derived organic matter. This partitioning of soil carbon by size on one hand and by age on the other provided a precise description of carbon turnover. The level towards which tend the organic pools under cultivation showed that the decay rates of soil carbon were one order of magnitude higher under cultivation than under forest. SOM can thus be considered as deprotected under cultivation. All size fractions appeared to be deprotected to the same extent. A progressive transfer of silt-sized C to clay-sized C was nevertheless suspected and attributed to the decreasing stability of fine silt-sized microaggregates with cultivation. SOM furthermore contained some very stable C present as silt-sized and possibly clay-sized particles. The turnover times of maize-derived organic matter was the same as that observed in similar soils cultivated for centuries. This indicated that the new conditions induced by cultivation were reached in the very first years after forest clearing and that the high initial SOM content and high mineralization rate of initial organic matter did not affect the dynamics of newly incorporated carbon.