Aggregate Size Fractions Research Articles

Clay content modulates differences in bacterial community structure in soil aggregates of different size

Aggregation of soil components is contributing to the formation of diverse habitats for microbial life. Soil aggregates of different size are known to promote niche differentiation. Moreover, aggregates differ in physical and chemical properties, therewith likely supporting biological diversity in soil. To evaluate whether different types of aggregates affect bacterial colonization, we analyzed the impact of aggregate size in dependence of soil clay content on the bacterial community composition. A further distinction was made between microaggregates that occur as free structures in the soil matrix and those that exist occluded within macroaggregates. Upon size fractionation of soil samples from a clay catena (19 – 34%), we analyzed the bacterial community composition in ten different aggregate fractions by amplicon sequencing of the 16S rRNA gene. Both the aggregate size and the clay content showed an influence on the bacterial community composition, although this influence explained only a minor amount of the total observed variation across all samples (approx. 10% each). The increasing clay content led to more distinct communities in aggregates of different size, especially the smallest microaggregate fractions (< 20 µm) became more distinct. This is explained by enhanced aggregate stability in soils of increasing clay content, resulting in the development of more distinct bacterial communities over time. Indicator species analysis revealed a preferential enrichment of copiotrophic bacteria in free microaggregates, while oligotrophic bacteria were enriched in occluded microaggregates, indicating that a further differentiation of the bacterial community occurs within this specific aggregate size fraction in dependence on the localization of the microaggregates in the soil matrix, being possibly related to differences in organic carbon quality. These findings show that further aggregate traits besides size, i.e. stability mediated by soil clay content and localization in the soil matrix, contribute to the diversification of bacterial communities in soil.

31-year contrasting agricultural managements affect the distribution of organic carbon in aggregate-sized fractions of a Mollisol

Evaluation of soil organic carbon (SOC) dynamics is often limited by the complexity of soil matrix. Quantitative information on the distribution of SOC within aggregate hierarchy will help elucidate the carbon flow in soil matrix. However, this knowledge still needs to be documented. Soils were sampled from a surface Mollisol with plots under 100 years of continuous cropping, 31 years of simulated overgrazing to severely degraded bareland, and grassland restoration from cropped soil. A combined density and chemical fractionation procedure within water-stable aggregate was utilized to quantify the distribution of OC after long-term different land use patterns. Results showed that grassland significantly increased total SOC and mean aggregate associated OC compared to initial soil in 1985 with total SOC (g kg−1 soil) from 46.1 to 31.7 and mean aggregate associated OC (g kg−1 aggregate) from 31.6 to 44.7. Converting cropland to grassland also enhanced the formation of macroaggregates (>0.25 mm) (from 34.7% to 52.2%) and increased the OC concentrations in density and humic fractions by 48.3%-75.9% within aggregates. But the proportions of OC in density and humic fractions to SOC only increased in macroaggregates in grassland. Alternatively, converting cropland to bareland caused substantial depletion of total SOC, macroaggregates and their associated OC concentrations. The SOC (g kg−1 soil) and mean aggregate associated OC (g kg−1 aggregate) significantly decreased from 31.7 to 25.7 and from 31.6 to 26.2, respectively. While the OC concentration of density and humic fractions within aggregates in bareland did not show significant decreases. Principal component analysis demonstrated that the soils were developed by contrasting land use changes, with the grassland soil being more associated with labile OC fractions within macroaggregats and bareland soil more associated with recalcitrant OC fractions within microaggregates and silt-clay units. These findings highlighted the favorable preservation of plant-derived carbon within soil aggregates, particularly in the labile OC fractions within macroaggregates under high plant inputs with 31 years of grassland conversion. For the cropland and bareland soils without organic inputs, more OC was stabilized within fine aggregates via organo-mineral interactions, tending to be more recalcitrant.

Complexity of bacterial and fungal network increases with soil aggregate size in an agricultural Inceptisol

Differently sized soil aggregates may provide spatially heterogeneous microenvironments for microorganisms, but little is known about the impacts of soil spatial structure on the co-occurrence patterns of bacterial and fungal taxa. Here, we investigated the compositions and networks of bacterial and fungal communities within different aggregate-size fractions (macroaggregates: 2000–250 μm; microaggregates: 53–250 μm; and silt + clay: <53 μm) of an agricultural Inceptisol to examine the micro-scale location and interaction of microbial populations. Our results showed that aggregate size was positively correlated with soil C, N and P, while negatively correlated with soil C/N ratio. Phospholipid fatty acid and high-throughput sequencing indicated that bacterial and fungal community structures were affected more strongly by aggregate size than by fertilization. Soil microbial biomass and community compositions were significantly correlated with environmental factors. The influences of fertilization treatments on soil microorganisms varied with aggregate size and the greatest were found for macroaggregates. Bacterial-fungal networks across different aggregate-size fractions were analyzed by MENA, CoNet and R software. Total links, average degree and average clustering coefficient increased with aggregate size, whereas average path distance followed the opposite trend. More bacterial-bacterial correlations were detected in macroaggregate network. Our results suggest that bacterial-fungal network was much more complex in macroaggregates than in microaggregates and silt + clay, indicating greater cooperation and competition among microbes in larger aggregates. Overall, our study demonstrates the significant effects of soil structure on microbial community structure and network.

Drought changed soil organic carbon composition and bacterial carbon metabolizing patterns in a subtropical evergreen forest

Subtropical forests are considerable carbon sinks in the northern hemisphere, yet are increasingly suffering from the impact of extreme drought. To better understand the dynamics and kinetics of forest soil carbon storage under long-term drought, a rainfall-reduction experiment was established in a subtropical evergreen forest of eastern China. Soil organic carbon (SOC) composition, microbial carbon metabolism and the interactions with soil microbial community structure were investigated across different soil aggregate size fractions. After five years' treatment of rainfall reduction, a significant loss of large macroaggregates, as well as an increase of microaggregates by over 100% was observed. Meanwhile, drought changed the composition of SOC, reducing the non-hydrolyzed carbon and humin contents in large- to medium-size macroaggregates. Microbial metabolizing capacity of polymeric compounds was also reduced especially in the above aggregate fractions, whereas the utilization of small-molecular compounds was more impacted in small macroaggregates and microaggregates. The changes in carbon metabolizing patterns were further associated with the abundance changes of specific microbial taxa, revealing the microbially mediated mechanism of soil carbon metabolism under long-term drought. In addition, carbon metabolism in microaggregates was particularly sensitive to the changes of soil moisture, suggesting long-term drought may continually influence the functional resistance of the microbial communities. Taken together, our results provide insights into how biotic and abiotic processes together influence the SOC metabolizing processes, continued monitoring and investigation of which shall contribute to better understanding of the dynamics and kinetics of SOC storage under the impact of long-term drought.

Long-term effects of straw return and straw-derived biochar amendment on bacterial communities in soil aggregates

Improving soil structure, fertility, and production is of major concern for establishing sustainable agroecosystems. Further research is needed to evaluate whether different methods of straw returning determine the variations of soil aggregation and the microbial community in aggregates in the long term. In this study, we comparatively investigated the effects of long-term fertilization regimes performed over six years, namely, non-fertilization (CK), chemical fertilization (CF), continuous straw return (CS), and continuous straw-derived biochar amendment (CB), on soil aggregation and bacterial communities in rice-wheat rotation systems. The results showed that straw/biochar application increased soil nutrient content and soil aggregate size distribution and stability at both 0–20 cm and 20–40 cm soil depths, compared with those of CF and CK; CB performed better than CS. CB increased bacterial community diversity and richness in 0–20 cm soil, and evenness in 0–40 cm soil (p < 0.05); CS had no significant effect on these aspects. Variations in the relative abundance of Actinobacteria, Chloroflexi, Bacteroidetes, Nitrospirae, Gemmatimonadetes, and Latescibacteria in specific aggregates confirmed the different effects of straw/biochar on bacterial community structure. The partial least squares discrimination analysis and permutation multivariate analysis of variance revealed that fertilization, aggregate size fractions, and soil depth affected the bacterial community, although their effects differed. This study suggests that CB may reduce chemical fertilizer usage and improve the sustainability of rice-wheat cropping systems over the long term, with a better overall outcome than CS.

Aggregates Characterization and Its Associated Organic Carbon in Two Contrasting Lowland Rice Soils of West Bengal

Aims: The present study investigated the effect of lowland rice soils of two regions viz. new alluvial and red-laterite on aggregate characterization and their associated organic carbon (SOC).&#x0D; Study Design: Randomized block design (RBD).&#x0D; Place and Duration of Study: New alluvial soils were collected from Jangipara block of Hooghly, West Bengal and Red-laterite soils were collected from Raghunathpur block of Purulia, West Bengal during 2017-18.&#x0D; Methodology: For each soil types (New alluvial and Red-laterite) five locations were identified and soil samples were collected from three depths i.e. 0-10, 10-20 and 20-30 cm. The aggregate characteristics i.e. water-stable aggregates (WSAs), mean weight diameter (MWD), aggregate stability and aggregate size fractions along with the distribution of carbon in those aggregate size fractions were critically studied.&#x0D; Results: The aggregate size as well as the stability decreased with increasing soil depth from 0 to 30 cm in both soils. New alluvial soils showed higher aggregate stability than red-laterite soils. Mean weight diameter (MWD) values of new alluvial soils were 34, 29 and 87% more than red-laterite soils at 0-10, 10-20 and 20-30 cm depth, respectively. Presence of higher amount of clay and organic matter in new alluvial made the difference in structural coefficient. The surface soil (0-10 cm) had more coarse aggregate (Cmac A &gt;2000μ) fraction, however, microaggregates (&lt;250μ) were dominant in lower depths in both soils. Water stable aggregates (WSA) in surface soils of new alluvial and red-laterite were 57 and 36%, respectively and were decreased with depth. Red-laterite produced higher micro aggregates as compared to new alluvial soils. Coarse macro aggregate fractions (&gt;2000μ) retained maximum amount of soil organic carbon in both soils however, coarse micro aggregate associated carbon (Cmic AC&lt;250μ) was captured in lower depths. New alluvial soils yielded aggregates with higher in diameter and stability coefficient that is due to higher amount of carbon stored in aggregates.&#x0D; Conclusion: The abundance of macro aggregate of New alluvial soils indicates better soil physical quality than Red-laterite soil which was dominated in higher micro aggregates leads to poor in structure and susceptible to water erosion.

Glyphosate and AMPA concentrations in the respirable dust emitted experimentally by soil aggregates, shortly after herbicide application

The objective of this study was to analyze glyphosate- and aminomethylphosphonic acid (AMPA) concentrations in the respirable dust (RD) emitted by different soil aggregates, shortly after herbicide application in a field approach. The relationships of both glyphosate and AMPA in the aggregates with organic matter- (OM), total iron- (Fe), and extractable phosphorus contents (P) as well as with pHCaCl2 values were analyzed. An Kastanozem haplic of the Semiarid Pampas (Argentina) was analyzed 7 days after glyphosate application. The RD (particulate matter <10 mm in diameter, PM10) was generated and collected for the following aggregate-size fractions: <0.42 mm, 0.42 to 0.84 mm, 0.84 to 2 mm, 2 to 6.4 mm, 6.4 to 19.2 mm, and >19.2 mm. OM, P, Fe and pHCaCl2 were analyzed in the aggregate fractions, and glyphosate- and AMPA contents were measured both in RD and in the aggregate fractions. Glyphosate concentration varied between 16 mg kg−1 (aggregates <0.42 mm) and 4.7 mg kg−1 (aggregates >19.2 mm) and increased with decreasing size of the aggregates. Glyphosate concentration in RD varied between 48.5 mg kg−1 (aggregates <0.42 mm) and 17.8 mg kg−1 (aggregates >19.2 mm). Finest aggregates emitted RD with higher glyphosate concentrations. A positive relationship was determined between glyphosate in aggregates and glyphosate in RD (p < 0.05), indicating that aggregates with higher glyphosate concentration emitted a more glyphosate-enriched RD. Glyphosate in the aggregate fractions was not related to Fe, OM or pH of aggregates. Glyphosate- and P concentrations in aggregates were positively correlated. This is explained because both substances share a phosphonic acid functional group and they have similar reactivities in the soil. AMPA concentration varied between 130 mg kg−1 (aggregates >19.2 mm) and 307 mg kg−1 (aggregates <0.42 mm) in aggregates and between 527 ug kg−1 and 746 ug kg−1 in RD. AMPA concentrations in RD were not related to AMPA concentrations in aggregates. The contents of AMPA were higher than those of glyphosate in both, RD and aggregates, due to the longer persistence of AMPA in relation to glyphosate. In all cases, glyphosate- and AMPA concentrations were higher in RD than in the aggregates. Our results suggest that the RD emitted by agricultural soils has higher glyphosate- and AMPA concentrations than the bulk soil. These results encourage future investigations to elucidate if other pesticides (and the surfactants used in their formulations) are present in the RD emitted by agricultural soils. This information is necessary in order to evaluate the effects of the particulate matter emitted by agricultural soils on the whole ecosystem and the human health.

Temporal change in soil physicochemical, microbial, aggregate and available C characteristic in dry tropical ecosystem

The holistic understanding of soil C dynamics with age of organic amendment is limited in dry tropical ecosystems, which may act as potential C sink if managed properly. Therefore, we studied soil physicochemical, microbial, aggregate and available C characteristic with age organic amendment across the season for mechanistic understanding of soil C accumulation. Organic systems under 1, 3, 5 and 10 years under organic amendment were selected along with a forest system as reference for this purpose. We observed that soil organic C and CO2 efflux (SCE) increased significantly after ten years of organic amendment (though with no change in SCE on unit C basis), and approached closer to reference forest site. Soil fine particles, moisture content, ammonium-N, ammonium-N/nitrate-N ratio (ANR), microbial biomass C, N and C/N, crude dehydrogenase, glucosidase and phosphomonoesterase activity, macro- and mesoaggregate, mean weight diameter, macroaggregate water stability, and total and labile C concentration and carbon management index across aggregate fractions significantly increased with age. However, soil nitrate-N, C/N ratio, water soluble C, specific glucosidase and phosphomonoesterase activity, microaggregate and mineral fraction decreased. The seasonality of total and labile C concentration and stock differed distinctly across aggregate size fraction, which showed its linkage with microbial and plant C input dynamics. Though, total and labile C stock increased across aggregate fractions with age, it shifted towards macro-and mesoaggregate, opposite to smaller size fractions. Macro-aggregate stability along with macro-aggregate labile C, dehydrogenase activity, microbial biomass C/N and mesoaggregate C majorly determined the SOC. However, specific glucosidase activity along with mesoaggregate, ammonium/nitrate ratio and microbial biomass majorly governed the SCE. The study indicates that macro-aggregate characteristic (including mesoaggregate) play a crucial role in soil C accumulation in dry tropical ecosystem under organic amendment, with microbial community characteristics and ANR. Moreover, the study indicates towards key role of macro-mesoaggregate equilibrium and inorganic N dynamics in the soil C dynamics of the studied ecosystems, in which seasonality and C input play a crucial role.

Performance of concrete targets mixed with coarse aggregates against rigid projectile impact

For effective structure and personnel protection against projectile impact, accurate prediction of the depth of penetration (DOP) of projectile into concrete is essential. Many empirical and semi-empirical formulae have been developed to yield accurate predictions as compared with testing data. In most of these formulae, the concrete target was assumed as homogeneous and the mechanical properties of concrete material were characterized by the compressive strength of concrete. The deviation of DOP predications is identified for the concrete targets mixed with coarse aggregates because of the homogeneous assumption which leads to errors in DOP prediction. In this study, the effects of mortar strength, aggregate strength, maximum size and volume fraction of coarse aggregate on penetration resistance are quantitatively studied by using 3D meso-scale modeling. The applicable condition of the homogeneous assumption and the upper bounds of aggregate size and volume fraction that affect the penetration resistance are determined with respect to the aggregate configurations. The factors dominating the penetration resistance of concrete target are investigated and it is found that the sensitivity of penetration resistance to aggregate size is lower than other mesoscopic factors, i.e., mortar strength, aggregate strength and volume fraction. In addition, a formula incorporating the effect of multi-factors is proposed to predict the DOP of rigid projectile penetration into concrete target mixed with coarse aggregates. The proposed formula, which considers the effects of the mesoscopic factors on the penetration resistance, can give better predictions of DOP into concrete targets.

Aggregate formation and organo-mineral association affect characteristics of soil organic matter across soil horizons and parent materials in temperate broadleaf forest

Precise assessment of soil organic carbon (OC) storage requires understanding of soil type and depth specific differences in organic matter (OM) stabilization. Therefore, we aimed to analyze OC dynamics down the soil profile and to clarify the effect of depth on the importance of aggregate formation and mineral adsorption for OC storage in mature beech forest soils developed from different parent materials. Aggregate size and density fractions were separated from samples of top and subsoil horizons, which were quantified and analyzed by infrared spectroscopy. We also determined the microbial biomass C (Cmic) and the amount of C decomposed within incubation experiments (CO2-C) for the bulk soil samples. OC stabilized via aggregate formation and mineral association significantly increased with soil depth. However, this stabilized pool seemed to fuel the labile OM stronger in the subsoil than in the topsoil because the CO2-C/SOC and CO2-C/Cmic ratios increased with depth. Measured differences in the magnitude of the detected stabilization and destabilization patterns were attributed to parent material and soil horizon, indicating pronounced spatial and vertical heterogeneity in the contribution of soils under temperate broadleaf forest to terrestrial C sequestration. Considering such site and depth specific differences will improve assessment and modelling of soil OC storage for areas covered with the same forest type but with high pedogenetic diversity.

Effect of biochar and compost on soil properties and organic matter in aggregate size fractions under field conditions

Aggregation affects soil properties crucial for sustainable soil management and productivity. However, longer-term studies (> five years) of treatments to enhance soil aggregation, such as addition of biochar with labile carbon to derive microbial binding agents, are limited, especially in temperate climates. To fill this gap, we established a field experiment with control, compost only, biochar only, and a mixed compost-biochar application (co-composted and only mixed) at low and high application rates (9–70 t ha−1) in southern Germany. After six years of agricultural use, surface (0−10 cm) and subsurface (10−30 cm) soil samples were analyzed for pH, microbial biomass carbon (Cmic), water holding capacity (WHC), and cation exchange capacity (CEC). Particulate organic matter (POM) and aggregate fractions were analyzed for organic carbon (OC) content and characterized using diffuse reflectance infrared Fourier transform spectroscopy. Biochar significantly increased OC and pH, while compost significantly increased OC, pH, Cmic, and CEC six years after application. Biochar also significantly increased OC storage in POM (10–32 times higher) and all aggregate fractions by 56–62 %, 32–47 %, and 29–32 % for the 2 - 0.25 mm, 0.25 - 0.053 mm, and < 0.053 mm fraction, respectively, while compost increased OC storage only in the 10−30 cm soil depth in the soil fractions > 0.053 mm. The proportion of reactive CO groups significantly increased in POM due to biochar and compost application, while only biochar affected the < 0.053 mm fraction. Our results suggest that six years after application, high rates of both biochar and compost are beneficial for soil properties affecting the sustainability of soil agro-ecosystems such as pH and CEC. For long lasting increase in soil C sequestration, our results indicate that only the application of biochar can be considered as a significant measure.

Enhanced soil aggregate stability limits colloidal phosphorus loss potentials in agricultural systems

BackgroundColloid-facilitated phosphorus (P) transport is recognized as an important pathway for the loss of soil P in agricultural systems; however, information regarding soil aggregate-associated colloidal P (Pcoll) is lacking. To elucidate the effects of aggregate size on the potential loss of Pcoll in agricultural systems, soils (0–20 cm depth) from six land-use types were sampled in the Zhejiang Province in the Yangtze River Delta region, China. The aggregate size fractions (2–8 mm, 0.26–2 mm, 0.053–0.26 mm and < 0.053 mm) were separated using the wet sieving method. Colloidal P and other soil parameters in aggregates were analyzed.ResultsOur study demonstrated that 0.26–2 mm small macroaggregates had the highest total P (TP) content. In acidic soils, the highest Pcoll content was observed in the 0.26- to 2-mm-sized aggregates, while the lowest was reported in the < 0.053 mm (silt + clay)-sized particles, the opposite of that revealed in alkaline and neutral soils. Paddy soils contained less Pcoll than other land-use types. The proportion of Pcoll in total dissolved P (TDP) was dominated by < 0.053 mm (silt + clay)-sized particles. Aggregate size strongly influenced the loss potential of Pcoll in paddy soils, where Pcoll contributed up to 83% TDP in the silt + clay-sized particles. The Pcoll content was positively correlated with TP, Al, Fe, and the mean weight diameter. Aggregate-associated total carbon (TC), total nitrogen (TN), C/P, and C/N had significant negative effects on the contribution of Pcoll to potential soil P loss. The Pcoll content of the aggregates was controlled by the aggregate-associated TP and Al content, as well as the soil pH value. The potential loss of Pcoll from aggregates was controlled by its organic matter content.ConclusionWe concluded that management practices that increase soil aggregate stability or its organic carbon content will limit Pcoll loss in agricultural systems.

Molecular characterization of soil organic carbon in water-stable aggregate fractions during the early pedogenesis from parent material of Mollisols

The aims of this study were to investigate how short-term agricultural practices varying in vegetation cover, tillage, and organic amendments affected the chemistry of aggregate-associated C and particulate organic matter (POM) after 8-year soil restoration from parent material (PM) of a Mollisol. The research allowed indicating the effects of perennialization and organic amendments on the chemical compositions of soil organic carbon (SOC) in both aggregates and POM in short-term and thereof helped to unveil the mechanisms of C sequestration and stabilization. The field surface soils (0–20 cm) under 8-year natural perennials and an arable soil with organic amendments were collected. Soil aggregate fractionation method was applied to obtain three aggregate-sized fractions: macroaggregate (> 250 μm), microaggregate (53–250 μm), and silt-clay unit ( 250 μm), microaggregate (53–250 μm), and silt-clay units within macroaggregate, the fine POM (fPOM, 53–250 μm), and silt-clay units within microaggregate were further separated. In all, 10 subgroups of aggregates were obtained to determine organic carbon content and chemical structure using 13C-NMR spectroscopy technique. The PM for the establishment of the experiment was also used. The chemical composition of SOC differed among aggregate sizes, representing different organic compositions with different decomposition stages. Compared with PM, soil restoration under natural perennial and organic amendments increased SOC in both POM and aggregate fractions; higher proportion of POM with larger proportion of plant-derived O-alkyl C was observed in arable soil than in perennials. Meanwhile, the silt-clay units, sequestrating the most of the organic C, contained larger proportions of alkyl C, aromatic C, and carbonyl C, probably due to the microbial-derived by-products. The data partially supported our hypothesis that continuous organic amendments to PM likely promoted aggregate formation and subsequently affected the chemical composition of C therein. Perennialization and organic amendments increased not only the total SOC stocks in bulk soils and aggregate fractions but also changed the SOM chemistry by varying quantity and quality of plant residues. The silt-clay units contributed largely to the stock and stability of SOC during the soil development stage from PM of a Mollisol. The main mechanisms of plant-derived C sequestrated and SOC accrued were controlled probably not only by the physiochemical protection of soil aggregates but also by the microbial C sequestration capacity in silt-clay fraction at the initial stage of soil formation.

A 3D multi-phase meso-scale model for modelling coupling of damage and transport properties in concrete

A 3D multi-phase meso-scale model is proposed to analyse the evolution of damage and transport properties in concrete. This model includes five phases: aggregates, mortar, voids, cracking and interfacial transition zones (ITZs) between aggregates and mortar. They are generated using measurable characteristics, such as shape, size and volume fraction of aggregates and voids. Mechanical part of the model is validated through comparing the predicted and experimental stress-strain curves and cracking patterns. Transport part of the model is validated by the comparison of predicted and experimental diffusivities of chloride ions in concrete. A new interface element type is also proposed to represent cracking in transport analysis which can simulate transport not only in normal directions but also in tangential directions. The effects of parameters relevant to aggregates, voids, ITZs and cracking patterns on the mechanical and transport responses of concrete are also analysed. The derived qualitative understanding of the relation between meso-structural features and macroscopic properties will help future advances in modelling and formulation of experimental programmes to improve the calibration and validation of meso-scale models. The proposed sequential mechanical-transport coupling model is applicable to a large class of composite materials with appropriate selection of phases and their properties.

Effect of Tillage and Water Management on Aggregate Stability of a Gleyic Cambisols

The objective of this study was to determine the effect of tillage and water management on the physical stability of irrigated lowland rice field, in Kwalkwalawa, Sokoto State. The experiment was carried out in a farmer`s field, near the Usmanu Danfodiyo University Teaching and Research Farm, Kwalkwalawa, Sokoto State. The coordinates of the area were taken using global positioning system (GPS) model Garmin etrex 20.0, which shows that the area is located on (N13°05.963”E005°12.650” and at 252 m asl). The soils of the study area were classified as Aeric Endoaquepts at subgroup level in the USDA Soil Taxonomy System which correlated with Gleyic Cambisols in the World Reference Base. The treatments consisted of factorial combination of two tillage systems (conventional tillage (CT) and reduced tillage (RT), three irrigation water managements (Alternate one, two and three days irrigation intervals, which were carried out from one week after transplanting to hard dough stage) and three rice varieties (FARO 44, 60 and 61) all laid in a split-plot design and replicated three times. After harvest, disturbed soil samples were collected with the aid of soil auger, prepared and passed through 5 mm sieve for aggregate stability determination. Result reveals that a consistent trend in aggregate size fraction was observed between the two tillage systems in both years, were a significant decrease in values of aggregate fractions of both the CT and RT in 2019 compared to in 2018. Aggregate size fraction of 5-2 mm had a significantly high value of RT compared to CT, 2-0.25 mm fractions were at par (0.41) while a greater value of aggregate fractions for CT in both 0.25-0.005 mm and ˂0.005 mm were observed compared to RT. Alternate days to irrigation and sampling depths increase with a corresponding in all the aggregate size classes in the two years of this study. A significant difference in mean weight diameter (MWD) and geometric mean diameter (GMD) between the two tillage, water management and depth was noticed in both years of the trial where RT and alternate one day had high value while MWD and GMD increase with increasing depth.

Labile soil carbon fractions as indicators of soil quality improvement under short-term grassland set-aside

Grassland set-asides (GLSA) are fields that are taken out of intensive annual crop production and seeded with a mixture of grasses and legumes for one to four years to improve soil quality. The objectives of this study were to evaluate (i) the relationships among soil organic carbon (SOC), permanganate oxidisable C (POXC), dilute-acid extractable polysaccharides (DAEP) and aggregate stability to determine if they may be used as proxies for one another, (ii) whether these indicators could be used to predict aggregate stability, (iii) if differences in soil quality after short-term GLSAs, detected with aggregate stability, could instead be detected with POXC or DAEP and (iv) potential use of diffuse Fourier transform spectroscopy (FT-MIR) to predict POXC, DAEP and aggregate stability in the Fraser River Delta region of British Columbia, Canada. There were strong relationships among SOC, POXC and DAEP, but the relationship between DAEP and SOC (R2 = 0.60, P &amp;lt; 0.0001) was less strong than that observed between POXC and SOC (R2 = 0.71, P &amp;lt; 0.0001). All three soil C fractions were significantly predicted with the 2–6 mm aggregate size fraction but the correlations for DAEP (R2 = 0.43) and POXC (R2 = 0.36) were stronger than that for SOC (R2 = 0.29). Predictions of soil quality indicators using FT-MIR produced R2 = 0.92 for POXC, R2 = 0.93 for DAEP and R2 = 0.62 for the 2–6 mm aggregate size fraction. These results suggest that FT-MIR holds promise as a low-cost method to determine labile soil C fractions that are better proxy soil quality indicators for aggregate stability than SOC.

Impact of vegetation restoration on soil organic carbon stocks and aggregates in a karst rocky desertification area in Southwest China

The objectives of the study were as follows: (a) to determine the response of soil organic carbon (SOC) fractions to vegetation restoration; (b) to examine the contributions of aggregate-associated OC to total soil OC accumulation along vegetation restoration, (c) to identify the factors that affect SOC accumulation along natural vegetation restoration in a karst region in Southwest China. Four vegetation restoration stages, namely, grassland, shrubland, shrub-arbor mixed forestland, and arbor forestland, were compared with cropland (CL). Soil samples were collected at depths of 0–10 cm and separated into five aggregate size fractions. SOC, light fraction OC (LFOC), easily oxidizable OC (EOC), and aggregate-associated OCs were determined for different aggregate sizes and total soil. Natural vegetation restoration increased macroaggregate amount but decreased the fractions of meso- and microaggregates. Vegetation restoration significantly increased total SOC, EOC, and LFOC concentrations and stocks and soil aggregate-associated OC concentrations. The responses of EOC and LFOC in total soil and soil aggregates were more sensitive than those of SOC along with vegetation restoration. Aggregate-associated OC concentrations generally increased with a decrease in aggregate size. Macro- and microaggregate-associated OC stocks increased, but mesoaggregate-associated OC stocks decreased following the conversion of CL to a natural vegetation ecosystem. The increase in SOC stocks was primarily attributed to the macroaggregate-associated OC stocks and their changes. OC concentrations and stocks in total soil and the soil aggregates were significantly positively related to exchangeable calcium content. Vegetation restoration considerably affects the amount of soil aggregates, OC concentrations, and stocks in total soil and soil aggregates. Changes in OC concentrations and stocks can be more pronounced in the liable carbon fraction than in total SOC. The increase in SOC was mostly attributed to OC accumulation in macroaggregates. Exchangeable calcium also affected soil OC accumulation in total soil and soil aggregates.

Impact of different fallow durations on soil aggregate structure and humus status parameters

Soil aggregate structure and soil organic matter are closely interrelated and commonly considered as key indicators of soil quality. The aim of this study was to evaluate the effects of different fallow durations on indices of soil structure and humus status indicators. Studies were conducted on abandoned agricultural fields (15, 20 and, 35 years after abandonment). As a reference site, we used a cultivated field in the area. The experimental soil fields are classified as Gleyic Cambisols. Soil macroaggregates were separated with the sieve (dry sieve) to seven aggregate size fractions, i.e.&amp;gt; 10, 10–5, 5–2, 2–1, 1–0.5, 0.5–0.25 and &amp;lt; 0.25 mm. The humus status parameters of soils included the following indicators: soil organic carbon (C&lt;sub&gt;org&lt;/sub&gt;), humus reserves (Q&lt;sub&gt;H&lt;/sub&gt;), the degree of humification of organic matter (SOM&lt;sub&gt;dh&lt;/sub&gt;), fractions of humic acids (HA) (free and bound with monovalent cations and Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, bound with Са&lt;sup&gt;2+&lt;/sup&gt; which forms humates, bound with clay minerals), fulvic acids (FA) (free aggressive) and ratio of HA to FA (C&lt;sub&gt;HA&lt;/sub&gt;&lt;sub&gt; &lt;/sub&gt;: C&lt;sub&gt;FA&lt;/sub&gt;). After a fallow period of more than 20 years on the surface formation of a sod layer. A long-term fallow period had an impact on the mean weight diameter of the aggregates (MWD) and agronomically valuable aggregates (AVA). Fallow soils have a significantly better structure than soils under a cultivated field. Long-term cultivation leads to the deterioration of soil structure and the formation of large aggregates (&amp;gt;10 mm). The C&lt;sub&gt;org&lt;/sub&gt; content remains at the level of the background content when the soils are left fallow for less than 15 years and increases over time. The C&lt;sub&gt;org&lt;/sub&gt; in the upper 0–20 cm soil layer has been shown to increase from 3.55 to 8.74% on arable land that has been fallow for 35 years and has been largely associated with significant accumulation of organic matter within the plant root mass. Mature sites are characterized by an increase of fulvic acids in the humus composition in comparison with their arable analogues. The abandonment of soil agricultural use and the cessation of mechanical tillage results in the restoration of the natural structure of soils and the improvement of their agrophysical properties. Such studies have not been previously conducted in the Primorsky region of the Russian Far East.

Effects of freeze-thaw cycles and initial soil moisture content on soil aggregate stability in natural grassland and Chinese pine forest on the Loess Plateau of China

Ongoing global warming is decreasing the thickness of snow covers and increasing initial soil moisture content (SMC), which increase the number of freeze-thaw cycles (FTCs) at mid-high latitudes and high elevations, especially at temperate regions. FTCs substantially affect the stability of soil aggregates, which may increase soil erosion. A comprehensive understanding of aggregate stability under different types of vegetation restoration in response to FTCs, however, has not yet been attained. We evaluated the effects of number of FTCs (0, 1, 3, and 9) and initial SMC (40, 60, and 80% field capacity) on aggregate distribution and stability in cropland, natural grassland, and Chinese pine forestland, the three typical types of vegetation on the Loess Plateau in China. The experiment was conducted under simulated conditions in the laboratory using disturbed soil samples. Most aggregate-size fractions and mean weight diameters (MWDs) were significantly (P < 0.05) affected by FTCs, initial SMC, and vegetation types. FTCs significantly decreased MWD by 3.6–18.1% through disrupting larger macro-aggregates, which increased with SMC. Increased SMC increased MWD by 2.0–53.0% through binding soil particles, the effect of which was much larger than the disruptive effects under the freeze-thaw conditions, especially at high SMC. MWD was in the order of cropland < Chinese pine forestland < natural grassland under each freeze-thaw condition. The results indicated that natural vegetation succession was better than Chinese pine forest plantation for resisting seasonal FTCs and that aggregate stability may increase due to increased initial SMC under scenarios of future global warming.

Spatial variations of aggregate-associated humic substance in heavy-textured soils along a climatic gradient

Humic substances (HS), as important soil components, are crucial to soil fertility and ecology. The composition of HS is interlinked within the formation of aggregates, and jointly influences soil biogeochemical processes. However, little detailed and quantitative information is available on the association of the HS composition with aggregate distribution for soils spanning a broad range of climatic gradients (e.g., temperature and precipitation). Herein, the spatial distribution of HS and its chemical composition (humic acids, HAs; fulvic acids, FAs; iron-linked humin, HMi; clay-combined humin, HMc; and insoluble humin residue, HMr) associating with different sizes of aggregates (>0.25, 0.25∼0.05, and <0.05 mm) from six typical zonal soils (two Luvisols, Alisols, Acrisols, Plinthosols, and Ferrosols) of a heavy texture (silty clay loam, silty clay, and clay) under a constant land use from temperate to tropical climate in central-southern China (34°48′N ∼ 19°52′N), were investigated. Results indicate that soil weathering degree (characterized by free iron oxides Fed) increased with the increase of annual temperature and precipitation along the latitude transect, which indirectly reflects the influence of climatic gradient. The spatial distribution of HS was significantly influenced by soil type, profile horizon, and its interaction (p < 0.001), while topsoil with higher proportion of macroaggregates (>0.25 mm) and higher concentration of HS was more significantly affected by climatic conditions than subsoil (F > 21, p < 0.001). Additionally, the composition of HS was dominated by insoluble humin residue (3.4 ∼ 14.38 g kg−1), which tended to decrease with the increase of weathering gradient except for Rhodi-Humic Ferrosols, whereas fulvic acids in the second rank (0.9 ∼ 5.96 g kg−1) demonstrated an opposite spatial trend along the latitude transect. With respect to the concentration of aggregate-associated HS, it was higher in macro- and micro- aggregates, especially for Luvilsols, and had a positive correlation to its concentration in bulk soils (r>0.52, p < 0.01) and the proportion of its corresponding aggregate fraction (p < 0.05). Moreover, aggregate-associated fulvic acids were positively correlated with sesquioxides, clay content, 14 nm intergrade mineral and kaolinite, but negatively correlated with soil pH, bulk density, silt content, and vermiculite (p < 0.05). However, these physicochemical properties had an opposite effect on the variation of humic acids in macro- and micro- aggregates, and made different contributions to humins up to the subdivided composition and the associated aggregate size fraction. These results would contribute to the prediction accuracy of HS storage across scales, especially when a wide size-range aggregation and various geochemical factors are considered.