Soil Aggregate Size Research Articles

The Adsorption and Aging Process of Cadmium and Chromium in Soil Micro-aggregates.

The particle size and components of soil aggregates have played important roles in the migration and transformation of heavy metals. The present study focused on the adsorption behavior and aging characteristics of cadmium (Cd) and chromium (Cr) in various granular aggregates and soil components, which were studied by analyzing the adsorption isotherm, adsorption kinetics, and heavy metal speciation distribution. The results showed that, compared with other aggregates, clay aggregates (0-0.002 mm) had the strongest adsorption effect on Cd and Cr and that there was no significant positive correlation between the adsorption amount and the particle size of aggregates for Cd and Cr. In general, the influence of three components on Cd was organic matter > amorphous iron > free iron oxide, and the influence on Cr was free iron oxide > amorphous iron > organic matter. The adsorption isotherm showed that the correlation coefficient of the Langmuir model (R2 ) was higher than that of the Freundlich model (R2 ), indicating that the adsorption of Cd and Cr by soil aggregates can be well described by the Langmuir model with monolayer adsorption behavior. Kinetic adsorption studies showed that quasi-first-order kinetics and quasi-second-order kinetics were more consistent with the actual adsorption amounts of Cd and Cr in soil aggregates, respectively. At the same time, the forms of Cd and Cr gradually transformed from unstable to stable after entering the soil. After 60 days, Cd was mainly oxidized, and residual, and Cr was mainly reduced and residual. These results provide a theoretical basis for assessing the environmental risks of Cd and Cr and providing prevention and treatment methods. Environ Toxicol Chem 2022;41:975-990. © 2022 SETAC.

Crop\u2013livestock integration enhanced soil aggregate-associated carbon and nitrogen, and phospholipid fatty acid

Integrated crop–livestock (ICL) production enhances diversification and provides ecosystem benefits by improving nutrient cycling and energy efficiency, thus, increasing overall farm productivity. However, a detailed study is needed to understand the influence of crop diversification and grazing animals on soil aggregation and associated carbon (C) and nitrogen (N), and microbial properties, especially compared with a grazed native pasture. We investigated the soil aggregate size distribution and associated C and N fractions, glomalin-related soil protein, and soil phospholipid fatty acid (PLFA) to understand the collective influence of livestock grazing of crop residue and cover crops (CC) and compared it with native pasture and non-grazed traditional production systems. The study was conducted in South Dakota at four different locations consisting of three long-term (> 30 years) on-farm sites: 1 (Salem), 2 (Bristol), 3 (Bristol) with three treatments that included ICL (corn, Zea Mays L.-soybean, Glycine max L.-oats, Avena sativa L.-CC with cattle grazing); natural ecosystem (NE) or native pasture; and control (CNT) (corn–soybean-without grazing or CC). Experimental site 4 (Beresford) with study duration of 3-year consisted of oats, oats with CC, oats with CC + grazing, and grazed pasture mix. Soil samples were collected from 0 to 5 cm depth at all four sites in summer 2019. Data showed that at sites 1 and 2, ICL had significantly (P ≤ 0.5) greater fractionation of 0.053–0.25 mm and > 4 mm aggregates compared with NE and CNT. At site 1, ICL showed significantly higher soil organic carbon (SOC, 36–49%) and higher nitrogen (33–44%) in > 4 mm aggregates than NE and CNT. At site 2, ICL had 32–41% higher SOC than NE and CNT for 0.25–0.5 mm aggregates. At site 1, NE enhanced total phospholipid fatty acid (PLFA), total bacterial biomass, gram (+), gram (−) bacteria than CNT, however, it did not vary significantly than ICL. Grazed pasture mix at site 4 had higher total PLFA (40.81 nmol g−1 soil) than the other treatments. The principal components 1 and 2 accounted for 33% and 22% of the variation, respectively, where the majority of the microbial compositions and aggregate-associated C and N were influenced by ICL and NE compared with corn–soybean without grazing or short-term oats/CC/grazing treatments. Integrated crop–livestock system and NE enhanced C and N concentrations in macroaggregates as well as in microaggregates. It is concluded that ICL and NE systems are sustainable prospects in enhancing overall soil health. Integrating crop and livestock improved physicochemical and microbial properties compared to the traditional corn–soybean system.

Mixture of N2-fixing tree species promotes organic phosphorus accumulation and transformation in topsoil aggregates in a degraded karst region of subtropical China

Many studies have shown that introducing N2-fixing tree species into plantations can increase soil nitrogen (N) availability, via biological N2 fixation and faster N cycling. However, the effects and regulatory mechanisms of N2-fixing tree species in planting single species vs a combination of two species on phosphorus (P) accumulation and transformation in degraded karst soils remain poorly understood and appear site-dependent. This study aimed to determine the effects of introducing N2-fixing tree species (via single species vs a combination of two species) into a degraded karst region on organic phosphorus (Po) accumulation and transformation in topsoil aggregates. A comparative experiment was performed involving 8-year-old pure plantations of Dalbergia odorifera (PD) and Acrocarpus fraxinifolius (PA), a mixed plantation of Dalbergia odorifera and Acrocarpus fraxinifolius (MP), and an adjacent natural weed field (no trees, CK) in Mashan, Guangxi, subtropical China. The results showed that the contents of soil organic carbon (SOC), nitrate nitrogen (NO3−-N), available phosphorus (AP) and C:N ratios in bulk soil and aggregates, increased significantly (P < 0.05) in MP compared to CK. The levels of soil microbial biomass N (MBN) and microbial biomass P (MBP) in bulk and aggregate soils increased significantly (P < 0.05), except for micro-aggregates (<0.25 mm) in MP samples. In contrast, no significant differences of MBN or MBP were found among PD, PA and CK plots in bulk and most aggregate soils. The phospholipid fatty acid (PLFA) contents of all microbes, bacteria, fungi, arbuscular mycorrhizal (AM) fungi and actinomycetes were also significantly higher (P < 0.05) in MP than those in CK and PD in bulk and most aggregate soils. Labile Po and highly resistant Po, and the activities of all tested N and P hydrolytic enzymes, improved in bulk soil and most aggregate size classes in all forest types, especially in MP samples. Finally, redundancy analysis (RDA) indicated that Po fractions were primarily driven by alkaline phosphatase (ALP), MBN and AM. Our findings suggest that introducing N2-fixing tree species (especially MP) may be an effective method for increasing N availability and AM colonization of roots, and thereby promoting Po accumulation and transformation in degraded karst regions of southwest China.

The Theoretical Approach to the Modelling of Gully Erosion in Cohesive Soil

The stochastic gully erosion model (STOGEM) is based on a combination of deterministic mechanics and a stochastic description of the erosion control factors. The main proposition in the model is that the depth of the active surface layer of eroded cohesive soil is equal to one particle diameter, and the deposition of eroded particles is negligible. The erosion rate at the gully bed is calculated directly from the equation of the balance between driving and resistance forces acting on soil particles in flowing water using the probability density functions of stochastic variables: flow velocity, soil aggregate size and cohesion. Probability density functions of cohesion in the model vary through time and space during the erosion event due to the changes in soil composition—armoring and loosening. This theory is still far from achieving practical application, but opens up a new way for better understanding the experimental results of gully erosion and shows the direction for future investigations.

Effects of corn stalk biochar and pyrolysis temperature on wheat seedlings growth and soil properties stressed by herbicide sulfentrazone

The residual toxicity of persistent herbicide sulfentrazone to rotation crops and non-target soil organisms represents a significant soil health-related issue worldwide. Therefore, it is necessary to explore effective methods to minimize the impacts of sulfentrazone on plant and soil systems. Considering the positive effects of biochar on soil quality, first, we prepared corn stalk biochars at 300, 500, and 700 °C. Then, we investigated the effects of these biochars on growth and physiology of wheat seedlings, composition of soil aggregate-size classes, and bacterial community structure at three application rates (0.1, 0.5, and 1.5% by mass) under sulfentrazone stress (0.3 mg/kg). Results showed that sulfentrazone inhibited growth and altered the physiological functions of wheat seedlings though it did not damage root cells. The soils amended with biochars (300 and 500 °C) at rates of 0.5% and 1.5% reduced the phytotoxicity of sulfentrazone. While 0.1% application rate of biochar prepared at 700 °C also showed similar results. The soils amended with biochars (700 °C) at rates of 0.5% and 1.5% reduced the phytotoxicity of sulfentrazone and improved the growth of wheat seedlings. Moreover, the soil-applied biochar improved soil macro-aggregation and bacterial community structure. In summary, lower application rate of biochar prepared under high-temperature processing is a better strategy to reduce the phytotoxicity of sulfentrazone and improve plant performance and soil health.

Microscale heterogeneity of soil bacterial communities under long-term fertilizations in fluvo-aquic soils

Differently sized soil aggregates, with non-uniform distribution of space and nutrients, provide spatially heterogeneous microenvironments for microorganisms and are important for controlling microbial community ecology and biogeochemistry in soils. Here, we investigated the prokaryotic communities within different aggregate-size fractions: macroaggregate (>0.25 mm), microaggregate (0.053–0.25 mm) and silt + clay (< 0.053 mm). These were isolated from fluvo-aquic soils under 39-year fertilization strategies: no fertilizer (CK), chemical fertilizer (NPK), manure fertilizer (M), and combination of manure and chemical fertilizers (MNPK). The results showed that the proportion of macroaggregate, soil aggregate-associated organic carbon (SOC) content and aggregate stability were all significantly increased by both manure and chemical fertilizations. Organic fertilizations (M and MNPK) more effectively boosted formation and stability of macroaggregates and enhanced SOC concentration than NPK. The distribution patterns of microorganisms in aggregates were primarily shaped by fertilization and aggregate size. They explained 76.9% of the variance in bacterial community compositions. Fertilizations, especially with organic fertilizers primarily transitioned bacterial communities from slow-growing oligotrophic groups (e.g., Chloroflexi) dominance to fast-growing copiotrophic groups (e.g., Proteobacteria and Bacteroidetes) dominance across all aggregate sizes. Macroaggregates possessed a more stable bacterial community and efficiency of resource transfer, while smaller aggregates increased antagonism and weakened mutualism among bacterial communities. Overall, combination of manure and chemical fertilizers was crucial for increasing SOC content and aggregation, leading to a clear shift in bacterial community structures at aggregate scale.

Soil microbial biomass and oxy-hydroxides contribute to aggregate stability and size distribution under different land uses in the Central Andes

Context Agricultural intensification leads to land use changes with potential consequences for soil aggregate stability and size distribution, affecting nutrient and water retention capacity, aeration, sequestration of soil organic carbon, and biogeochemical cycling. Aims This study evaluated soil aggregate stability and size distribution under potato, fallow and Eucalyptus globulus L. land uses in Cambisols of the eastern branch of the Central Andes, Bolivia. We also investigated the relation between aggregates and total C, extractable C, oxy-hydroxides, microbial biomass and activity. Methods Aggregate stability, size distribution and oxy-hydroxides were measured in soil samples from eight plots of each land use. Key results Compared to fields cultivated with potato (Solanum tuberosum L.), Eucalyptus increased aggregate stability, megaaggregate content, and C and N in the free silt + clay fraction. Fallow did not lead to significant changes in soil structure. Soil aggregate stability was related to both microbial biomass and oxy-hydroxides. Microbial biomass C, microbial activity and dithionite extractable Fe were positively related to megaaggregates and aggregate stability. Oxalate extractable Fe and Mn were related to microaggregates. Conclusions The plantation of Eucalyptus is suitable for soil structural amelioration and C sequestration, but its introduction to cultivated areas should be carefully evaluated due to its effects on soil chemistry and microbiology. Short-term fallowing did not contribute to the maintenance of soil structure. Implications In a context of land uses change, modifications of microbial biomass and activity would affect megaaggregate formation and stability. Alternative management practices are required to maintain soil structure and optimize sustainable land use of cultivated and fallow fields.

Comparative Effects of Different Tillage Practice and Crop Residue on Soil Organic Carbon, Bulk Density and Soil Aggregate Size Distribution

Comparative Effects of Different Tillage Practice and Crop Residue on Soil Organic Carbon, Bulk Density and Soil Aggregate Size Distribution

Developing an automated monitoring system for fast and accurate prediction of soil texture using an image-based deep learning network and machine vision system

To guarantee proper seedbed preparation, it is important to assess and control soil aggregate size in tillage operations. Doing so would lead to higher crop yield and more efficient resource use. This study proposes a portable smartphone-based machine vision system using convolutional neural network (CNN) for the classification of soil texture images taken from 20, 40 and 60 cm heights. The proposed CNN model consists of two blocks with several different layers. The first block (feature extraction) includes Conv, Max-pooling, drop out and batchnormalization layers. The second block (classifier) consists of fully connected layers, flatten and SVM classifier. Also in this study, ANN, SVM, RF and KNN algorithms were used to compare the proposed CNN results with otherclassifiers. The proposed CNN model was able to successfully predict soil images in distances of 20, 40 and 60 cm with the accuracies of 99.89, 99.81 and 99.58%, respectively. The results showed that the best performance was obtained when using fully preprocessed images at the height of 20 cm. Ultimately, a graphical user interface was designed in form of a user-friendly software to predict soil texture based on CNN model. The results revealed the proposed CNN method could quickly and accurately predict the type of soil texture on large scale farms and thus be a good alternative to the costly and time-consuming laboratory methods.

Pedotransfer Functions, Terrain Attributes and Digital Soil Mapping for Characterizing Soil Aggregate Size Distribution

Pedotransfer Functions, Terrain Attributes and Digital Soil Mapping for Characterizing Soil Aggregate Size Distribution

The microbial-driven C dynamics within soil aggregates in walnut orchards of different ages based on microbial biomarkers analysis

Soil aggregates and microbial attributes are of great importance to clarify soil C dynamics in agroecosystems. However, whether and how walnut plantation with different ages affects microbial-driven soil C dynamics, especially at the aggregate scales, remains unclear. In this study, the microbial community (as measured by phospholipid fatty acids analysis), exo-enzyme activities (micro-plate enzyme method), and microbial residues (amino sugar analysis) were determined in soil aggregates, which were collected from surface soils in walnut plantations of various ages (0-, 7-, 14-, and 21-year) in the areas of Lincheng county, China. According to the optimal-moisture sieving method, soil aggregates were divided into three parts, including > 2 mm, 2–0.25 mm, and < 0.25 mm aggregates. Results showed that long-term walnut cultivation (14- and 21-year) improved soil aggregate stability as well as its associated nutrient conditions, and these changes were mainly reflected in the larger (>2 mm and 2–0.25 mm) aggregates. Besides, long-term walnut cultivation was favorable to promoting microbial growth (as indicated by total PLFAs), functions (hydrolase activities), and metabolism (microbial residues C, MRC) within aggregates. Irrespective of the walnut plantation ages, the above-mentioned microbial attributes increased with increasing soil aggregate sizes, whereas oxidase activities decreased. We also found that the increased plantation ages and decreased aggregate sizes could enhance the microbial contributions to soil organic C (SOC) accumulation (as indicated by the MRC/SOC ratios), although higher SOC contents have been found in the larger aggregates. Notably, the partial least squares path model further emphasized that walnut plantation ages and aggregate sizes had strong effects on microbial attributes (e.g., microbial communities, exo-enzyme activities, and microbial residues), as well as their importance for soil C dynamics. Consequently, the mature walnut plantations were beneficial for improving soil structure, microbial attributes, and SOC sequestration in the East Taihang Mountains in Hebei, China.

Effects of long-term (70 years) nitrogen fertilization and liming on carbon storage in water-stable aggregates of a semi-arid grassland soil

Grasslands cover up to 40.5% of the world's landmass and store 30% terrestrial carbon (C). Various practices, including mineral fertilization and liming, are used to manage these ecosystems with potential long-term effects on the size and distribution of soil aggregates and inevitably carbon dynamics. The objective of this study was to examine the long-term effects of nitrogen fertilization and liming on soil carbon storage and its dynamics in water-stable aggregates of a semi-arid grassland. Soil samples (0–10 cm) were collected from Ukulinga long-term grassland trial in Pietermaritzburg, South Africa where nitrogen fertilizers have been applied annually and lime every five years for 70 years. Ten treatments were studied: the control (0 kgN/ha and unlimited), lime at 2250 kg/ha (L), ammonium sulphate at 70 kg/ha (AS70) and 211 kg/ha (AS211); ammonium nitrate at 70 kg/ha (AN70) and 211 kg/ha (AN211); AS70 + lime (AS70L); AS211 + lime (AS211L); AN70 + lime (AN70L) and AN211 + lime (AN211L).Nitrogen fertilizers significantly reduced soil pH and increased total soil N. Liming increased soil pH with no effect on total soil N. Lime and lime + N fertilizer treatments had no effect on mean weight diameter (MWD) while separate N application decreased MWD and large macro-aggregates (LMA). Lime only treatment had no effect on water stable aggregate (WSA) fractions. Nitrogen fertilization and liming (separately or in combination) did not affect total C concentration and stocks. Overall, soils had very high total soil organic carbon ranging from 49.7 – 57.6 g/kg across treatments. Nitrogen fertilization decreased organic carbon in LMA in AS70 (1.52%) and AN211 (1.67%) treatments compared to the control (3.40%) which was in concert with increases in C associated with small macro-aggregates (SMA) and micro-aggregates (MiA and SCA). Organic carbon in SMA was 2.67 % (AS70); AS211 (2.62 %); AN70 (2.02 %); AN211 (2.49 %) compared to 1.26 % in the control. Lime + N fertilizer treatments increased C storage in all aggregate fractions compared to N fertilizer only treatments. The lack of response in total SOC to 70 years of N fertilization and liming suggests possible C saturation given the high soil C concentration. Changes in C associated with WSA fractions suggests their importance as diagnostic indicators of N fertilization and liming induced changes in SOC. Findings also show that ammonium-based N fertilization is associated with soil acidification, dispersion of LMA resulting in an increase of microaggregates and C stored in them. Liming can counteracts acidifying and the dispersive effect on NH4+ associated with ammonium-based fertilizers thus restoring macro-aggregation in N fertilized grasslands. These findings suggests that long-term N addition may result in poor soil physical condition and possible stabilization of C in stable fractions.

The temporal and spatial changes in the structural and aggregate composition of typical chernozem with different anthropogenic stress

The paper aimed to establish features of formation and regularities of structural and aggregate content change in the cultivated layer of low-humus typical chernozem under constant application of various systems of fertilizing and the basic cultivation of soil in short-rotation 4-feld crop rotations with 75 % saturation with grain (winter wheat, barley or spring barley) and 25 % oilseeds (sunflower, spring rape). The research was carried out in a long stationary experiment of the Department of Agrosoil Science and Soil Microbiology of the NSC «Institute of Agriculture of NAAS» and the laboratory of agriculture of the Panfly Research Station of the National Scientifc Center «Institute of Agriculture of the National Academy of Agrarian Sciences of Ukraine». The research results reveal the efciency of different fertilizing systems (fertilizers with crop by-products) combined with tillage tools that differ in their intensity and principle of action. The fertilizing system affected the biomass formation and the tools effect involved layer-by-layer distribution of new crop biomass and formation of soil aggregates of certain shape and size, their share in the formation of structural properties formation which testifes the impact of various anthropogenic stress. The study defnes the direction of soil processes, features of structure formation for long-term implementation of different depth and methods of basic tillage with preservation and anti-erosion and intensive use of 0–30 cm tillage layer (plowing 25–27 cm – control) and long-term use of no–till technology for growing crops that vary on the principle of localization of the crops root system. Key words: structural and aggregate composition, typical chernozem, method of basic tillage, differentiation.

Isotope analysis reveals differential impacts of artificial and natural afforestation on soil organic carbon dynamics in abandoned farmland

A multitude of studies analyzed the dynamics of soil organic carbon (SOC), but their methodology does not provide sufficient understanding of the differential impact of artificial and natural afforestation on SOC dynamic. We investigated the SOC dynamics following artificial (AR) and natural (NQ) afforestation on abandoned farmland (AF) in China’s Loess Plateau in an attempt to evaluate the effects of these afforestation methods. We characterized soil structure and stoichiometry using stable isotope carbon and radiocarbon models. We aim to compare SOC dynamics, clarify SOC sources under different afforestation, examine comparability of the study areas and ascertain how soil aggregate size classes control SOC dynamics. The 0-10 cm and 10-20 cm SOC stocks were significant higher in NQ than AR and AF. At other depths, there is no significant difference among the three land-use systems. Total topsoil SOC stocks, C:N and C:P of differently sized soil aggregates significantly increased following afforestation. Carbon isotope analysis results indicated that the SOC decomposition rate and the new SOC input rate were lower under NQ than AR. Afforestation can lead to SOC accumulation in soil depths up to 1m mainly because the topsoil (0-20 cm) changes significantly. SOC resources are mainly from macroaggregate formation provided by fresh plant residues. The comparability of study sites is validated, so the“space-for-time substitution” method is applicable in this study.

Improved digital image-based assessment of soil aggregate size by applying convolutional neural networks

Assessing and control of soil aggregate size during tillage operations are important to guarantee a proper seedbed preparation and in consequence a high crop yield, as well as to achieve a high resource use efficiency. In this research, a convolutional neural network (CNN) called SoilNet is proposed to classify images of nine different soil aggregate sizes taken from three different heights of 60, 80 and 100 cm (the higher the height, the lower the images’ resolution). SoilNet combines a residual architecture with attention modules. Specifically, convolutional block attention modules are added after a series of convolution layers to highlight important local regions and extract more discriminative features. Moreover, features from multiple layers are combined by means of hypercolumns to increase predictive performance. In addition, fine-tuning of three pre-trained CNNs, VGG16, ResNet-50 and MobileNetV2 were applied. Based on the results, the performance of the pre-trained CNNs decreased with decreasing the image resolution increasing the photography height. The best mean overall classification accuracies (MCAs) obtained by the pre-trained CNNs were 96.97, 96.15 and 93.81%, for the photography heights of 60, 80 and 100 cm, respectively. However, the SoilNet showed a more satisfactory performance for classifying the images captured at greater heights with lower resolutions. The MCAs obtained by the network for the imaging heights of 60, 80 and 100 cm were 97.34, 97.06 and 97.13%, respectively. The SoilNet improved the overall classification accuracies obtained by applying the classical image processing on the same datasets in a previous study, up to 25.30, 29.18 and 30.46%, at imaging heights of 60, 80 and 100 cm, respectively.

Organic carbon distribution and soil aggregate stability in response to long-term phosphorus addition in different land-use types

Understanding the relationships among phosphorus (P), soil organic carbon (SOC) and aggregate stability is vital for improving soil fertility and P and C cycling in different land-use types. Here, we assessed the responses of SOC distribution in different aggregate fractions to P addition in two different land-use types categorized in either a P-limitation or non-P-limitation status based on the stoichiometric ratios of microbial enzymes. Four different long-term fertilization managements (no fertilizer [control, non-P limitation]; nitrogen and potassium [NK treatment, P limitation]; phosphorus [P treatment]; and nitrogen, phosphorus, and potassium [NPK treatment]) were established in upland and paddy soils to compare the effects of pre-assessed non-P- and P-limited soil conditions in combination with different fertilizer additions. A wet-sieving method was used to obtain four sizes (> 2-, 0.25–2-, 0.053–0.25- and < 0.053-mm) of soil aggregates. The concentrations of SOC and iron and aluminum (Fe/Al) oxides in the forms extractable by DCB (Fed/Ald), oxalate (Feo/Alo) and polyphosphate decahydrate (Fep/Alp) were measured. The results showed that compared to the non-P-limited control, the P treatment reduced soil mean weight diameter (MWD) by 13.1% in upland and 9.7% in paddy soils. The MWD of NPK treatment was significantly higher by 24% than that of the P-limited NK treatment in upland soil, while the difference between the same two fertilization treatments was not significant in paddy soil. Compared with that of the control, SOC concentration in each size was greater in the P-treated upland soil, while in the paddy soil, SOC concentration of the >2-mm size were lower in the P and NPK treatments than in the control and NK treatments, respectively. Phosphorus addition decreased Fep/Alp of each aggregate size in upland soil, while it increased Fep/Alp of each aggregate size in paddy soil. To better quantify the effects of P on SOC and aggregate stability, we used a partial least squares path model (PLS-PM). The results indicated that P addition had a direct negative effect on MWD (−0.797, P < 0.01) under the non-P limitation condition (control vs. P treatment). The addition of P in upland soil had a high positive effect on MWD (0.565, P < 0.05) under the P-limitation condition (NK vs. NPK treatments), but the effect was not significant in paddy soil (0.047, P > 0.05). Thus, strategies aiming to improve soil carbon cycling and aggregate stability by regulating phosphorus addition should consider land-use type and soil P-limitation status.

Sediment transport mechanisms and selective removal of soil particles under unsteady-state conditions in a sheet erosion system

Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies. Most agricultural soils are located on gentle slopes where fertility is high; however, until now, the main attention on sediment transport mechanisms was paid to high-slope gradients, where soil erosion is intense, but soils are less productive. Despite the importance of sediment size distribution (SSD) and transport mechanisms under unsteady-state conditions, few studies have been done on this issue. Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions. To address this issue, sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions. Six inflow rates (75, 100, 125, 150, 175, and 200 mL/s) were applied on two contrasting soils, including an adjacent agricultural soil and a not cultivated soil (namely Cropland soil and Control soil, respectively) at two low slope gradients (1.5% and 2%). No rill formation was observed during the experiments. The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions. The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size. The contribution of suspension-saltation (SS) to the total sediment load for the Cropland and Control soils varied from 22% to 68% and from 35% to 59%, respectively, while up to 78% and 65% of soil particles were transported by bed load (BL), respectively. However, SS and BL indicated a reverse trend with stream power. The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles (0.042 mm), whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine (≤ 0.084 mm) and coarse (1.5 mm) particles. The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers. The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils, where small stream powers can remove fine and fertile soil particles.

Effect of site hydrological conditions and soil aggregate sizes on the stabilization of heavy metals (Cu, Ni, Pb, Zn) by biochar

Biochar is a popular material that would effectively immobilize heavy metals in soil, which can greatly decrease the health risk of heavy metals. Although many previous studies have studied the immobilization of heavy metals by biochar, the influence of hydrological conditions on the immobilization effect is still not clear. This paper carried out column experiments to study the effect of fluctuating groundwater table on Cu, Ni, Pb, Zn distribution and speciation with the addition of biochar from pyrolysis of swine manure. Experimental results showed that biochar could significantly decrease the leaching toxicity of Cu and Ni by 24.4% and 44.7% respectively, while the immobilization effect of Pb and Zn was relatively insignificant. The average reduction percentage of bioavailable Cu was 14.5%, 39.5% and 33.3% in the unsaturated zone, fluctuating zone and saturated zone respectively, showing the better immobilization effect in the fluctuating zone and saturated zone. The residual fraction of heavy metals increased significantly after the addition of biochar, and the increase of residual fraction was larger in small soil aggregates. This study helped illustrate the influence of hydrological conditions and soil aggregate sizes on the stabilization effect of heavy metals by biochar, which could be used to guide the remediation process of sites contaminated by heavy metals.

Changes of small strain shear modulus and microstructure for a lime-treated silt subjected to wetting-drying cycles

Lime treatment can enhance the workability and hydro-mechanical properties of soil through different physical-chemical reactions. Nevertheless, the beneficiary effect of lime treatment can be altered when the soil was exposed to wetting-drying cycles, depending on the wetting fluid and soil state. In the present study, the changes in small strain shear modulus (Gmax) and microstructure of a compacted lime-treated silt under wetting-drying cycles were studied. The untreated state of soil was also considered for comparison. Meanwhile, the effects of wetting fluid (deionized water and synthetic seawater) and maximum soil aggregate size (Dmax = 0.4 mm for S0.4 and 5 mm for S5) were investigated. Results showed that Gmax increased significantly for the lime-treated soil over curing, and increased slightly for the untreated soil. The untreated specimens were softened and damaged by wetting-drying cycles, while the lime-treated specimens exhibited good resistance with the pore size distributions almost kept reversible along the wetting-drying paths. The subsequent more intensive drying resulted in a significant fabric alteration with occurrence of shrinkage-related fissures of the clay part. However, these fissures were almost healed with rewetting. Thereby, the Gmax of lime-treated soil showed a constant decreasing trend with wetting and an increasing trend with drying. Moreover, Gmax decreased slightly with wetting-drying cycles for the lime-treated specimens wetted by deionized water, suggesting that the wetting-drying indeed softened the soil. However, Gmax increased for the lime-treated specimens wetted by synthetic seawater, due to the more production of cementitious compounds promoted by salts. The lime-treated specimens S0.4 wetted by synthetic seawater had higher Gmax than those wetted by deionized water, while the wetting fluid had insignificant effect on the Gmax of specimens S5 due to the limited promotion of pozzolanic reaction and negligible soil aggregation induced by salts.

Impact of Cover Crop Monocultures and Mixtures on Organic Carbon Contents of Soil Aggregates

Cover crops are considered an integral component of agroecosystems because of their positive impacts on biotic and abiotic indicators of soil health. At present, we know little about the impact of cover crop types and diversity on the organic carbon (OC) contents of different soil aggregate-size classes. In this study, we investigated the effect of cover plant diversity on OC contents of different soil aggregates, such as macro- (&lt;2000–500 μm), meso- (&lt;500–250 μm), and micro-aggregates (&lt;250 μm). Our experiment included a total of 12 experimental treatments in triplicate; six different monoculture treatments such as chickling vetch (Vicia villosa), crimson clover (Trifolium incarnatum), hairy vetch (Vicia villosa), field peas (Pisum sativum), oilseed radish (Raphanus sativus), and mighty mustard (Brassica juncea), and their three- and six-species mixture treatments, including one unplanted control treatment. We performed this experiment usingdeep pots that contained soil collected from a corn-soybean rotation field. At vegetative maturity of cover plants (about 70 days), we took soil samples, and the soil aggregate-size classes were separated by the dry sieving. We hypothesized that cover crop type and diversity will improve OC contents of different soil aggregate-size classes. We found that cover plant species richness weakly positively increased OC contents of soil macro-aggregates (p = 0.056), whereas other aggregate-size classes did not respond to cover crop diversity gradient. Similarly, the OC contents of macroaggregates varied significantly (p = 0.013) under cover crop treatments, though neither monoculture nor mixture treatments showed significantly higher OC contents than the control treatment in this short-term experiment. Interestingly, the inclusion of hairy vetch and oilseed radish increased and decreased the OC contents of macro- and micro-aggregates, respectively. Moreover, we found a positive correlation between shoot biomass and OC contents of macroaggregates. Overall, our results suggest that species-rich rather than -poor communities may improve OC contents of soil macroaggregates, which constitute a major portion of soil systems, and are also considered as important indicators of soil functions.