Quaternary Red Clay Research Articles

Phases and rates of iron and magnetism changes during paddy soil development on calcareous marine sediment and acid Quaternary red-clay

Dynamic changes in Fe oxides and magnetic properties during natural pedogenesis are well documented, but variations and controls of Fe and magnetism changes during anthropedogenesis of paddy soils strongly affected by human activities remain poorly understood. We investigated temporal changes in different Fe pools and magnetic parameters in soil profiles from two contrasting paddy soil chronosequences developed on calcareous marine sediment and acid Quaternary red clay in Southern China to understand the directions, phases and rates of Fe and magnetism evolution in Anthrosols. Results showed that paddy soil evolution under the influence of artificial submergence and drainage caused changes in soil moisture regimes and redox conditions with both time and depth that controlled Fe transport and redistribution, leading to increasing profile differentiation of Fe oxides, rapid decrease of magnetic parameters, and formation of diagnostic horizons and features, irrespective of the different parent materials. However, the initial parent material characteristics (pH, Fe content and composition, weathering degree and landscape positions) exerted a strong influence on the rates and trajectories of Fe oxides evolution as well as the phases and rates of magnetism changes. This influence diminished with time as prolonged rice cultivation drove paddy soil evolving to common pedogenic features.

RUSLE erodibility of heavy‐textured soils as affected by soil type, erosional degradation, and rainfall intensity: A field simulation

AbstractSoil erosion is a serious problem world‐wide for the environment or humankind, exacerbating land degradation. However, little systematic knowledge is available about the erodibility of various types of soils at different erosional degradation levels, especially at the field scale. In this research, the spatiotemporal variations of the erodibility for 5 soil types (calcic luvisol, ferric luvisol, plinthic alisol, plinthic acrisol, and acric ferralsol) from temperate to tropical climate were investigated with field rainfall simulation experiments on prewetted bare fallow heavy‐textured soils (silty clay loam, silty clay, and clay) derived separately from loess deposits, quaternary red clays, and basalt. The experiments were performed in 3 erosion classes (noneroded, moderately eroded, and very severely eroded) and at 2 rainfall intensities (45 and 90 mm hr−1). Soil erodibility was represented by the K factor of the Revised Universal Soil Loss Equation. Soil erodibility was the lowest for the ferralsols among all the soil types and was significantly lower in the very severe than in the no or moderate erosion classes. Soil erodibility tended to be larger at the high than at the low rainfall intensity (p < .05) except for the very severely eroded luvisols, and their difference between rainfall intensities in the erodibility varied with rainfall duration. Soil erodibility could be well predicted by the combination of illite, dry aggregate stability, and amorphous aluminium oxides (adjusted R2 = .51, p < .001), and its temporal variations were significantly related with particle density, 1.4‐nm intergrade mineral, and capillary porosity (p < .05). Furthermore, soil properties selected to account for soil erodibility varied with rainfall intensity. The integrated data indicated that soil erodibility was mainly influenced by clay minerals at the region scale and soil degradation degree at the pedon scale.

Effects of soil type and rainfall intensity on sheet erosion processes and sediment characteristics along the climatic gradient in central-south China

Soil erosion poses a major threat to the sustainability of natural ecosystems. The main objective of this study was to investigate the effects of soil type and rainfall intensity on sheet erosion processes (hydrological, erosional processes and sediment characteristics) from temperate to tropical climate. Field plot experiments were conducted under pre-wetted bare fallow condition for five soil types (two Luvisols, an Alisol, an Acrisol and a Ferralsol) with heavy textures (silty clay loam, silty clay and clay) derived separately from loess deposits, quaternary red clays and basalt in central-south China. Rainfall simulations were performed at two rainfall intensities (45 and 90mmh−1) and lasted one hour after runoff generation. Runoff coefficient, sediment concentration, sediment yield rate and sediment effective size distribution were determined at 3-min intervals. Runoff temporal variations were similar at the high rainfall intensity, but exhibited a remarkable difference at the low rainfall intensity among soil types except for tropical Ferralsol. Illite was positively correlated with runoff coefficient (p<0.05). Rainfall intensity significantly contributed to the erosional process (p<0.001). Sediment concentration and yield rate were the smallest for the tropical Ferralsol and sediment concentration was the largest for the temperate Luvisol. The regimes (transport and detachment) limiting erosion varied under the interaction of rainfall characteristics (intensity and duration) and soil types, with amorphous iron oxides and bulk density jointly enhancing soil resistance to erosive forces (Adj-R2>88%, p<0.001). Sediment size was dominated by <0.1mm size fraction for the Luvisols and bimodally distributed with the peaks at <0.1mm and 1–0.5mm size for the other soil types. Exchangeable sodium decreased sediment size while rainfall intensity and clay content increased it (Adj-R2=96%, p<0.01). These results allow to better understand the climate effect on erosion processes at the spatial-temporal scale from the perspective of soil properties.

Development of a soil quality index for Camellia oleifera forestland yield under three different parent materials in Southern China

Camellia oleifera is used to produce tea oil, which is also known as “oriental olive oil”. Camellia oleifera forestland in southern China displays low yield per unit area, which seriously hinders the production of tea oil. This decreased production is primarily a result of improper planting site and lack of scientific guidance. However, during soil quality assessment, selection of soil indicators from a single soil physical and chemical attribute or specific site resulted in little practical guidance. To assess soil quality scientifically, a total of 142 sampling sites in southern China under three types of parent materials, killas, sandstone, quaternary red clay, were investigated for 23 indicators representing soil physical, chemical and biological properties. Data were analyzed by factor analysis, discriminant analysis and regression analysis. Killas had the highest total nitrogen (TN), available potassium (AK), available magnesium (AMg), available manganese (AMn), available copper (ACu), available zinc (AZn), urease and sucrase, while quaternary red clay had the highest bacteria, actinomycetes and fungi concentrations. Seven indicators, bulk density (BD), sand content, soil organic matter (SOM), available phosphorus (AP), AK, available iron (AFe), and acid phosphatase (ACP) were combined in a minimum data set (MDS) for assessing soil quality under different parent materials. The soil quality index (SQI) of soils that originated from killas, sandstone, quaternary red clay were 0.71, 0.62, 0.54, respectively, which were significantly correlated with the SQI based on all 23 indicators (R2=0.733) and oil yield (R2=0.694). Soils originating from killas were considered most suitable for planting of C. oleifera since they had the highest soil quality among those investigated herein. Measures should be taken to improve soil fertility of soils originating from quaternary red clay, which showed nutrient deficiency and had the lowest SQI.

A modified depositional hypothesis of the Hanjiang Loess in the southern Qinling Mountains, central China

Geomorphologically, alluvial deposits in river systems are expected to be older on higher terraces than on the lower terraces. However, loess deposits of aeolian origin may also occur on the surface of terrace systems and as seen in the Chinese Loess Plateau (CLP). Such loess is continuously, rather than episodically deposited. This study presents numerous dating results of loess cover on the Hanjiang River terrace system in the southern Qinling Mountains, an atypical loess deposit belt outside of the CLP. We name this “Hanjiang Loess”. Results indicate that the Hanjiang Loess deposited on the high, middle, and low terraces are also the oldest, old, and most recent, respectively, apparently analogous with alluvial expectations. We thus propose a modified depositional hypothesis, whereby terrace loess reflects the deposition of aeolian loess and other material, subsequent fluvial reworking. This depositional hypothesis should also be applicable to loess deposits on the river terraces in the Xiashu Loess, Quaternary Reticulate Red Clay, and Quaternary Red Clay in southern China, and other atypical loess areas outside of the CLP.

Effects of erosion degree and rainfall intensity on erosion processes for Ultisols derived from quaternary red clay

Abstract Soil erosion poses a major threat to the sustainability of soil and water resources. Soil horizons differ in their properties due to pedogenic differentiation. Currently, limited research attempts have been made to assess the impact of the removal of topsoil horizons on the subsequent erosion process. The main objective in this study was to investigate the effects of erosion degree and rainfall intensity on erosion process and sediment transport mechanism. Field plot experiments were conducted on pre-wetted bare fallow Ultisols (derived from quaternary red clay) under four erosion degrees (no (E0), moderate (E1), severe (E2), and very severe (E3)) and two rainfall intensities (60 and 120 mm h−1). The erosion degrees were judged according to the outcrop of eluvium, illuvium (B1, B2) and parent material horizons. The simulated rainfall lasted one hour after runoff generation, and runoff and sediment were sampled at 3-min intervals to determine the runoff coefficient, sediment concentration, soil detachment rate and sediment effective size distribution. Runoff coefficient was negatively correlated with bulk density (r = −0.76, p E0 > E2 > E3 at the high rainfall intensity. Rainfall intensity and clay content played the negative and positive roles in the erosion process, respectively (Adj-R2 > 0.80, p

Effect of hydrochloric acid on grain size analysis of the Quaternary red clay rich in tiny ferromanganese nodules, Jiangxi province, southern China

There are many tiny ferromanganese nodules that cannot be found and eliminated easily in Quaternary red clay in southern China. The existence of these tiny ferromanganese nodules may influence grain size analysis results. Therefore, this paper focuses on the effects of the ferromanganese nodules on the grain size analysis and the effects of hydrochloric acid (HCl) on removing these nodules. The experimental results showed that the nodules clearly influenced the results of grain size analysis. The tiny ferromanganese nodules increased the coarse fraction and show a low peak in the coarse fraction of >100 μm on the frequency curves. The addition of HCl in the pretreatment process of grain size analysis can eliminate the effects of these tiny nodules effectively. Additionally, the HCl reacts with clay minerals and causes a reduction of the <4 μm fraction.

Correlations Between Different Extractable Cadmium Levels in Typical Soils and Cadmium Accumulation in Rice

Pot experiments were used to study the correlations between different extractable cadmium levels in typical soil and cadmium accumulation in rice. To analyze the pH, Cd in soil solution(SSE-Cd), TCLP extractable Cd level(TCLP-Cd), and Cd accumulation in rice at different growth stages of rice, we conducted pot experiments which selected the reddish clayey soil(developed from quaternary red clay parent materials) and purple paddy field(developed from purple sandy shale parent materials), meanwhile added with exogenous Cd with the gradients of 0, 0.5, 1, 2, 5, 10 mg·kg-1. The results showed that, during the rice growth period, the content of SSE-Cd in reddish clayey soil was in the range of 0 and 2.5 μg·L-1, and the average content was 0.57 μg·L-1; TCLP-Cd was in the range of 0 and 0.25 μg·L-1 with the average content of 0.10 mg·kg-1;The content of SSE-Cd in purple paddy field was in the range of 0 and 1.6 μg·L-1 with the average content of 0.48 μg·L-1; TCLP-Cd was in the range of 0 and 0.2 mg·kg-1, and the average content was 0.07 mg·kg-1. It showed that the cadmium concentrations in soil solution and the TCLP extractable Cd levels were both significantly reduced in two types of soil with the extension of rice growth period, and the content in reddish clayey soil was higher than that in purple paddy. The TCLP extractable Cd level was significantly positively correlated with Cd concentration in soil solution. The total Cd accumulation in rice plants gradually increased with increasing exogenous Cd concentration. There were significant positive correlations between Cd concentration in soil solution and Cd concentration in rice, Cd concentration in soil extracted by TCLP method and Cd concentration in rice and total Cd accumulation in rice plant. The soil environmental capacities of the two different parent materials varied greatly,and the safety threshold of Cd in purple paddy field was 2.06 times of that of reddish clayey soil. There were significant differences in Cd uptake and accumulation in different soils, so different measures may be needed to control Cd pollution in different parent materials. TCLP extractable Cd was more relevant with total Cd accumulation in rice, and had more extraction amount. Therefore, the TCLP method can more accurately evaluate the biological availability of soil Cd.

Developing equations to explore relationships between aggregate stability and erodibility in Ultisols of subtropical China

A soil aggregate represents a key soil structural unit that influences several physical soil properties such as water infiltration, runoff and erosion. The relationships between soil aggregate stability and interrill and rill erodibility are critical to process-based erosion prediction models yet remain unclear, likely due to the difficulty of distinguishing between interrill and rill-eroded sediment during the erosion process. This study was designed to partition interrill and rill erosion rates and relate them to the aggregate stability of Ultisols in subtropical China. Six kinds of rare earth elements (REEs) were applied as tracers mixed with two cultivated soils developed over Quaternary red clay or shale at six slope positions. Soil aggregate stability was determined by the Le Bissonnais (LB)-method. Simulated rainfall of three intensities (60, 90 and 120mmh−1) was applied to a soil plot (2.25m long, 0.5m wide, 0.2m deep) at three slope gradients (10°, 20° and 30°) for a duration of 30min after runoff initiation. The results indicated that rill and interrill erosion rates in the soil developed over shale were considerably greater than those in the soil developed over Quaternary red clay. Equations using an aggregate stability index As to replace the erodibility factor of interrill and rill erosion in the Water Erosion Prediction Project (WEPP) model were constructed after analysing the relationships between estimated and measured rill and interrill erosion data. The results show that these equations based on As have the potential to improve methods for assessing interrill and rill erosion erodibility synchronously for subtropical Ultisols by using an REE tracing method.

Sedimentary environment of vermicular red clay in South China

Increasing interest in recent years has focused on vermicular red clay (VRC) in southern China due to its controversial sedimentary environment and provenance. Grain size is a useful way to determine sedimentary environment and provenance. Fisher Linear Discriminant Analysis (LDA) is a common and widely used method for multivariate statistical analysis. Based on a proper training sample set, the LDA can be used to discuss the sediment provenance. In this study, grain size data for 77 Malan loess samples and 41 floodplain deposit samples were used as a training sample set to deduce a Fisher linear discriminant function. Then, 299 VRC samples from 6 Quaternary red clay profiles were analyzed using the discriminant function. Grain size parameters and microscopic images of quartz grains separated from the VRC were evaluated in detail to determine the VRC sedimentary environment in south China. The results show that VRC profiles can be classified into two regions: the Chiang-nan Hilly Region and Wuyi Mountains Region. The VRC samples in the Chiang-nan Hilly Region originated from eolian dust deposits. This VRC is characterized by a higher content of fine particles (<20 μm) and lower average transport kinetic energy than loess in a C-M plot. The quartz grain sizes and microscope images of this VRC suggest that it could be a polyphyletic mixture of far-sourced and near-sourced eolian deposits. The far-sourced eolian deposits share similar provenance with Xiashu loess and were transported by the East Asian winter monsoon. The near-sourced eolian deposits were dust emitted from the adjacent floodplain. In the Wuyi Mountains Region, the rugged topography weakened the dustfall and strengthened the reconstructive effect of hydrodynamic forces during the Quaternary glacial periods. The VRC in this region was reworked strongly by water and retained typical hydraulic characteristics no matter the source.

亚热带几种典型稻田与旱作土壤中外源输入秸秆的分解与转化差异

PDF HTML阅读 XML下载 导出引用 引用提醒 亚热带几种典型稻田与旱作土壤中外源输入秸秆的分解与转化差异 DOI: 10.5846/stxb201607211481 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目（41371252，41430860，41671242）；国家重点研发计划项目（2016YFD0300902）。 Decomposition and transformation of input straw in several typical paddy and upland soils in subtropical China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:选取亚热带四种典型母质（花岗岩风化物、第四纪红色粘土、板页岩风化物、近代河流沉积物）发育的稻田土壤，以毗邻的旱作土壤为对比，通过室内模拟培养试验研究45%田间持水量（WHC）条件下稻田和旱作土壤中外源输入秸秆矿化和转化的特征与差异。结果表明：在180 d的培养期内，所选4种稻田土壤中外源输入秸秆的累积矿化率（18%-21%）均显著低于对应的旱作土壤（21%-28%），外源秸秆的输入对土壤原有有机碳矿化的激发效应也是以稻田土壤（5%-30%）明显低于对应的旱作土壤（17%-65%）。外源秸秆在土壤中的分解产物主要向颗粒有机碳（POC）和铁铝结合态有机碳（Fe/Al-OC）分配，分配比例分别为9%-21%和12%-24%，其次为腐殖质碳（HMC）（11%-15%），而向微生物生物量碳（MBC）和溶解性有机碳（DOC）分配的比例极小，分别仅为2%-7%和0.1%-0.7%。与旱作土壤相比，稻田土壤中外源秸秆的分解产物向POC、Fe/Al-OC和MBC分配的比例较高，分别为15%-21%、17%-24%和6%-7%，而旱作土壤为9%-17%、13%-18%和2%-4%。此外，外源秸秆分解产物向2000-250 μm水稳性粗团聚体分配的比例也以稻田土壤（10%-13%）高于旱作土壤（6%-7%），其它粒径中稻田与对应的旱作土壤之间并无显著差异。本研究结果说明，稻田土壤中外源输入秸秆的矿化率低于旱作土壤的现象在不同母质类型的土壤中可能普遍存在，这可能与稻田土壤中外源秸秆分解产物受水稳性团聚体的物理保护、与氧化铁铝的化学键合以及向有机碳稳定组分的分配作用较强有关，从而贡献于稻田土壤较高的有机碳积累。 Abstract:Decomposition and transformation of input straw in four types of paddy soils were investigated under incubation at 25℃ and 45% water holding capacity (WHC) for 180 d. The soils were derived from different parent materials (weathered granite, quaternary red clay, weathered shale, and river alluvial) in subtropical China, and the adjacent upland soils were selected as a control. During the 180 d period, the mineralization ratios of input straw in the selected paddy soils (18%-21%) were lower than those in the corresponding upland soils (21%-28%). The priming effects of straw amendment on native soil organic carbon mineralization were also lower in the paddy (5%-37%) than in the corresponding upland soils (23%-65%). The decomposed products of input straw were mainly distributed in particulate organic carbon (POC, 9%-21%) and Fe/Al-bound organic carbon (Fe/Al-OC, 12%-24%), followed by humus carbon (HMC) (11%-15%), whereas only a small part was distributed as microbial biomass carbon (MBC, 2%-7%) and dissolved organic carbon (DOC, 0.1%-0.7%). In paddy soils, the conversion ratios of input straw in POC, Fe/Al-OC, and MBC (15%-21%, 17%-24%, and 6%-7%) were higher than those in upland soils (9%-17%, 13%-18%, and 2%-4%). In addition, the 2,000-250 μm coarse water-stable aggregates in paddy soils tended to receive more decomposed products of input straw than those in upland soils (10%-13% vs. 6%-7%), whereas no significant difference was observed between paddy and upland soils in other small sizes of aggregates. The results indicated that the mineralization of input straw may be lower in paddy than in upland soils derived from different parent materials, possibly owing to stronger physical protection in soil coarse aggregates, chemical protection by binding with Fe/Al oxyhydrates, and larger transformation to stable fractions of input straw during its decomposition in paddy soils. This fate of input straw decomposition may contribute to a higher organic carbon accumulation in paddy than in upland soils. 参考文献 相似文献 引证文献

Effect of water content, bulk density, and aggregate size on mechanical characteristics of Aquults soil blocks and aggregates from subtropical China

Soil aggregate mechanical characteristics can significantly affect soil strength and are important soil properties to predict soil erodibility. However, in most research, the aggregate mechanical strength is always measured under air-dried condition, and limited information is available about the mechanical strength of aggregates and soil blocks with different water contents. This study evaluated the effects of water content, bulk density, and aggregate size on mechanical properties of soil blocks and aggregates. Shear strength (τ) parameters (φ and c) of soil blocks in different states (undisturbed and remoulded) and tensile strength (TS) of aggregates were determined in the laboratory on two soils derived from Quaternary red clay (Q) and shale (S) with variations in water content, bulk density, and aggregate size. The results indicated that the φ values were higher in drier and denser soil and showed no obvious variation with varying aggregate size. The c values increased first and then decreased with increasing water content and decreasing aggregate size and increased with increasing bulk density. The water content corresponding to the rapid decrease of the c value appeared to be related to soil properties. Tensile strength increased with decreasing water content in all sizes of aggregates. It decreased with increasing aggregate size at a relative low water content (3.2–7.3 %), but increased with increasing aggregate size at a relative high water content (10.6–14.8 %). The effect of soil moisture on soil strength varied with soil states. Thus, water content, bulk density, and aggregate size have significant effects on the mechanical properties of the soil blocks and aggregates. The result from this research may contribute to a better understanding of the soil erosion resistance of Aquults from the perspective of soil mechanics.

Aging process of arsenite [As(3)]in soils originated from different parent materials.

An incubation test was conducted to study the bioavailability and fractionations of exogenous arsenite [As(3)] during the aging period in three soils originated from different parent materials, including quaternary red clay, purple sandy shale and granite. The results indicated that the exogenous arsenite As(3) were totally transformed into As(V) after 120 d aging. The available As content in soils derived from the three soils generally decreased sequentially throughout the aging period in the order of purple sandy shale, granite and quaternary red clay. The pseudo-second-order model well fitted the change of available As content with aging time (P＜0.05). Soil pH, organic matter (SOM) content and the concentrations of Fe, Al and Mn oxides were the main factors influencing the red soil arsenic in aging, especially, Mn oxides played a more crucial role than Fe and Al oxides in As aging (P＜0.05). Correlation analysis revealed that the non-specially and specially absorbed As constituted the primary forms of available As.

Effects of Subsetting by Parent Materials on Prediction of Soil Organic Matter Content in a Hilly Area Using Vis-NIR Spectroscopy.

Assessment and monitoring of soil organic matter (SOM) quality are important for understanding SOM dynamics and developing management practices that will enhance and maintain the productivity of agricultural soils. Visible and near-infrared (Vis–NIR) diffuse reflectance spectroscopy (350–2500 nm) has received increasing attention over the recent decades as a promising technique for SOM analysis. While heterogeneity of sample sets is one critical factor that complicates the prediction of soil properties from Vis–NIR spectra, a spectral library representing the local soil diversity needs to be constructed. The study area, covering a surface of 927 km2 and located in Yujiang County of Jiangsu Province, is characterized by a hilly area with different soil parent materials (e.g., red sandstone, shale, Quaternary red clay, and river alluvium). In total, 232 topsoil (0–20 cm) samples were collected for SOM analysis and scanned with a Vis–NIR spectrometer in the laboratory. Reflectance data were related to surface SOM content by means of a partial least square regression (PLSR) method and several data pre-processing techniques, such as first and second derivatives with a smoothing filter. The performance of the PLSR model was tested under different combinations of calibration/validation sets (global and local calibrations stratified according to parent materials). The results showed that the models based on the global calibrations can only make approximate predictions for SOM content (RMSE (root mean squared error) = 4.23–4.69 g kg−1; R2 (coefficient of determination) = 0.80–0.84; RPD (ratio of standard deviation to RMSE) = 2.19–2.44; RPIQ (ratio of performance to inter-quartile distance) = 2.88–3.08). Under the local calibrations, the individual PLSR models for each parent material improved SOM predictions (RMSE = 2.55–3.49 g kg−1; R2 = 0.87–0.93; RPD = 2.67–3.12; RPIQ = 3.15–4.02). Among the four different parent materials, the largest R2 and the smallest RMSE were observed for the shale soils, which had the lowest coefficient of variation (CV) values for clay (18.95%), free iron oxides (15.93%), and pH (1.04%). This demonstrates the importance of a practical subsetting strategy for the continued improvement of SOM prediction with Vis–NIR spectroscopy.

Estimating the influence of related soil properties on macro- and micro-aggregate stability in ultisols of south-central China

BackgroundStable macro- or micro-aggregates are important for preventing soil degradation. The interactions among soil aggregates and stabilizing agents—like clay, soil organic matter (SOM), Fe, and Al oxides—are complex and have not been fully understood. MethodsEight ultisol samples were collected from the surface (0–10cm) and subsurface layers (10–20cm). The macro-aggregate stability was determined by wet sieving, and the micro-aggregate distribution was determined via particle size distribution analysis; however, no chemical dispersant (sodium hydroxide) was applied. Using the PLSR models, the main soil properties that affect macro-aggregate and micro-aggregate stability were estimated. ResultsAll soils were strongly acidic (pH4.28–5.56) with low SOM content (<20gkg−1). The dithionite-citrate-bicarbonate extractable Fed and Ald were the dominant forms in Fe and Al oxides, much greater than acid ammonium oxalate extractable Feo and Alo. For most soils, the percentage of >5mm aggregates was the highest, and the percentage of 2–1mm aggregates was the lowest after wet sieving. Soil parent materials had a significant effect on the particle size distribution of the micro-aggregates. The stability of macro-aggregates and micro-aggregates from Quaternary red clay was stronger than that from Shale (p<0.05). Regardless of the soil parent materials, the water stability of surface cropland soil macro-aggregates was significantly lower than that of the other land-use types, but the micro-aggregate stability exhibited no trend across different land use types. ConclusionAld was the most important binding agent of the macro-aggregates, and clay was the main binding agent of the micro-aggregates, followed by the Fed, Alo, CEC and SOM, while Feo was the weakest agent.

Effect of aging on the bioavailability and fractionation of arsenic in soils derived from five parent materials in a red soil region of Southern China

The effects of aging time and soil parent materials on the bioavailability and fractionations of arsenic (As) in five red soils were studied. The results indicated that As bioavailability in all soils decreased during aging, especially with a sharp decline occurring in the first 30 days. After aging for 360 days, the highest available As concentration, which accounted for 12.3% of the total, was observed in soils derived from purple sandy shale. While 2.67% was the lowest proportion of the available As in soils derived from quaternary red clay. Furthermore, the best fit of the available As changing with aging time was obtained using the pseudo-second-order model (R2 = 0.939–0.998, P < 0.05). Notably, Al oxides played a more crucial role (R2 = 0.89, P＜0.05) than did Fe oxides in controlling the rate of As aging. The non-specially and specially absorbed As constituted the primary forms of available As.

Evaluation of soil aggregate microstructure and stability under wetting and drying cycles in two Ultisols using synchrotron-based X-ray micro-computed tomography

Soil structure and the soil pore system are important for many soil environmental processes. However, little is known about the influence of intra-aggregate microstructure on aggregate stability. The objective of this study was to evaluate the effect of soil pore characteristics on wet aggregate stability and aggregate tensile strength (TS) using synchrotron-based X-ray micro-computed tomography (SR-μCT) under wetting and drying cycles. Undisturbed topsoils (200cm3) derived from shale (S) and Quaternary red clay (Q) were submitted to three wetting and drying treatments under controlled laboratory conditions. 3–5mm aggregates were collected from different treatments and scanned at 3.7μm voxel-resolution for the reconstruction of 3D micro tomography images. The wet aggregate stability and TS were measured after each treatment. The relationships among pore characteristics, wet aggregate stability, and TS were analyzed using partial least squares regression (PLSR). The results indicated that porosity (P), percentage of pores >100μm (Pd>100), and fraction of elongated pores (FEP) all significantly increased with an increasing number of wetting and drying cycles, while the opposite trends were observed for the total number of pores (TNP), the percentage of pores with a diameter of 75–100, 30–75, and <30μm. Decrease in wet aggregate stability and TS of both soils was mainly due to higher P, a more extensive and complex pore network, which caused increased air pressure, increased rate of water entry, and high probability of crack propagation and interaction. The TNP, Pd75–100, P, and Pd>100, were identified as the primary factors controlling the wet aggregate stability and TS according to PLSR. The pore characteristics and soil clay content accounted for as much as 99% of the variation in wet aggregate stability and TS. This study provides insights for improved understanding of the change in topsoil microstructure; however, the effect of pore characteristics on the aggregate breakdown mechanism requires further investigation.

Effects of soluble organic carbon addition on CH4 and CO2 emissions from paddy soils regulated by iron reduction processes

An incubation experiment with the addition of glucose was conducted to evaluate the effects of carbon and iron (Fe(III)) reduction on methane (CH4) and carbon dioxide (CO2) emissions from paddy soils. Soils of a rice–rapeseed (Brassica napus) rotation and rice–fallow/flooded rotation were collected from Qianjiang (QR and QF, respectively) and Xianning (XR and XF). Incubation was conducted under flooding at 25°C ± 1°C with or without (CK) glucose over 40 days. With glucose addition, cumulative CH4-C emissions from QR, QF, XR and XF soils were 5.31, 35.26, 13.92 and 27.58 mg kg–1, respectively, and cumulative CO2-C emissions were 594.33, 620.49, 549.42 and 792.46 mg kg–1. Compared with CK, glucose addition significantly (P &lt; 0.05) increased cumulative CH4 fluxes in QR and QF soils 11.07-fold and 1.39-fold, respectively, and cumulative CO2 fluxes 0.41-fold and 0.44-fold, whereas the effects of glucose addition on CH4 and CO2 fluxes in XR and XF soils were negligible. In addition, the soil Fe(II)/(Fe(II) + Fe(III)) fraction correlated positively with CH4 fluxes during the major emission period (P &lt; 0.05), and the Fe(II) production rate was positively correlated with the CO2 flux during the whole incubation period. Furthermore, Fe(III) reduction strongly competed with CH4 emission, especially in XR and XF soils, which derived from quaternary red clay. The results suggest that Fe(III) reduction plays a key role in mediating the carbon cycle of paddy soils.

Parental material and cultivation determine soil bacterial community structure and fertility.

Microbes are the key components of the soil environment, playing important roles during soil development. Soil parent material provides the foundation elements that comprise the basic nutritional environment for the development of microbial community. After 30 years artificial maturation of cultivation, the soil developments of three different parental materials were evaluated and bacterial community compositions were investigated using the high-throughput sequencing approach. Thirty years of cultivation increased the soil fertility and soil microbial biomass, richness and diversity, greatly changed the soil bacterial communities, the proportion of phylum Actinobacteria decreased significantly, while the relative abundances of the phyla Acidobacteria, Chloroflexi, Gemmatimonadetes, Armatimonadetes and Nitrospira were significantly increased. Soil bacterial communities of parental materials were separated with the cultivated ones, and comparisons of different soil types, granite soil and quaternary red clay soil were similar and different with purple sandy shale soil in both parental materials and cultivated treatments. Bacterial community variations in the three soil types were affected by different factors, and their alteration patterns in the soil development also varied with soil type. Soil properties (except total potassium) had a significant effect on the soil bacterial communities in all three soil types and a close relationship with abundant bacterial phyla. The amounts of nitrogen-fixing bacteria as well as the abundances of the nifH gene in all cultivated soils were higher than those in the parental materials; Burkholderia and Rhizobacte were enriched significantly with long-term cultivation. The results suggested that crop system would not deplete the nutrients of soil parental materials in early stage of soil maturation, instead it increased soil fertility and changed bacterial community, specially enriched the nitrogen-fixing bacteria to accumulate nitrogen during soil development.

Pedogenetic evolution of clay minerals and agricultural implications in three paddy soil chronosequences of south China derived from different parent materials

This study aims to understand how clay minerals change sequentially with paddy cultivation age and how parent materials (or original soils) affect the clay mineral behavior of paddy soils. Three paddy soil chronosequences in the hilly regions of South China, derived from purple sandy shale (PS), Quaternary red clays (RC), and red sandstone (RS), were selected to explore the dynamic changes in clay mineralogy, by comparing physical, chemical, and mineralogical properties of soil sequences. For RC and RS soils, both of which have a low K content, there was little change in the clay minerals. Long-term paddy cultivation can promote formation of illite-like minerals; however, this form of K storage was limited under present farming conditions. In PS soils, which are abundant in K-bearing minerals, the depotassication was strong, accompanied by marked transformation of clay minerals. Kaolinite-like minerals gradually decreased with paddy cultivation age; by contrast, derivative clay minerals such as secondary chlorite and halloysite gradually increased. Strong depotassication mainly occurred in the nonclay fractions. The rate of depotassication and the generation of clay fractions were much faster than in natural soils. The clay minerals of paddy soils mainly followed the feature of their original soils. Their evolutions could be distinguished based on their constituents, which are greatly affected by their parent materials. Moreover, paddy cultivation is able to modify clay mineralogy, according to the original mineralogy and paddy soil management.