中国亚热带红壤团聚体稳定性与土壤化学性质的关系

The stability of aggregates in the surface soil is crucial to soil erosion and runoff generation. Thus, to understand the stability and the breakdown mechanisms of soil aggregates as well as the relationship between aggregate stability and selected soil chemical properties, such as different forms of Fe and Al oxides, organic matter, CEC and clay content, the aggregates of slightly and severely eroded red soils derived from Quaternary red clay in subtropical China were analyzed using the routine wet sieving and the Le Bissonnais methods. The results indicated that the aggregates of the severely eroded soils were more stable than those of the slightly eroded soils. Different aggregate breakdown mechanisms resulted in different particle size distribution. The slaking from entrapped air in aggregates severely destroyed the soil aggregates, especially in the slightly eroded soils. Meanwhile, mechanical breakdown and microcracking had little effect on the aggregates compared to slaking. The fragments resulting from slaking were mainly microaggregates that increased in size with increasing clay content. The main fragment size of the slightly eroded soils was 1.0-0.2 mm, while for the severely eroded soils it was 5.0-2.0 mm and 1.0-0.5 mm. Overall, more than 20% of the fragments were smaller than 0.2 mm. In addition, aggregate stability was positively and often significantly correlated with Fe_d, Al_d, Fe_o and clay content, but significantly and negatively correlated to SOC.     

鄂南第四纪粘土红壤团聚体的稳定性及其稳定机制初探

用湿筛法和LeBissonnais法研究了鄂南第四纪红粘土母质发育的两种侵蚀程度的红壤团聚体的稳定性,并且分析了影响供试土壤团聚体稳定性的土壤性质。结果表明,轻度侵蚀的耕作土壤团聚体的稳定性较低,在水的作用下易崩解成较小粒径的水稳性团聚体;强度侵蚀的土壤表层团聚体的稳定性较高,崩解后产生较多的水稳性大团聚体。引起土壤团聚体破坏的主要作用机制是土壤团聚体中的闭蓄空气爆破引起的消散作用;研究区第四纪红壤团聚体的主要胶结物质是土壤中的粘粒、游离氧化铁铝和无定形铁。由于供试土壤中有机质含量很低,在本研究中,有机质含量与土壤团聚体稳定性之间没有显著正相关关系。     

前期土壤含水率对红壤团聚体稳定性及溅蚀的影响

为了探究前期含水率对南方红壤团聚体稳定性及溅蚀的影响,选取泥质页岩和第四纪红黏土发育的4个典型红壤为研究对象,就5个前期含水率(3%、5%、10%、15%、20%)条件下3~5 mm团聚体水稳定性特征及其与溅蚀的关系进行了初步的探讨。结果表明,消散作用是团聚体破碎最有效的机制,土壤前期含水率越大,团聚体破碎程度越小。随着前期含水率的升高,泥质页岩发育的2种红壤水稳性团聚体平均质量直径(MWDwa)显著增大;第四纪红黏土发育的2种红壤水稳性团聚体平均质量直径(MWDwa)先增大后减小,拐点出现在含水率为15%条件下。泥质页岩发育2种红壤溅蚀量随前期含水率的升高显著减小;第四纪红黏土发育2种红壤溅蚀量随前期含水率的升高呈现先减小后增大的趋势,在含水率为15%时达到最小。团聚体水稳性较高的土样,溅蚀粒径分布呈双峰曲线分布,主要分布1~0.5和0.05 mm范围内,且前期含水率越高,0.05 mm溅蚀颗粒含量越大;而团聚体水稳定性较差的土样,除前期含水率为20%外,溅蚀粒径分布呈单峰曲线分布,主要分布在0.25~1mm。该结果为红壤区农业水土工程及机侵蚀机理研究提供一定的参考,对完善坡面水蚀模型具有一定的参考价值。     

湿润速率和粘粒含量对红壤沟间侵蚀的影响

An aggregate stability test and a simulated rainfall test were conducted on four representative Ultisols from southeastern China. The soils selected, with clay contents ranging between 117 and 580 g kg-1 , were derived from shale and Quaternary red clay. The stability of aggregates (2–5 mm in diameter) obtained from the soil samples were determined by the Le Bissonnais method. For determination of infiltration, runoff, and erosion, the soil samples were packed in 30 cm × 60 cm trays, wetted at rates of 2, 10, and 60 mm h-1 , and then exposed to simulated rainfall at 60 mm h-1 for 1 h. The results indicated that both aggregate stability and slaking caused by fast wetting increased with increasing clay content. The effect of wetting rate (WR) on infiltration and seal formation varied with clay contents. In the soil with low clay content (sandy loam), the infiltration rate was affected slightly by WR due to low aggregate stability and slaking. In the soils with medium clay content (silt clay loam and clay), WR affected infiltration significantly due to the high aggregate slaking force. In the soil with high clay content, the effect of WR on infiltration was significant, but not as evident as in the soils with medium clay content, which may be related to high aggregate stability by wetting partially compensating for slaking force. The effect of WR on soil loss was similar to that of runoff, but more pronounced. The findings from this study indicated that the relationship between wetting rate and clay content should be considered when predicting interrill erosion in Ultisols.     

Primary particle size distribution of eroded material affected by degree of aggregate slaking and seal development

Primary particle size distribution (PSD) of eroded sediment can be used to estimate potential nutrient losses from soil and pollution hazards to the environment. We studied eroded sediment PSDs from three saturated soils, packed in trays (20 × 40 × 4 cm), that had undergone either minimal aggregate slaking (MAS) or severe aggregate slaking (SAS) prior to a 60 mm simulated rainstorm (kinetic energy, 15.9 kJ m⁻³; droplet diameter, 2.97 mm) and collected runoff at regular intervals. The degree of aggregate slaking was controlled by the rate at which soils were wetted to saturation. The PSDs of eroded materials and of parent soils were determined using a laser particle size analyser. For each soil, PSD frequency curves of eroded sediments and parent soils were generally of a similar shape but most eroded sediments had larger clay contents than their parent soils. In the SAS treatment, cumulative clay enrichment in the eroded materials was inversely related to the parent soil clay content, these being 28.5, 26.6 and 22.8% richer in clay than their parent soils for the loam, sandy clay and clay, respectively. Generally, total clay loss was greater from soils with SAS than from those with MAS because of erosion rates; however, clay enrichment of sediments, compared with parent soil clay contents, was mostly greater in samples with MAS. Greater clay enrichment took place during the early seal development stage in the loam, but could not readily be associated with specific stages of seal development for the clay. In the sandy clay, the relation between seal development and clay enrichment in the eroded material depended on the initial degree of aggregate slaking. The observed large preferential loss of clay by erosion in cultivated soils re-emphasizes the need to employ erosion control measures.     

Aggregate breakdown dynamics under rainfall compared with aggregate stability measurements

Aggregate breakdown due to rainfall action causes crusting and interrill erosion. Erodibility is seemingly determined by the capacity of surface aggregates to resist the effects of rainfall. In this paper, we evaluated the relevance of an aggregate stability measurement, which comprises three treatments, in order to characterize aggregate breakdown dynamics. Two cultivated soils were studied: a clay loam slightly sensitive to erosion and a more susceptible silt loam. We compared the size distributions of microaggregates produced by the three aggregate stability treatments with the results from a rainfall simulation. The behaviour of four initial aggregate size classes (< 3 mm, 3–5 mm, 5–10 mm and 10–20 mm) was also compared to study the influence of the initial aggregate size on the nature of resulting aggregates. The mean weight diameter was from 200 to 1400 µm for the silt loam and from 600 to 7000 µm for the clay loam. The two experiments – aggregate stability measurements and aggregate breakdown dynamics under rainfall – yielded similar results. Qualitative analysis showed that for both soils the sizes of fragments produced by breakdown with the aggregate stability tests and under rainfall were similar and seemed to be qualitatively independent of the size of initial aggregates. We first schematized the structural organization of aggregates in cultivated horizons with a simple hierarchical model at two levels: (i) < 250 µm microaggregates and (ii) > 250 µm macroaggregates made by the binding together of microaggregates. We then developed a model of aggregate breakdown dynamics under rainfall which gives, for various rainfall durations, the size distributions of resulting fragments on the basis of aggregate stability measurements. We obtained a correlation coefficient, r, of 0.87 for the silt loam and of 0.91 for the clay loam, showing that the experimental and predicted mass percentages were linearly related for each size fraction.     

降雨过程中红壤表土结构变化与侵蚀产沙关系

通过人工模拟降雨和表土微形态观测,研究了发育于泥质页岩、第四纪红粘土和花岗岩3种母质的红壤在降雨侵蚀过程中表土土壤结构的变化及其对侵蚀的影响。结果表明:降雨过程中,泥质页岩红壤极易形成土壤结皮,增加径流,响应结皮的形成,径流速率和含沙量较高,且迅速达到最大值,随后径流稳定而含沙量持续下降。第四纪红粘土红壤团聚体稳定,较难形成结皮,且结皮易被破坏,导致侵蚀过程中产流产沙量较低,均随降雨时间的延长而呈缓慢上升趋势。花岗岩红壤基本上不能产生结皮,粗化现象严重,因此产流量和产沙量也较低;由于土壤团聚体稳定性差以及径流的选择性运移,泥质页岩红壤和花岗岩红壤侵蚀泥沙中细颗粒(<0.02mm)含量远高于土壤中该粒径颗粒。而第四纪红壤侵蚀泥沙中粗颗粒较多,以多级团聚体的团聚体为主。     

雨滴机械打击和消散作用对土壤团聚体的破坏特征

土壤团聚体是土壤结构的基本单元,其稳定性是描述土壤抵抗侵蚀过程中破坏作用的重要指标。但溅蚀过程中,雨滴对团聚体的消散和机械打击两种破坏作用的相对贡献及其破坏机制尚未明晰。利用酒精和超纯水作为雨滴形成材料,模拟机械打击单独作用及消散和机械打击共同作用,分别在五个高度(0.5、1、1.5、2和2.5 m)对塿土和黄绵土进行溅蚀实验。结果表明:当降雨动能相同时,塿土的溅蚀率均小于黄绵土。同时,超纯水雨滴对土壤的机械打击和消散共同作用所导致的溅蚀率均大于酒精雨滴单一机械打击作用的溅蚀率。随着降雨动能增加,两种雨滴对两种土壤的溅蚀率均呈幂函数增加;团聚体因消散破坏作用和机械打击作用的溅蚀率均亦随之增加。但两种土壤的消散破坏作用和机械打击作用的贡献率分别随着降雨动能增加而减小和增加。在相同降雨动能时,塿土消散破坏作用的贡献率均大于黄绵土,而机械打击作用贡献率均小于黄绵土。研究结果对深入理解溅蚀过程中团聚体破坏机理及评价溅蚀过程中团聚体稳定性具有重要意义。     

几种侵蚀红壤中有机质和团聚体的关系

以3种侵蚀红壤（轻度、中度、严重）为供试材料,利用干湿筛法获得其不同粒径的水稳性团聚体（〉4 mm,2～4 mm,1～2 mm,0.5～1 mm,0.25～0.5 mm）,分析团聚体中有机质的分布。结果显示,随着侵蚀程度的增强,〉0.25mm水稳性团聚体的含量逐渐下降,而团聚体的分散度却依次升高;无论何种侵蚀程度,湿筛后团聚体的组成均以小粒径团聚体（〈0.25 mm）占优势。轻度和中度侵蚀的红壤,其有机质含量随着团聚体粒径的增大而增大,严重侵蚀的则相反;侵蚀红壤有机质含量和〉0.25 mm水稳定性团聚体呈显著正相关,3种侵蚀红壤团聚体对土壤有机质的贡献为轻度侵蚀〉中度侵蚀〉严重侵蚀。     

Comparison of two methods for assessment of aggregate stability of agricultural soils in southeast Norway

Abstract

Soil aggregate stability is a crucial property regarding soil erodibility. However, results from different stability-assessment methods differ. The objective of this study was to compare two such methods on a set of southeast Norwegian agricultural soils. Traditionally, a raindrop-impact method has been used to determine the soil aggregate stability of Norwegian soils. Here, a more common, less destructive method was also used, a wet-sieve method using one sieve. Two soil fractions were studied, aggregates of 0.6–2 mm and 2–6 mm in diameter. The soil samples were chosen to give a wide range in clay content (5.7–68.0%) and soil organic matter (1.7–8.5% SOM). Using the wet-sieve method two different pre-treatments were performed; either using air-dried aggregates or slowly pre-wetted, air-dried aggregates. The aggregate stability found using the wet-sieve method gave generally higher stability values for soil aggregates 2–6 mm in diameter than using the standard raindrop-impact method (21% higher on average). Soils with high contents of silt and very fine sand (0.002–0.2 mm), however, were found to be more susceptible to destruction due to raindrop impact than to wet-sieving. Hence, using the wet-sieve method, the silty soils were ranked as more stable than by using the standard raindrop-impact method. Aggregate stability was positively correlated with the SOM and Al-oxides content and negatively correlated with the silt and the very fine sand content (0.002–0.02 mm). Using slowly, pre-wetted air-dried aggregates, which induce rebonding and reduce the effect of slaking, resulted in distinctively higher stability values than using air-dried aggregates in the wet-sieving method (34% higher on average). The wet-sieve method is less laborious and more widely used elsewhere, but the lower disruptive effect on silty soils should be kept in mind when using this method.     

Mechanisms of aggregate stabilization in Ultisols from subtropical China

The quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size. The objectives of this study were to: (i) determine the aggregate stability in Ultisols from subtropical China applying the Le Bisssonais Method; (ii) determine the effect of initial aggregate size on its stability, and (iii) interpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges (5–3, 3–2 and 2–1 mm) were obtained by dry sieving. After the wetting treatments, the dominant fraction of fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdown was in the order, slaking>mechanical breakdown>micro-cracking. They differed with soil type and composition. The normalized mean weight diameter (NMWD) of the aggregates after fast wetting and wet stirring were more correlated with soil properties, such as degree of micro-aggregation (DOA), cation exchange capacity (CEC), K₂O, Fe₂O₃ or Al₂O₃ rather than clay and soil organic carbon (SOC) content. The binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals.The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams from sandstone (Sc and Sw) were the weakest soils while the purple mudstone (Pp) was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.     

Effect of sodicity and salinity on disaggregation and tensile strength of an Alfisol under different cropping systems

The influence of electrolyte concentration (EC) and sodium adsorption ratio (SAR) on the tensile strength and aggregate stability via flocculation and dispersion behaviour of an Alfisol varying in organic carbon content due to different cropping systems was assessed using a split-split plot experiment involving eight soils, three levels of EC and seven levels of SAR.

Generally, at a given SAR value, mean weight diameter (MWD) increased with organic matter status of the soil in the following order: virgin pasture>wheat>wheat-fallow. As MWD decreased, the amount of dispersible clay increased at a given SAR indicating that more surfaces exposed due to slaking of aggregates led to more clay dispersion. Statistical analysis of changes in tensile strength with various factors showed that an increase in organic matter decreased the magnitude of changes in strength induced by sodicity because organic matter tends to increase aggregate stability (higher MWD). While individual soils had significant relationships between the tensile strength of the aggregates and the amount of spontaneously dispersible clay, this relationship was poor when the results of all soils were pooled together. The amounts of dispersible clay from dry aggregates were higher than from wet aggregates and dispersive breakdown of the aggregates of sodic soils occured irrespective of the mode of wetting. The most important factor in determining the soil strength was the amount of clay dispersed during wet-sieving analysis followed by MWD.     

中南地区典型地带性土壤团聚体抗张强度的变化及影响因素

团聚体的力稳性是决定土壤抗侵蚀能力的关键因素。为探究地带性土壤团聚体抗张强度的变化规律及其影响因素,自北向南选取我国中南部地区6种典型地带性土壤(褐土、黄褐土、棕红壤、红壤、赤红壤和砖红壤)的不同粒径(1~2,2~3,3~5,5~10mm)团聚体作为研究对象,通过测定团聚体的抗张强度(TS),探究其与土壤基本理化性质的关系,揭示该区域团聚体抗张强度的变化特点和稳定机制。结果表明:(1)供试土壤皆为黏性土壤,自北向南,随着水热条件的增加,土壤的pH值逐渐降低,高岭石含量和游离氧化物(Fe_d、Al_d)呈现明显的增加趋势;有机质含量随土壤深度的增加而降低。(2)同种土壤团聚体的抗张强度随着粒径的增大而逐步减小,从北至南,相同粒径团聚体的抗张强度整体呈现减小的趋势。(3)TS与pH、粉粒含量、蛭石含量呈极显著正相关(r0.63,p0.01),TS与黏粒含量、1.4nm过渡矿物含量、高岭石含量、游离氧化铁、铝(Fe_d、Al_d)呈显著负相关(r-0.53,p0.05)。(4)逐步回归分析表明,Fe_d和CEC可以较好的预测和评价3~5mm团聚体的抗张强度(R~2=0.80,p0.01)。总体而言,黏土矿物类型及其含量是影响地带性土壤团聚体力稳性的重要因素。研究结果可为该区域土壤侵蚀预测提供一定的理论依据。     

Investigating the effects of organic and conventional management on soil aggregate stability using X-ray computed tomography

Soil aggregate stability is an important measure for assessing soil structural quality. Here we compare stable and unstable soil aggregates from organically and conventionally managed soils that have similar organic matter contents to determine the role of management in aggregate stability. Aggregate stability was determined by slaking, which proved to be an accurate estimator of the rate of aggregate turnover. The results indicate that, in soil aggregates released by slaking, the organic matter is held in aggregates > 0.3 mm diameter. A relatively short aggregate turnover rate will prohibit the production of stable micro-aggregates within macro-aggregates and thus gradually reduce aggregate stability, as was observed in soils under conventional management. Data obtained by X-ray μCT suggested that intra-aggregate porosity did not significantly affect stability. We observed cracks and elongated pores connected to the surface of stable soil aggregates that can provide an escape route for entrapped air, thus reducing pressure build-up and potentially reducing slaking. Our results show that organic management has the potential to develop more stable aggregates compared with conventionally managed soils for the soil type studied. Thus, conventional soils require particular attention to management practices that increase the rate of aggregate turnover in order to reduce the production of unstable aggregates that can contribute to crusting, erosion and runoff.     

不同修复措施下红壤水稳性团聚体中有机质分布特征

试验设置作物对照(CK)、作物+PAM(聚丙烯乙酰)(C1)、作物+稻草覆盖(C2)、作物+带状牧草(C3)、作物+PAM+带状牧草(C4)和作物+稻草覆盖+带状牧草(C5)6个处理,分别于种植花生前(A)、花生收获后(B)和小麦收获后(C)采集表层土样,研究不同修复措施下鄂东南侵蚀红壤团聚体的稳定性及水稳性团聚体中有机质的分布特征。结果表明,C时期各处理>0.25mm水稳性团聚体含量的平均值比A时期提高了24.78%,以C2处理的水稳性团聚体含量最高。不同时期土壤和各级水稳性团聚体中有机质含量表现为B>C>A。随着团聚体粒径(>4mm,4～2mm,2～1mm,1～0.5mm,0.5～0.25mm)减小,水稳性团聚体中有机质含量逐渐降低。土壤有机质含量与各粒径水稳性团聚体有机质含量呈线性正相关,小粒径(2～0.25mm)水稳性团聚体的有机质含量与土壤有机质含量呈极显著正相关(相关系数为0.779 2～0.828 6)。综合考虑土壤有机质含量和团聚体稳定性2个因素,稻草覆盖措施对侵蚀红壤的修复效果较好。     

土粒表面电场对土壤团聚体破碎及溅蚀的影响

团聚体是土壤结构的基本单元,其稳定性是评估土壤抗侵蚀能力的重要指标。土壤团聚体破碎是降雨溅蚀发生的关键一步。土粒表面电场对团聚体稳定性具有重要影响,必然也会深刻影响降雨溅蚀过程。该文以黄土母质发育的黄绵土和塿土为研究对象,采用不同浓度的电解质溶液定量调控土粒表面电场,研究不同电场强度对团聚体破碎及溅蚀的影响。结果发现:1)随电解质浓度的降低,土粒表面电位升高,表面电场增大,黄绵土和塿土团聚体平均重量直径减小,团聚体稳定性降低,降雨溅蚀量增大。2)电解质浓度小于10-2 mol/L,黄绵土和塿土表面电位绝对值分别高于202.0和231.6 mV,此时团聚体稳定性和溅蚀量变化不明显,表明表面电位202.0和231.6 mV分别是影响黄绵土和塿土团聚体稳定性及溅蚀的关键电位。3)随着土粒表面电场的减弱,团聚体破碎后释放的<0.15 mm微团聚体含量减小,>0.25 mm大团聚体含量增加,团聚体倾向于破碎为更大粒级的团聚体。4)电场作用下团聚体的破碎特征对降雨溅蚀具有重要的影响,溅蚀量与团聚体破碎释放的<0.15 mm微团聚体含量呈显著正相关,与>0.25 mm大团聚体含量呈显著负相关。上述结果表明,当降雨进入土壤后,对于干燥的土壤而言,土壤溶液电解质浓度被迅速稀释,土粒表面产生强大的电场,该电场通过影响团聚体破碎程度进而影响降雨溅蚀。该研究有助于加深对降雨溅蚀的科学认识,同时也为土壤团聚体稳定性及降雨溅蚀的人为调控提供了一定的理论依据。     

有机质影响溅蚀破坏土壤团聚体的主要作用机制

土壤有机质是团聚体的重要组成部分,其在团聚体形成过程中作用机制已有大量研究,但有机质在团聚体破坏过程中起何种作用尚未明晰,其对团聚体破坏过程中雨滴机械打击和消散作用贡献的影响也有待深入研究。为研究不同有机质含量对土壤团聚体的影响,选取5种不同退耕还林年限的土壤为研究对象,利用95%酒精和超纯水作为降雨液体,分别在4个高度下(0.5、1、1.5和2 m)对其进行溅蚀试验。结果表明:随有机质含量的增加,团聚体稳定性逐渐增强,土壤对雨滴机械打击和消散作用的敏感程度越来越弱,土壤抵抗侵蚀的能力越来越强;降雨前后大团聚体数量随着有机质含量的增加逐渐趋近相似,且酒精雨滴作用下的大团聚体含量明显大于超纯水作用下的大团聚体含量;在相同的降雨动能条件下,有机质含量增加使消散作用的贡献率呈现减小的趋势,且其对消散作用的影响在降雨动能较小时较为显著。研究结果对深入理解溅蚀过程中团聚体稳定性评价以及团聚体破坏因子具有重要作用。     

湘东地区典型土壤团聚体稳定性的影响因素

[目的]揭示湘东地区土壤团聚体的分布规律及其影响稳定性因素,为研究南方土壤团聚体胶结机制提供一定的理论依据。[方法]在湘东地区选择3种常见成土母质(第四纪红土、花岗岩风化物、板岩风化物)上发育的典型自然林地、水田与旱地,采集表层土壤(0—20cm)与底层土壤(40—60cm),利用湿筛法测定土壤水稳性团聚体组成。[结果]土地利用方式、母质类型及土层部位显著影响土壤团聚体的组成及稳定性,且它们的交互作用也非常明显。不同利用方式下土壤团聚体稳定性总体表现为:水田林地旱地;但利用方式对团聚体稳定性的影响仅局限于表层土壤,同时在花岗岩风化物发育质地较砂的土壤上表现不明显。不同母质间土壤团聚体稳定性呈现出如下规律:第四纪红土板岩风化物花岗岩风化物,但表层土壤或旱地、林地利用下该规律不甚明显。同时,表层土壤团聚体稳定性一般要显著高于底层土壤,但对于第四纪红土发育质地较为黏重的土壤或水田利用方式下并非如此。[结论]成土母质、利用方式、土层部位通过对土壤质地、有机质、氧化物等的影响间接影响着湘东地区土壤团聚体的分布。     

铵、钾同时存在时, 土壤对铵的优先吸附

The water stability of aggregates in various size classes separated from 18 samples of red soils under different managements, and the mechanisms responsible for the formation of water-stable soil aggregates were studied. The results showed that the water stability of soil aggregates declined with increasing size, especially for the low organic matter soils. Organic matter plays a key role in the formation of water-stable soil aggregates. The larger the soil aggregate size, the greater the impact of organic matter on the water stability of soil aggregates. Removal of organic matter markedly disintegrated the large water-stable aggregates (> 2.0 mm) and increased the small ones (< 0.25-0.5mm) to some extent, whereas removal of free iron(aluminium) oxides considerably destroyed aggregates of all sizes, especially the < 0.25-0.5 mm classes. The contents of organic matter in water-stable aggregates increased with aggregate sizes. It is concluded from this study that small water-stable aggregates (< 0.25-0.5 mm) were chiefly cemented by Fe and Al oxides whilst the large ones (> 2.0 mm) were mainly glued up by organic matter. Both free oxides and organic matter contribute to the formation and water stability of aggregates in red soils.     

Soil slaking sensitivity as influenced by soil properties in alluvial and residual humid tropical soils

Purpose

In the humid Caribbean region characterized by high-intensity tropical rainfall, soil aggregate breakdown and pore blocking due to slaking pressures are major land degradation mechanisms. In this research, we investigated the susceptibility of soils to slaking pressures under rapid wetting as influenced by soil properties and the depositional origin from which the soil is formed using water-stable aggregates (WSAr) and percolation stability (PSc) as indices of the strength of aggregate inter-particle cohesion.

Materials and methods

Wet sieving and percolation stability analyses were employed to investigate WSAr and pore blocking, respectively. The combined effect of soil properties of clay, organic matter (OM), cation exchange capacity (CEC), and exchangeable sodium percentage (ESP) was used to determine the slaking sensitivity score (SSc) of 14 physiogeographically important soils in Trinidad, comprising of nine alluvial and five residual soils.

Results and discussion

Results showed that irrespective of alluvial or residual depositional nature of the parent material, samples had high SSc with an average WSAr of 37.8% and PSc of 6.0 mm/10 min. The linear relationships between SSc with WSAr (r²?=???0.12) and SSc with PSc (r²?=???0.012) of all the 14 soils although negative were weak. Clay content accounted for 94.0% of the variation in CEC in alluvial soils and had strong negative relationships with WSAr (r²?=???0.74) and PSc (r²?=???0.79) in residual soils. Additionally, OM with WSAr (r²?=?0.52) and PSc (r²?=?0.24), and CEC with WSAr (r²?=?0.46) and PSc (r²?=?0.39) showed significant positive linear relationships in residual soil.

Conclusions

The predominantly micaceous and kaolinitic clay mineralogy of these soils, coupled with the low OM contents, increases the proneness of the soils to slaking. This suggests that clay mineralogy is responsible for the high slaking sensitivity rather than clay content or just the depositional origin of the soils. As CEC increases, an accompanying increase in OM is required to increase inter-particle cohesion and to impart partial hydrophobicity, which in turn decreases mineralogically induced susceptibility of individual aggregates to slaking.