Assessment of the burrow system of Lumbricus terrestris, Aporrectodea giardi, and Aporrectodea caliginosa using X-ray computed tomography

Artificially packed soil columns were inoculated with individuals of different earthworm species (Lumbricus terrestris, Aporrectodea giardi or Aporrectodea caliginosa) and placed under controlled conditions in the laboratory. At the end of the incubation period, which lasted 8 months, three-dimensional reconstructions of the burrow system of each species were obtained using X-ray computed tomography. The particular features of the three burrow systems and the differences between them are discussed in terms of density, orientation and distribution of the burrows and the complexity of the network in relation to variations in soil depth and soil density. Received: 5 February 1997     

Characterization of the burrow system of the earthworms Lumbricus terrestris and Aporrectodea giardi using X-ray computed tomography and image analysis

 The burrow systems of two earthworm species (Lumbricus terrestris and Aporrectodea giardi) were studied in artificially packed soil columns placed in controlled conditions in the laboratory. At the end of the incubation, which lasted 246 days, the burrow systems were characterized on the undisturbed columns using X-ray computed tomography. This method provided a set of digitized images corresponding to horizontal 3-mm-thick sections. The following parameters were measured using image analysis on each section: number of biopores, their volume, and their individual orientation calculated using an elliptical model of the earthworm channel. The profiles of these parameters through the columns showed that the burrow systems of L. terrestris and A. giardi, which are both anecic species, were very different in terms of total volume, number of burrows, burrow orientation and extension with soil depth. These results led us to conclude that the burrow system of L. terrestris can be considered as a permanent structure whereas that of A. giardi is closer to the burrow system of endogeic species. Received: 12 June 1998     

Quantitative analysis of earthworm burrow systems with respect to biological soil-structure regeneration after soil compaction

 On arable land, tilled with conventional tillage (CT) and conservation tillage (CS) respectively, plots were compacted by wheeling them 6 times with a 5 Mg wheel load in spring 1995. Immediately after compaction, undisturbed soil monoliths were excavated from the compacted and uncompacted plots. The monoliths were defaunated and inoculated with either Lumbricus terrestris or Aporrectodea caliginosa. One monolith from each plot remained uninoculated as a control. After 6 months the monoliths were defaunated again and then scanned with X-ray helical computed tomography. The data were transformed, the void systems inside the monoliths were reconstructed and visualised, and the parameters total void length, total void volume, tortuosity and continuity were quantified. The parameters' values were generally lower in the controls than in the inoculated monoliths. Differences in burrow construction could be explained by the different life strategies of the two earthworm species. Changes in burrow morphology due to tillage system and soil compaction were minor. Only the continuity of the burrow systems clearly changed: decreasing for L. terrestris and increasing for A. caliginosa. This can be explained by a change in the earthworms' burrowing activity to minimise energy expenditure in compacted soil. By extrapolating field data, we concluded that earthworms have great potential for biologically regenerating the soil structure after a single compaction event. Due to higher earthworm abundances in soil managed by CS the regeneration of the soil structure is assumed to be better in these plots than those tilled by CT. Received: 17 December 1997     

聚丙烯酰胺增加土壤降雨入渗减少侵蚀的模拟试验研究 Ⅰ.入渗

土壤 (特别是黄绵土 )在雨水作用下 ,表面会形成结皮 ,显著地降低降雨入渗率 ,聚丙烯酰胺 (PAM)能有效地抑制降雨过程中土壤结皮的形成 ,提高降雨入渗率。为了获得PAM使用量和覆盖率与入渗率之间的关系 ,进行了一系列的人工降雨模拟试验。试验采用了三个雨强 :5 0、1 0 0和 1 5 0mmh- 1,四个坡度 :8.74%、1 7.63 %、3 6.4%和 46.63 % ,及五种地表处理 :对照、秸秆覆盖和三个PAM覆盖率 (A、B和C) ,试验设三次重复。试验分Ⅰ、Ⅱ两个降雨阶段进行 ,第Ⅱ阶段在第Ⅰ阶段降雨结束土壤放置 2 4h后进行。分析建立了降雨入渗率与PAM的覆盖率间的相关关系 ,确定了PAM的覆盖率对降雨入渗的影响。     

确定侵蚀细沟土壤临界抗剪应力的REE示踪方法

近年来兴起的REE示踪方法 ,可以研究土壤侵蚀的发生和发展的过程 ,及其沿坡面变化的规律。本研究采用Dy、La、Sm、Yb、Ce、Eu、Nd、Tb等 8个REE、3个雨强 (50mmh- 1,10 0mmh- 1和 150mmh- 1)和4个坡度 (8.74% ,17.63 % ,3 6.4%和 46.63 % )进行了一系列人工模拟降雨试验 ,展示了土壤侵蚀沿坡面变化与坡面细沟发展的关系 ,分析了土壤表面水流动力特征与土壤侵蚀之间的动态平衡。由土壤侵蚀沿坡面的变化 ,确定了土壤细沟侵蚀发生的临界距离 ,并由此得到侵蚀细沟土壤临界剪应力。对不同试验条件下最大细沟侵蚀率与径流剪应力和细流力之间的关系进行了回归分析     

Residue cover and rainfall intensity effects on runoff soil organic carbon losses

Soil cover and rainfall intensity (RI) are recognized to have severe impacts on soil erosion and an interaction exists between them. This study investigates the effect of rainfall intensity (RI) and soil surface cover on losses of sediment and the selective enrichment of soil organic carbon (SOC) in the sediment by surface runoff. A field rainfall simulator was used in the laboratory to produce 90 min rainfall events of three rainfall intensities (65, 85 and 105 mm h^− 1) and four cover percentages (0%, 25%, 50% and 75%) on soil material at 9% slope. A strong negative exponential relation was observed between cover percentage and RI on sediment loss under 85 and 105 mm h^− 1 of rain, while under RI of 65 mm h^− 1, the highest sediment loss was observed under 25% cover. Overall, higher RI and lower cover produced higher sediment and consequently higher nutrient loss, but resulted in a lower SOC enrichment ratio (ER_SOC) in the sediment. The amount of runoff sediment rather than the ER_SOC in the sediment was the determinant factor for the amount of nutrients lost. The values of ER_SOC were high and positively correlated with the ER values of particles smaller than 20 µm (p < 0.01). Although the sediment contained substantially more fine fractions (fine silt and clay, < 20 µm), the original soil and runoff sediment were still of the same texture class, i.e. silt clay loam.     

Erodibility and runoff-infiltration characteristics of volcanic ash soils along an altitudinal climosequence in the Ecuadorian Andes

Soil erosion is a widespread phenomenon in Andean South America, where many regions are covered with soils derived from volcanic parent materials. Climate-induced differences in the genesis of these soils have been demonstrated along toposequences on volcanic slopes. This research was conducted to study the impact of such differential pedogenesis on erodibility and runoff-infiltration characteristics along an altitudinal Entisols–Inceptisols–Andisols sequence in the Andes of northern Ecuador. Surface soils were packed into small pans and placed on a 9% slope, and a simulated rainstorm with varying intensities was applied for a duration of 30 min. The runoff-erosion behaviour of the studied volcanic ash soils is strongly affected by their pedological development. Accumulation of organic matter and precipitation of active amorphous materials at high elevations have led to the formation of well-developed Andisols with very stable aggregate structure. These soils remain wettable when air-dried, show very high infiltration capacity and, consequently, low potential for runoff generation and soil erosion. Low organic matter contents and absence of active amorphous materials at low elevations have led to the formation of weakly aggregated Entisols and Inceptisols. These soils are susceptible to surface crusting, which lowers their infiltration capacity and increases their erodibility. However, in comparison with other soils of different origin and composition, the interrill erodibilities determined for these more erodible low-elevation soils are classified as low. The findings of this study suggest that upland soil erosion is not a major threat to sustainability in the studied volcanic landscape, which is generally confirmed by field observations.     

The influence of the soil conditioner ‘Agri-SC' on splash detachment and aggregate stability

The influence of the soil conditioner ‘Agri-SC' on splash detachment and water-stable aggregation of an erodible clay Vertisol from Oahu, HI, was assessed. Laboratory rainfall simulation was used to assess splash detachment from soil treated with 0 (untreated control), 0.3, 3.0, 30, and 300 l ha⁻¹ of Agri-SC. Results indicated that the quantity of sediment splashed was significantly lower for Agri-SC application rates of 0.3 and 3.0 l ha⁻¹ (rates are equivalent to 1 and 10 times the manufacturer's recommended rates, respectively), than for the control, or for Agri-SC applied at 30 and 300 l ha⁻¹ (100 and 1000×, respectively). A second experiment was designed to test the influence of Agri-SC on water-stable aggregation of the Vertisol. Aggregates were subjected to rapid immersion in solution, shaken and washed through a series of sieves. Data indicated that there were no statistically significant differences in geometric mean aggregate diameter between the untreated and treated aggregates. The effect of the active ingredient, ammonium laureth sulfate (an anionic surface active agent) on splash and erodibility are discussed. These preliminary results indicate that further testing of Agri-SC is warranted on a variety of soils with different textures and mineralogies.     

Field measurement of infiltration rate using an oscillating nozzle rainfall simulator in the cold, semiarid grassland of Mongolia

Recent intensive grazing in Mongolia may be significantly reducing the infiltration rate of rangeland. This study measured infiltration rates using simulated rainfall with high raindrop impact for small plots established on steppe grassland, desert grassland, and shrubland sites in Mongolia. The response of the infiltration rate to short-term livestock removal was also investigated. On the steppe grassland, a high infiltration rate was measured on an ungrazed plot with relatively dense vegetation cover; a statistically significant correlation was found between the total surface cover and final infiltration rate, indicating that surface cover by rock fragments also increased the infiltration rate to some extent. For desert grassland and shrubland, however, the surface cover condition was not a major factor controlling the final infiltration rate. After 4 years of livestock removal, the surface vegetation cover of the ungrazed plot was greater than that of the grazed plot, but no appreciable change occurred in soil penetration resistance. These results suggest that the high infiltration rate on the ungrazed plot was maintained mainly by the recovery of surface vegetation cover after the short-term livestock removal; this may indicate a potential mechanism of recovery from desertification processes for Mongolian rangeland.     

坡度和雨强对崩岗崩积体侵蚀泥沙颗粒特征的影响

不同侵蚀条件下崩积体的侵蚀产沙特性是阐明崩积体侵蚀机理的关键。采用人工模拟降雨试验,研究不同坡度和雨强条件下崩积体坡面侵蚀泥沙颗粒的变化特征。结果表明:随着雨强和坡度的增大,泥沙粗颗粒含量及粗颗粒的富集率均增加;侵蚀物质随降雨过程逐渐变粗,后趋于稳定,大雨强条件下细沟侵蚀阶段表现为对供试土壤的\"整体搬运\";侵蚀泥沙颗粒的平均重量直径(Mean weight diameter,MWD)随雨强的增大而增大,1.00 mm min-1和1.33 mm min-1雨强下,细沟间及细沟侵蚀泥沙的MWD随坡度变化均存在临界坡度(30°~35°之间),其他雨强条件下则无此种情况;雨强对侵蚀泥沙MWD的影响大于坡度。     

前期含水量对坡面降雨产流和土壤化学物质流失影响研究

在防止土壤侵蚀条件下,利用室内人工降雨实验,研究了土壤初始含水率对坡面降雨入渗、产流、溶质迁移规律的影响。结果表明:初始含水率越高,产流越快,达到稳定入渗率的时间也越短。在初始含水率不同时次降雨过程中养分的流失量与初始含水量具有较强的相关性。磷浓度的变化范围主要在5cm以内,随着含水量的增加有效磷向土壤中迁移的总量增加。随着含水量的增加土壤钾离子向土壤中迁移的总量减小。在相同条件下,EDI值大小顺序是:磷<钾<溴。     

不同雨强和覆盖度条件下崩积体侵蚀泥沙颗粒特征

崩积体坡面侵蚀泥沙颗粒的变化特征及过程研究是揭示崩岗崩积体侵蚀机理的关键。基于崩岗崩积体土质疏松、粗颗粒含量高、极易被侵蚀的特性,通过室内人工模拟降雨试验,研究30°坡度条件下,不同覆盖度(0,25%,50%,75%,100%)和雨强(60,90,120 mm h-1)组合坡面侵蚀泥沙颗粒特征。结果表明:降雨过程中,坡面径流优先搬运的是粒径较小的泥沙颗粒;侵蚀泥沙中粗颗粒(砾石和粗砂)泥沙含量随着覆盖度的增加呈先减小后增大趋势;侵蚀泥沙颗粒的平均重量直径(MWD)与覆盖度之间呈极显著相关;当覆盖度达到50%时,坡面粗颗粒泥沙的减少效果最明显,75%覆盖度坡面较容易发生崩塌。以上结果表明,侵蚀泥沙颗粒的大小与坡面秸秆覆盖度的高低密切相关,50%左右的秸秆覆盖度可以达到较好的减沙效果。