Destabilization of soil during the production of earthworm (Lumbricidae) and artificial casts

The mechanisms by which soil is destabilized in the digestive tract of endogéic earthworms were investigated with artificial casts, which were moulded with a syringe from slurries of a silty loam with or without gypsum and organic matter treatments, and compared to casts produced by Aporrectodea rosea (Lumbricidae). Both types of casts generally had the same levels of mechanical dispersion, observations of slaking, and particle size distribution when the casts were fresh, aged-moist for 30 days, or air-dried. Fresh casts were significantly more dispersive than the uningested soil despite the addition of gypsum or organic matter to the soil. However, the dispersion from aged-moist or air-dried casts was not substantially greater than that of uningested soil. Air-drying was more effective than moist-ageing in increasing the stability of casts and uningested soil. The concentration of soluble carbohydrate was greater in artificial casts produced from soil treated with sheep dung or xanthan gum, and in earthworm casts produced from soil treated with xanthan, than in the uningested soil of the same treatments. An increase in the concentration of soluble carbohydrate was related to an increase in dispersion. An attempt was made to simulate the addition of mucus to soil in the digestive tract of earthworms, by the addition of sucrose or xanthan gum to the slurry during the production of artificial casts. The addition of xanthan, but not sucrose, to the slurry increased mechanical dispersion relative to that of the uningested soil in the fresh treatment. Although the production of artificial casts destabilized soil to the same degree as earthworm casts, the artificial casts did not simulate all chemical, biochemical, and microbiological aspects of digestion. Received: 24 November 1995     

Relationships between casts of geophagous earthworms (Lumbricidae,Oligochaeta) and matric potential

A laboratory study was conducted to determine the effects of matric potentials between -25.4 and-2.0 kPa on the mechanical dispersion of fresh and aged casts produced by Aporrectodea caliginosa, A. rosea, and A. trapezoides. We also examined the effects on dispersion of the matric potential at which the air-dried casts were rewetted. Dispersion from subsurface casts up to 6 days old and uningested aggregates was influenced, but dispersion from fresh casts was not influenced, by the matric potential of the soil core from which the samples were collected. Dispersion from subsurface casts and uningested aggregates increased as the matric potential of the soil core increased from -25.4 to -2.0 kPa. Cast dispersion was not influenced by the earthworm species which produced the cast. Little or no clay was mechanically dispersed after the casts had been air-dried and rewetted, and the matric potential at which casts were rewetted had little influence on dispersion.     

A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat or lucerne straw

The mechanisms responsible for stabilization of earthworm casts were investigated in a laboratory study. Earthworms (Aporrectodea caliginosa) were fed soil or soil amended with either ground wheat straw or lucerne hay. Cast material and uningested soil material were incubated for 56 days, and changes in aggregate stability (measured by wet sieving), soil biological activity and macronutrient availability were measured periodically. In general, aggregate stability and microbial biomass C tended to increase during the incubation for both cast and soil material, whereas hot-water extractable carbohydrate content declined. For amended treatments, basal respiration rate and microbial metabolic quotient were large at the first sampling (7 days) but declined rapidly thereafter. There was a transitory increase in extractable P in fresh casts compared with uningested material, a sustained increase in mineral N concentrations but no change in exchangeable K content. For unamended treatments, the casts were less stable than soil material, but this difference diminished during incubation. Drying aggregates before analysing them did not reverse this trend. The casts contained more microbial biomass C than soil material did, but the basal respiratory rate, respiratory quotient and hot-water extractable carbohydrate content were less. By contrast, for wheat- and lucerne-amended treatments casts were more stable than soil material, and the microbial biomass was less at all sampling times. For wheat treatments, respiratory quotient and hot-water extractable carbohydrate content were larger for cast than soil material, but the opposite was the case for lucerne treatments. We attribute the stability of casts in amended treatments to the intimate mixing of part-decomposed organic fragments with comminuted soil particles, binding by microbial mucilage associated with the organic fragments and linking and binding by fungal hyphae.     

麦秸还田和结实期灌溉方式对超级稻籽粒结实和米质的影响

以大穗型超级杂交粳稻‘甬优8号’为材料,设置麦秸还田(麦秸全量还田与麦秸不还田)和结实期灌溉方式(浅水层灌溉、轻干湿交替灌溉和重干湿交替灌溉)两因素试验,研究其对大穗型超级稻籽粒结实和主要米质性状的影响。结果表明:结实期灌溉方式对籽粒结实性状有显著影响。在麦秸还田与麦秸不还田下,与浅水层灌溉相比,轻干湿交替灌溉显著提高了‘甬优8号’的千粒重、结实率、充实度,而重干湿交替灌溉则相反;麦秸还田处理下籽粒结实性状均优于秸秆不还田处理,麦秸还田与结实期轻干湿交替灌溉互作可以改善超级稻‘甬优8号’的结实性状;与麦秸不还田相比,麦秸还田提高了籽粒的整精米率、粗蛋白含量和消减值,降低了垩白率、垩白度、直链淀粉含量、胶稠度和崩解值;与浅水层灌溉相比,轻干湿交替灌溉降低了直链淀粉含量、粗蛋白含量和消减值,提高了整精米率、垩白率、垩白度、胶稠度和崩解值,但重干湿交替灌溉则使米质明显变差;两处理对弱势粒的主要稻米品质的调控作用大于强势粒。在麦秸还田下,结实期轻干湿交替灌溉可以较好地协调‘甬优8号’结实性状和稻米品质间的关系。     

Determination of ATP in soils: Effect of soil treatments

The adenosine triphosphate (ATP) contents of seven soil samples were determined after air-drying, freeze-drying, storage, incubation with glucose or water. The amount of ATP extracted was rapidly reduced after air-drying of the field moist soils, but a short period of wetting of the air-dried soils increased their ATP contents significantly. Addition of an ATP-uncoupler to the air-dried soils indicated that the additional amount of ATP extracted after wetting may not be due to synthesis during the wetting, but from some other processes. Freeze-drying of moist soils reduced the amounts of ATP extracted from soils to a lesser extent than air-drying. Storage of the freeze-dried soils at 25° and — 15°C led to substantial losses of ATP.Incubation of soils with and without glucose rapidly increased the ATP contents of soils, particularly those which had been air-dried previously.Biomass C: ATP ratios in two soils declined during the first few days of incubation and then became relatively constant as incubation proceeded, although significantly different for a loam and a clay soil.     

EFFECT OF LIVING ROOTS OF DIFFERENT PLANT SPECIES ON THE AGGREGATE STABILITY OF TWO ARABLE SOILS

The influence of root growth and activities on soil aggregate stability was investigated using five crop species and two soils. Single plants were grown in pots for 6 weeks or less to minimise any possibility of changes in aggregate stability caused by decomposition of dead roots. Planted soils were compared with fallow controls. Aggregate stability was estimated by a turbidimetric technique (used for fresh and air-dried samples) and by wet sieving (used for air-dried samples only). Root growth of perennial ryegrass and of lucerne for 42 days was generally associated with increases in aggregate stability whether the soil was tested in a fresh or an air-dried condition. These beneficial effects were associated with periodate-sensitive (probably polysaccharide) materials produced in the rhizosphere. Growth of maize, tomato and wheat roots for 25 days decreased the stability of fresh soil aggregates, although the effects of tomato and of wheat were not consistent. However, the deleterious effects of these three species on aggregate stability were not apparent after air-drying. The restabilization of maize soils (relative to fallow controls) on air-drying appeared to be caused by increased stabilization by periodate-sensitive materials. The results suggest that the growth and activities of living roots may be a major factor controlling the overall direction and magnitude of changes in aggregate stability under arable or ley crops.     

Characterization of the water soluble soil organic pool following the rewetting of dry soil in a drought-prone tallgrass prairie

To better understand the nature of the C flush that follows the rewetting of dry soil, we chemically characterized the water soluble pools following rewetting of soil dried to several different water potentials. To assess the impact that historical soil water status has on the size of the rewetting labile soluble pool, a laboratory water stress gradient was applied to soils that were collected from drought-prone and irrigated tallgrass prairie soils. In the laboratory, soils were either incubated at −33 kPa or dried steadily over a 0.6, 1, 2, or 3 day period to −1.5, −4, −15, and −45 MPa respectively. On the 4th day, samples were wetted back to −33 kPa and immediately assayed for soluble, microbial, or respiratory pools of carbon. After extraction, samples were also assayed using NMR, GC-MS, and LC-MS to assess carbohydrate, amino acid, osmolyte and sugar pools. The greater the degree of drying before rewetting was associated with greater concentrations of microbial, soluble and respiratory pools of carbon, increasing by 50, 400 and 250%, respectively, in the most water stressed compared to continuously moist soil. Compared to drought-prone soils, the amount of soluble C released as a result of rewetting was 30 to 50% greater in soils that were irrigated for 11 years. The pool of organics was not completely characterized and only small amounts of TBDMS and TMS derived compounds accounting for 2-4% of the soluble C pool were detected. In contrast, oligosaccharides constituted approximately 20-25% of the sample C. Our results suggest that the flush of C following wetting of a dry soil is not dominated by common microbial osmolytes (e.g. proline, glycine betaine, ectoine, glycerol, mannitol, trehalose). In light of this finding more research is needed to better understand the adaptations that microbial communities utilize to respond to the rewetting of dried soil.     

施用秸秆对土壤有机碳组成和结构稳定性的影响

在室内条件下培养 6个月时随秸秆添加量的增加 ,土壤中总有机碳 (TOC)、焦磷酸钠提取有机碳 (SPPC)、水解碳水化合物 (HDC)、热水提取碳水化合物 (HWC)的含量和水稳性团聚体含量极显著增加 ,粘粒分散率除一个盐土土样外其余 3个土样显著降低。虽然玉米秆和小麦秆分别对TOC、SPPC、水稳性团聚体和粘粒分散性的影响之间没有显著差异 ,但是有小麦秆处理比玉米秆处理HDC和HWC含量高的趋势。TOC、SPPC、HDC和HWC分别与水稳性团聚体显著正相关 ,与粘粒分散率显著负相关 ,但总体而言 ,HWC与团聚体的稳定性更相关 ,HDC和粘粒的分散性更相关。     

Microbial biomass dynamics and soil wettability as affected by the intensity and frequency of wetting and drying during straw decomposition

Water repellency is influenced by soil management and biological process. We carried out a 60‐day laboratory incubation experiment to evaluate the effects of straw amendment, together with the intensity and frequency of wetting and drying (W/D), on microbial processes and water repellency. One W/D cycle consisted of 1.5‐day wetting at −0.03 kPa from the soil core bottom and different drying lengths in a temperature‐controlled laboratory, resulting in different drying intensities. At a regular interval, soil respiration rate (SRR) on drying and wetting, soil microbial biomass C and N (SMB‐C and N), and soil water repellency (SWR) after the wetting were measured simultaneously. Rice straw amendment had a greater effect on SRR, but smaller influences on SMB and SMB‐C : N than W/D frequency and drying intensity. The first W/D caused the largest decrease in soil respiration and the soil respiration recovered partly in the subsequent W/D cycles. The increase in SMB and SMB‐C : N as well as metabolic quotient with W/D frequency and intensity suggested a shift of microbial community from bacterial dominance to fungal dominance. SWR was significantly related to SMB‐C (R²= 0.689, P < 0.001). However, this study was limited to these indirect measurements. Direct measurements of fungal biomass and microbial community are needed in the future. The results suggest that rice straw amendment in dry season may increase C sequestration due to reduced decomposition and stabilize soil structure due to the enhancement of microbial induced water repellency.     

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

Naturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha^–1, 6.2 t ha^–1, and 12.4 t ha^–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha^–1 and 12.4 t ha^–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha^–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.     

Mechanisms of stabilization of earthworm casts and artificial casts

Summary Fresh casts were collected from the earthworm species Aporrectodea caliginosa, and artificial casts were also made. The casts were subjected to ageing, drying-rewetting, and sterilization by hexanol vapour. Clay dispersion was determined, as a measure of the lack of stability of the casts. Two soils were used, the topsoil of a recently reclaimed polder soil in the Netherlands and the topsoil from a South Australian duplex soil. For both soils the fresh worm casts had higher dispersible clay than the artificial casts. During ageing, both types of casts became more stable. There are strong indications that this was mainly due to changes on the surface of the casts. Fungi developed on the surface of 6-day-old worm casts made of Australian soil. This gave a higher stability to the casts compared to artificial casts of the same age without fungal growth. With both types of casts, hexanol inhibited fungal growth on the surface of the casts, reducing the stabilizing effect of ageing. The fungus did not develop on Dutch soil casts until after 42 days, and the development of a higher stability with age was also less marked than in the Australian soil. When the casts were subjected to a drying and rewetting cycle before analysis, they became much more stable than the casts that were analyzed wet. The drying-rewetting cycle removed most of the differences between the worm casts and the artificial casts, and also removed any effects of ageing.Dedicated to the late Prof. Dr. W. Kühnelt     

Influence of temperature and cycles of wetting and drying on modulus of rupture

Effects of temperature and cycles of wetting and drying on soil modulus of rupture were studied for 2 calcareous soils varying in their tendency to form crusts under field conditions. Modulus of rupture decreased with increase in temperature of wetting and drying. The modulus of rupture of a silty clay loam soil from Al-Raid decreased with the increase in the number of wetting and drying cycles, whereas that for soil from Al-Wahda increased during the first 4 cycles of wetting and drying and decreased during the subsequent cycles. The reduction in modulus of rupture under different treatments of wetting and drying was not associated with any improvement in the stability of soil aggregates. Modulus of rupture correlated strongly with the degree of shrinkage of soil briquets. Low shrinkage in soil briquets coincided with a low modulus of rupture.     

Effect of sample pretreatment on aggregate stability measured by wet sieving or turbidimetry on soils of different cropping history

The effects of cropping history (pasture or arable) and sample pretreatment (field-moist, air-dried or air-dried and then tension or vacuum rewetted) on aggregate stability as measured by wet sieving or turbidimetry were compared. When field-moist samples were used there was a tendency for aggregate stability, as measured by wet sieving, to decline with increasing time under arable cropping (i.e. decreasing soil organic matter content). Air-drying samples caused a pronounced decline in stability of soils from under arable management and as a consequence there was a marked decline in stability with increasing time under arable. Use of tension or vacuum rewetted samples resulted in high values of stability which were unaffected by cropping history. For turbidimetry, there was a marked decline in measured stability with increasing time under arable cropping when field-moist samples were used. Air-drying caused an increase in measured stability that was relatively greater for the less stable samples. In comparison with air-dried samples, tension and vacuum rewetting caused a decrease in stability values for relatively unstable soils. It is suggested that, upon air-drying (and contraction of aggregates), additional intermolecular associations were formed between soil constituents thus conferring greater stability on aggregates. This resulted in reduced dispersion (and the release of particles <0.04 mm in diameter) from the surfaces of aggregates and slaked aggregate fragments following rapid rewetting. As a consequence stability as measured by turbidimetry was increased by drying. For aggregates from a predominantly arable history, this stabilization was not great enough to prevent slaking occurring following rapid rewetting, with the formation of a large proportion of stabilized fragments <0.5 mm in diameter. The stability of these aggregates as measured by wet sieving was therefore decreased by drying.     

Shrinkage properties of differently managed clay soils in Finland

Soil cracking is a well-known phenomenon, also seen in clay soils in the boreal climatic zone. This study was carried out to quantify soil shrinkage properties in six differently managed clay soils in Finland (Vertic Cambisols, 51% clay). Cylinder samples (100 cm³) were taken in spring from two depths (0–5 and 5–10 cm), then saturated with water and dried as a function of applied suction. The heights of the sample were measured after each drying step and the volume of soil was calculated assuming isotropic shrinkage. The volume loss by shrinkage at a suction of −50 kPa was 1.6–3.8% and the total shrinkage was 5.2–10.5% of the total soil volume, respectively. All shrinkage curves showed structural shrinkage which occurred in the matric potential range from saturation to around −6 kPa. The shrinkage curves were characterized by minor proportional and wide residual shrinkage zones. Eight of twelve sites showed a steeper shrinkage in the proportional shrinkage zone than the theoretical 1:1 line. Large slope values, up to 3.0, reflect the collapse of inter-aggregate pore space due to shrinkage pressure. The results indicate significant particle rearrangement and structural changes, e.g. structural collapse and changes in inter-aggregate pore space due to shrinkage pressure. Continuous water saturation and variable periods of freezing between spring and autumn are mostly responsible for soil weakness against increasing effective stress as soil dries. It is presumed that shrinkage behaviour will change substantially with increases in drying and wetting cycles.     

A porous-matrix sensor to measure the matric potential of soil water in the field

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water‐filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ?85 kPa. In this paper, we report the development of a porous‐matrix sensor to measure matric potential in the approximate range ?50 to ?300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water‐filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ?80 kPa. The porous‐matrix sensors appeared to give reliable readings of matric potential in soil down to ?300 kPa and also responded appropriately to repeated wetting and drying. This porous‐matrix sensor has considerable potential to help understand plant responses to drying soil.     

Role of soil and residue microorganisms in determining the extent of residue decomposition in soil

The effects of residue (wheat straw or sewage-sludge compost) incorporation in soil and the relative contribution of microorganisms in the residues, or in the soil to decomposition of the added residue, (CO₂ production) was evaluated in an incubation experiment. All residues and soils were adjusted to 33 kPa moisture tension and maintained at 25°C under a constant flow of CO₂-free air for 72 days. Residue decomposition was determined by monitoring CO₂ evolution from the treatments.

Mixing an aged sewage-sludge compost (10%, 224 Mg ha⁻¹) with soil stimulated decomposition of the compost 1.64-fold when compared with any of the localized placements, and indicated that the indigenous soil microorganisms were the major contributors to the transformations of this mature compost. Wheat straw was populated with organisms capable of decomposing readily-available substrates in the straw during the first stage of the decomposition, whereas it appeared that soil organisms contributed to an acceleration of straw decomposition during the final stages. After 65 days approx. 30% of the added wheat straw C had been evolved as CO₂. Soil basidiomycetes doubled the extent of decomposition when the indigenous decomposers in wheat were inactivated by γ-irradiation. Model equations are presented for residue decomposition relative to time.     

Stability of urease in soils

Studies with surface samples of Iowa soils selected to obtain a wide range in properties showed that the following treatments of field-moist soils had no effect on urease activity: leaching with water ; drying for 24 h at temperatures ranging from 30 to 60°C ; storage for 6 months at temperatures ranging from ?20 to 40°C; incubation under aerobic or waterlogged conditions at 30 or 40°C for 6 months. No loss of urease activity could be detected when field-moist soils were air-dried and stored at 21–23°C for 2yr, but complete loss of urease activity was observed when they were dried at 105°C for 24 h or autoclaved (120°C) for 2h. Inactivation of urease in moist soils was detected at temperatures above 60°C.Treatment of field-moist soils with proteolytic enzymes which cause rapid destruction of jackbean urease did not decrease urease activity, but jackbean urease was destroyed or inactivated when added to sterilized or unsterilized soils.Although no decrease in urease activity could be detected when field-moist soils were air-dried, an appreciable (9–33%) decrease in urease activity was observed when air-dried soils were incubated under aerobic or waterlogged conditions. This decrease occurred within a few days, and prolonged incubation or repetition of the drying-incubation treatment did not lead to a further decrease in urease activity. Treatment of incubated air-dried soil with urease or glucose initially increased urease activity to a level exceeding that of the undried soil, but this activity decreased with time and eventually stabilized at the level observed for the undried soil.The work reported supports the conclusions from previous work that the native urease in Iowa soils is remarkably stable and that different soils have different levels of urease activity determined by the ability of their constituents to protect urease against microbial degradation and other processes leading to inactivation of enzymes.     

Rainfall impact effects on ageing casts of a tropical anecic earthworm

We investigated the erodibility of surface casts produced by an anecic earthworm of the Colombian savannahs by means of indoor rain simulations. The kinetic energy applied to samples, 21.62 J minute⁻¹ m⁻², was estimated to be equivalent to 41% of the energy of the more intense period of a local storm. The erodibility of casts was assessed at different stages of their ageing along with the effects of repeated wetting‐drying cycles. Bare soil cores and soil cores of the same size with a cast on their surface (soil + cast) were used as controls. Saturated hydraulic conductivity (HC) was measured to test whether casts enhance soil water permeability. Fresh, almost liquid, individual casts were completely dispersed by a 2‐hour rainfall simulation at a dispersion rate of 0.9 ± 0.5% of sample minute⁻¹. After 5 hours drying at 32°C and 79% relative humidity, casts were as moist as fresh casts (non‐significant Mann–Whitney U‐test) but had a solid appearance and were no longer dispersed by raindrop impact. Under simulated rainfall, dry casts were very slowly fragmented into large aggregates (> 5 mm). The HC of casts was increased by repeated cycles of wetting and drying but unaffected by length of air‐drying and natural ageing, and averaged 7 (3, n = 6), 20 (3, n = 7) and 32 (1, n = 45) cm hour⁻¹ for bare soil, soil + cast and all casts, respectively (standard error, sample size). Except for bare soil, these values were greater than the greatest rainfall intensities recorded locally. The consequences for soil erosion, nutrient losses and water infiltration are discussed.     

Straw decomposition and nitrogenase activity (C2H2 reduction): Effects of soil moisture and temperature

The effects of moisture and temperature on straw decomposition (CO₂ production) and nitrogenase activity (C₂H₂ reduction) were measured in laboratory experiments to evaluate the potential for nitrogenase activity at various times of the year in soils with added wheat straw. Soils collected from two areas (Gunnedah and Cowra) representative of large areas of the wheat belt of New South Wales, Australia, were examined. Straw decomposed over a wide range of temperatures (20–50°C, Gunnedah; 15–45°C, Cowra) and moistures (0.3–2.0 times ?10 kPa water content). Microbial populations that fix nitrogen were adapted to a broad range of temperatures (10–45°C, Gunnedah; 4–40°C; Cowra). However, nitrogenase activity with added glucose occurred at much lower soil moisture contents in the Gunnedah soil (0.5–1.75 times ?10 kPa water content) than in the Cowra soil (1.0–2.5 times ?10 kPa water content). Despite the differences between the soils the results show that there is potential for straw decomposition and nitrogenase activity throughout most of the year.     

Impact of biochars on swell–shrinkage behavior,mechanical strength,and surface cracking of clayey soil

Swell–shrinkage, cracking and stickiness of expansive clayey soils usually lead to their low yield. Improvement of these poor soil physical properties is a key goal for enhancing the crop productivity of expansive clayey soils. This article presents results of a study on the impact of three biochars produced from wheat straw (SB), woodchips (WCB), and wastewater sludge (WSB) on the swell–shrinkage behavior, mechanical strength, and surface cracking of a clayey soil. The soil was treated with biochars at the rate of 0, 20, 40, and 60 g biochar kg^?1 soil, respectively; and incubated for 180 d in glasshouse. Application of biochars decreased significantly (p < 0.01) the coefficient of linear extensibility (COLE) of the soil, the effect of SB being most prominent. The tensile strength (TS) of the clayey soil was originally 937 kPa, which decreased to 458 kPa, 495 kPa and 659 kPa for 6% SB‐, WCB‐, and WSB‐amended soils, respectively. Shear strength tests indicated that biochars significantly reduced cohesion (c) and increased internal friction angle (θ). Biochar significantly reduced the formation of soil surface cracks, surface area, and length of the cracks. The surface area density of cracks in the 6% biochar‐amended soils decreased by 14% for SB, 17% for WCB, and 19% for WSB, respectively, compared with control. The results suggest that biochar can be used as a soil amendment for improving the poor physical properties of the clayey soil, particularly in terms of reduction in swell–shrinkage, tensile strength and surface area density of cracking.