Soil erosion modelling of burned and mulched soils following a Mediterranean forest wildfire

Soil erosion modelling applied to burned forests in different global regions can be unreliable because of a lack of verification data. Here, we evaluated the following three erosion models: (1) Water Erosion Prediction Project (WEPP), (2) Morgan-Morgan-Finney (MMF) and (3) Universal Soil Loss Equation-Modified (USLE-M). Using field plots that were either untreated or mulched with straw, this study involved observations of soil loss at the event scale at a burned pine forest in Central Eastern Spain. The erosion predictions of the three models were analysed for goodness-of-fit. Optimization of the MMF model with a new procedure to estimate the C-factor resulted in a satisfactory erosion prediction capacity in burned plots with or without the mulching treatment. The WEPP model underestimated erosion in the unburned areas and largely overestimated the soil loss in burned areas. The accuracy of soil loss estimation by the USLE-M model was also poor. Calibration of the curve numbers and C-factors did not improve the USLE-M model estimation. Therefore, we conclude that an optimized MMF model was the most accurate way to estimate soil loss and recommend this approach for in Mediterranean burned forests with or without postfire mulching. This study gives land managers insight about the choice of the most suitable model for erosion predictions in burned forests.     

Simulation of soil organic carbon stocks in a Mediterranean olive grove under different soil-management systems using the RothC model

Agricultural soils play a very important role in regulating the carbon dioxide (CO₂) content of the atmosphere, and can behave either as carbon sources or sinks. We have simulated the dynamics of carbon in the soil under different land uses and soil-management systems in a Mediterranean olive grove with the Rothamsted carbon (RothC) model. To this end we chose patches of native vegetation (NV) and two different olive grove soils (chromic calcisols and calcic vertisols) under different soil-management systems: conventional tillage (T), and mulching with shredded olive-pruning debris and residues from olive-fruit cleaning (PD + CR). We measured the clay content, bulk density, soil organic carbon (SOC) and total nitrogen (N) in each patch. The SOC and N values decreased by more than 30% as a result of a change in soil use from NV to T olive grove. After adding PD + CR these values rose once more, even to levels above NV. The RothC model performed well for covered soils (NV and PD + CR) but overestimated the SOC values after the soil use was changed from NV to T olive grove, probably due to high carbon losses caused by erosion, common to T soils in the Mediterranean basin. As a result of mulching the soil with only pruning debris, CO₂ emitted to the atmosphere was reduced by >55% for both soils. Associated with this decrease in the emission rate, RothC estimated a potential carbon sequestration of 0.5 and 0.6 t C/ha/yr for chromic calcisols and calcic vertisols, respectively. The reuse of organic debris generated in the olive grove, such as pruning debris and residues from olive-fruit cleaning, is an efficient way of improving soil properties, diminishing CO₂ emissions and increasing the soil’s capacity to store carbon.     

中国亚热带不同种植制度下的土壤侵蚀

In order to optimise land use systems, to prevent erosion-induced degradation and to restore the degraded red soils in subtropical China, five cropping systems and four agroforestry systems were conducted in red soils with a slope of 7° from 1993 to 1995. The results showed that erosion risk period occurred from April to June, and the annual runoff and the losses of soil and nutrients with sediment were alarming for two conventional farming systems, whereas they were negligible for the farming systems with ridge tillage. Enrichment ratios of the lost soils from erosion were more than 1.20 for all nutrients with much higher values for hydrolysable N and organic matter. Compared with the control, the alley cropping systems also distinctly decreased runoff by 30% or 50%. However, the coverage of soil surface varied with alley cropping systems for the competition of nutrients and soil water, which made a profound difference in runoff. The cropping systems of sweet potato intercropped with soybean, the alley cropping systems and the measures of mulching and ridge tillage were the alternatives for red soil reclamation so as to prevent erosion-induced degradation.     

SCS-CN模型在具有高空间异质性小流域径流预测中的应用

For reasons of simplicity, the most commonly used hydrological models are based on the Soil Conservation Service Curve Number (SCS-CN) model, which is probably a good choice for the estimation of runoff on the Loess Plateau of China; however, the high spatial heterogeneity, mainly caused by a fragmented landform and variations in soil type, may limit its applicability to this region. Therefore, applicability of the SCS-CN model to a small watershed, Liudaogou on the plateau, was evaluated and the most appropriate initial abstraction ratio (Ia/S) value in the model was quantified by the inverse method. The results showed that the standard SCS-CN model was applicable to the estimation of runoff in the Liudaogou watershed and the model performance was acceptable according to the values of relative error and Nash-Sutcliffe effciency. The most appropriate Ia/S value for the watershed was 0.22 because with this modified Ia/S value, the model performance was slightly improved. The model performance was not sensitive to the modification of the Ia/S value when one heavy rainfall event (50.1 mm) was not considered, which implied that the model, using a standard Ia/S value, can be recommended for the Liudaogou watershed because single rainfall events exceeding 50 mm seldom occurred in that region. The runoff amount predicted for the Liudaogou watershed by the SCS-CN model, using the modified Ia/S value, increased gradually with increasing rainfall when rainfall values were lower than 50 mm, whereas the predicted amount increased rapidly when the rainfall exceeded 50 mm. These findings may be helpful in solving the problem of serious soil and water loss on the Loess Plateau of China.     

Analysis of the effects of soil management on runoff generation in olive orchards using a physically based model.

Abstract. A numerical model that incorporates the spatial variability of infiltration, surface storage and resistance to overland flow was developed, calibrated and validated for olive orchards. The model reproduced accurately amounts of runoff used in validation, and predicted runoff in olive orchards managed in different ways, in line with published results. The model was used to analyse the runoff generation in a virtual, 180 m length, 5% steep, olive grove, using 54 different scenarios which combined three different soil types, two tree canopy sizes and nine soil management techniques (four tillage scenarios: freshly or degraded tillage with and without a compacted plough layer; no-till, and four cover crops in strips differing in width and plant density). The results of the numerical experiment showed that no-till had the highest runoff coefficient, while a dense cover crop had the lowest. Recently tilled soils also exhibited some of the lowest runoff coefficients. The effects of increasing soil cover due to a greater tree canopy on runoff were significant and caused by the greater area of high infiltration beneath the canopy. Effects of tree canopy size were less important than the impact of soil management practices on runoff.     

基于前期雨量和降雨历时的SCS-CN模型改进

径流曲线法（Soil Conservation Service Curve Number,SCS-CN）对前期产流条件（Antecedent Moisture Condition,AMC）的概化,导致径流预测出现相应的突然跳跃,同时还忽略了降雨历时作为重要组成部分对地表径流的影响,影响了模型径流预测的精度。密云水库是北京市地表饮用水的重要来源,对其上游流域进行降雨径流预测有着重要的生态意义和经济意义。该研究将SCS-CN模型与前期雨量和降雨历时结合,采用API（Antecedent-Precipitation Index,前期降雨指数）模拟土壤前期水分条件,并且提出了一种考虑次降雨事件中土壤前期雨量和土壤入渗量的静态渗透方程,对SCS-CN模型进行了改进。其中,潜在最大蓄水等于前期土壤水分和土壤潜在蓄水量的和,最大静态渗透速度是流域土壤水分达到蓄满时的静态渗透速度,静态渗透系数是与土壤结构、土地利用等相关的无量纲。利用2006-2010年及2014-2020年石匣流域径流小区的200次降雨径流事件监测结果,对该研究改进的模型与原SCS-CN模型以及两种前人改进的SCS-CN模型进行了校准、验证和性能比较。结果表明,4种径流模型中,该研究改进的模型表现最好,纳什效率系数为0.77,决定系数为0.79,均方根误差为3.21 mm,相比于SCS-CN模型纳什效率系数、决定系数分别提高了319%、97.5%,均方根误差降低了107.5%。参数敏感性分析表明,潜在最大蓄水和静态渗透系数是最敏感的两个参数,最大静态渗透速度的参数敏感性一般,初损率的参数敏感性最差。该研究改进模型在密云水库上游潮白河流域降雨径流模拟中具有一定的适用性,可为其他地区产流计算提供参考依据。     

梯田对黄土区降雨径流过程的影响及SCS-CN模型应用与改进

梯田改变了流域下垫面,对降雨径流过程势必产生影响,径流曲线模型（Soil Conservation Service Curve Number,SCS-CN）在中国黄土区,特别是梯田分布广泛流域的适用性仍存争议。为此,该研究以位于甘肃天水的罗玉沟流域为研究对象,利用梯田修建前后2个时段（1985-1990年、2000-2010年）的降雨径流实测资料,分析梯田修建对降雨-径流过程的影响,验证SCS-CN模型的适用性,进一步考虑梯田修建的影响,针对不同降雨类型分别提出了基于降雨量和平均雨强的模型改进方法。结果表明：1）梯田修建对流域径流过程的影响与降雨强度关系密切。当雨强较小时,梯田显著削减流域洪峰和径流总量,削减幅度均在55%以上;而当降雨量和雨强较大时,梯田削减洪峰和径流总量效果并不显著。总体上,梯田修建后流域平均降雨径流系数显著减小至0.06,减小幅度约54%。2）基于最小二乘法优化与考虑前期土壤含水率修正的SCS-CN模型并不适用于梯田分布广泛流域的径流预报。引入降雨量和平均雨强改进后的模型模拟效果显著提高,此时流域暴雨量级以下和以上降雨事件的初损率分别为0.29和0.02。该研究基于降雨因子改进的模型在梯田分布广泛流域具有一定的适用性,可为流域产流计算及水土保持规划提供参考依据。     

三峡库区坡耕地免耕秸秆覆盖下氮磷流失特征

为有效防治三峡库区坡耕地氮磷流失,在三峡库区长坪小流域连续进行两年野外径流小区监测试验。在自然降雨条件下进行免耕秸秆覆盖(SM)和对照(CK)两个处理的田间试验,研究了免耕秸秆覆盖措施对地表径流和氮磷养分流失的影响。结果表明：(1)降雨量与径流量极显著正相关,径流主要由几次暴雨事件造成,需要重点防范玉米季暴雨造成的水土流失。免耕秸秆覆盖能减少地表产流产沙,相比对照处理,免耕秸秆覆盖减少19.1%的径流流失,减少63.6%的泥沙流失。(2)免耕秸秆覆盖分别降低了21.3%,25.8%的总氮和总磷流失量,径流量和氮磷流失量呈极显著正相关,免耕秸秆覆盖主要通过减少径流量来减少氮磷流失。(3)坡耕地油菜—玉米种植制的氮磷流失风险期为6—7月初,该时期CK处理41.3%的径流量贡献了81.4%,52.1%的总氮和总磷流失,SM处理38.4%的径流量贡献了75.2%,48.2%的总氮和总磷流失,在该时期通过免耕秸秆覆盖可分别减少17.5%,31.7%的总氮和总磷流失。研究表明,免耕秸秆覆盖是控制三峡库区坡耕地氮磷养分流失的良好水土保持措施。     

Erosion processes and erodibility of cultivated soils in North Rhine‐Westphalia under artificial rain. II. Results of field experiments and comparison with laboratory trials

In the densely populated state of North Rhine‐Westphalia, soil erosion by water causes substantial on‐site degradation and off‐site damages. The implementation of soil‐conservation measures is improved, if soil erodibilities and erosion processes are known. In a state‐wide investigation, we aimed to representatively assess soil‐erosion processes and erodibilities of cultivated soils. For this purpose, we measured surface runoff and soil‐loss rates of 28 cultivated soils with field plots under artificial rain. In the field experiments, surface runoff and soil loss indicated high sealing susceptibilities and high erodibilities on soils of quite different textures including a clay silt, a loam silt, a loam sand, a sand loam, and two standard loams. Rill formation causing high soil‐loss rates was observed on a clay silt (soil BM) and on a loam silt (soil RB), the latter yielding an empirical K‐factor of 1.66 t ha^–1 h N^–1. K‐factors of other silty soils ranged from 0.04 to 0.48, whereas sandy soils and clayey soils had K‐factors ranging from 0.00 to 0.32, and 0.00 to 0.12, respectively. Comparatively high erodibilities of two silt clays were due to saturation overland flow. Erosion processes and erodibilities of soils with similar texture varied to a large extent, possibly caused in part by seasonal differences in the timing of erosion tests. Surface runoff was different in field experiments compared with laboratory experiments (companion paper) conducted with topsoil material taken from the field plots. In addition, higher concentrations of suspended sediment were recorded on average in the field than in the laboratory. These differences might reflect the influence of the subsoil and are due to higher transport capacities on longer plots in the field. Thus, laboratory experiments can complement but not replace costly field trials for K‐factor determination. Empirical K‐factors derived from field and laboratory experiments are in general lower than K‐factors of other soils in Germany or calculated K‐factors derived from pedotransfer functions, which might be attributed to a more maritime‐type climate in North Rhine‐Westphalia. Since the temporal variability of erodibility was not assessed, the reported K‐factors should be regarded as preliminary.     

急陡坡土壤侵蚀试验研究

随着各项开发建设项目的大力开展,人为地产生了大量>25℃的急陡坡坡面,造成严重的土壤侵蚀.采用人工模拟降雨方法,进行了急陡坡对土壤侵蚀影响的试验研究.研究结果表明:随着坡度的增加,降雨产流的径流总量和流速随之减少;坡面的降雨侵蚀形式由径流侵蚀为主向击溅侵蚀为主转变,从而导致土壤侵蚀量的波动变化趋势.     

玉米秸秆覆盖对坡面产流产沙过程的影响

通过室内土槽模拟试验,研究不同降雨条件下玉米秸秆覆盖对沙坡地产流时间和产流产沙过程的影响。处理分为8种降雨强度和5个水平的秸秆覆盖度:0、15%、30%、60%和90%。40组模拟降雨结果表明:华北保护性耕作研究区域沙壤土条件下的产流时间与降雨强度符合幂函数关系;秸秆覆盖能延缓地表径流的产生,自然降雨(雨强10~80 mm/h)过程中,15%、30%、60%和90%秸秆覆盖较无覆盖分别推迟产流时间1.0~15.4、2.1~22.1、3.4~48.2和5.9~73.6 min;秸秆覆盖对地表径流和产沙影响显著,降雨历时1 h,30%秸秆覆盖减少径流总量17.9~/o~38.7%,减少产沙总量34.1%~48.0%0 30%秸秆覆盖水土保持效果显著,考虑到过多的秸秆覆盖(80%)会造成播种机堵塞,所以在保护性耕作研究实践中,建议保持30%~60%(1 400~3 100 kg/hm~2)的玉米秸秆覆盖,以达到较好的水土保持效果和播种质量,该研究可为保护性耕作研究区域适宜的玉米秸秆覆盖量提供依据。     

基于改进径流曲线数模型的北京密云坡地径流估算

密云区是北京重要的地表饮用水源地,准确模拟地表径流量,对于分析泥沙和污染物的运移十分重要。近年来,学者们运用径流曲线数(soil conservation service curve number,SCS-CN)模型计算本区地表径流量,但预报精度不理想;未考虑降雨过程和雨强对于产流过程的影响,可能是造成预报误差的重要原因。该文利用密云石匣小流域5个坡面径流小区共201场降雨产流资料,提出次产流径流曲线数计算方法,以改进SCS-CN模型并分析改进后模型模拟效果。结果表明,次产流径流曲线数与多年平均径流曲线数的比值和最大30 min降雨量与次雨量的比值之间呈显著幂函数递增关系,据此提出计算次产流径流曲线数的幂函数方程,以改进SCS-CN模型。当曲线数为0.02时,改进后模型模拟效果最好,效率系数为0.693,明显高于未改进的SCS-CN模型。改进后模型对裸地和耕地的产流模拟效果较好,但对林地的产流模拟效果不理想。今后需在深入分析产流机理的基础上,进一步提出与土壤特性有关的模型参数优化方法。     

径流曲线法在黄土区小流域地表径流预测中的初步应用

径流曲线法是目前国际上预测无径流观测资料地区降水地表产流的主要模型,由于气候、水文及下垫面的差异,在黄土高原地区的应用受到限制。利用黄土高原地区3个小流域的303场降雨径流资料,针对黄土高原降雨地表径流特点优化模型中的初损率λ,并提出降雨强度修正函数,将降雨强度因子引入径流曲线法。优化后的模型效率E达到0．812,实测径流深与预测径流深的线性回归决定系数R2达到0．822。改进后的SCS．CN模型可用于黄土区小流域降雨地表产流预报,对黄土高原无资料地区侵蚀产流预报、指导水土保持工程配置和设计具有重要的理论和工程实践意义。     

干旱半干旱区地表覆盖对土壤风蚀的影响

为了探究地表覆盖对干旱半干旱地区土壤侵蚀及表层特性的影响,探寻农业发展的新机遇,使用Meta分析的方法,通过搜集国内外干旱半干旱地区土壤覆盖已发表的研究数据,分析了3种地表覆盖方式：覆盖植物、秸秆覆盖和高留茬对土壤风蚀模数及表层干容重、含水量和有机碳含量的影响。结果表明：(1)地表覆盖使土壤年风蚀量降低77.96%,地表风蚀量与覆盖度密切相关,秸秆覆盖度最高,减轻风蚀效果最佳。(2)地表覆盖平均降低2.94%表层(0—20 cm)土壤干容重,从而提高孔隙度保水保肥,抑制土壤侵蚀;其中覆盖植物效果最好,能够降低5.26%。(3)地表覆盖能够增加6.26%年均表层土壤含水量,从而有效增大土壤颗粒间的黏聚力;在降水量≤450 mm的半干旱地区该值能达到10.62%。(4)地表覆盖能够提高23.54%表层土壤有机碳含量,增加土壤颗粒间的黏结性,从而提高土体的抗风蚀能力;高留茬的提升效果最好,为34.42%;覆盖植物中豆科作物的效果最好,平均提高30.76%。总之,3种地表覆盖措施均能不同程度地减轻风蚀,能够实现保土、保水和固碳的多重收益,是未来双碳背景下实现农业可持续发展的有效解决方案。     

Exploring the factors influencing the hydrological response of soil after low and high-severity fires with post-fire mulching in Mediterranean forests

Despite ample literature, the influence of the individual soil properties and covers on the hydrological response of burned soils of forests has not clearly identified. A clear understanding of the surface runoff and erosion rates altered by wildfires and prescribed fires is beneficial to identify the most suitable post-fire treatment. This study has carried out a combined analysis of the hydrological response of soil and its driving factors in burned forests of Central-Eastern Spain. The pine stands of these forests were subjected to both prescribed fire and wildfire, and, in the latter case, to post-fire treatment with mulching. Moreover, simple multi-regression models are proposed to predict runoff and erosion in the experimental conditions. In the case of the prescribed burning, the fire had a limited impact on runoff and erosion compared to the unburned areas, due to the limited changes in soil parameters. In contrast, the wildfire increased many-fold the runoff and erosion rates, but the mulching reduced the hydrological response of the burned soils, particularly for the first two-three rainfalls after the fire. The increase in runoff and erosion after the wildfire was associated to the removal of the vegetation cover, soil water repellency, and ash left by fire; the changes in water infiltration played a minor role on runoff and erosion. The multi-regression models developed for the prescribed fire were accurate to predict the post-fire runoff coefficients. However, these models were less reliable for predictions of the mean erosion rates. The predictions of erosion after wildfire and mulching were excellent, while those of runoff were not satisfactory (except for the mean values). These results are useful to better understand the relations among the hydrological effects of fire on one side and the main soil properties and covers on the other side. Moreover, the proposed prediction models are useful to support the planning activities of forest managers and hydrologists towards a more effective conservation of forest soils.     

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。     

黔西喀斯特区秸秆覆盖对坡耕地产流产沙特征的响应

基于2018-2019年野外径流小区定位观测资料,以玉米单作无秸秆覆盖处理(M0)为对照,探索了玉米单作处理和不同秸秆覆盖量(M1~M5,1111,2222,3889,5556,6944 kg/hm^2)对贵州省黔西喀斯特区坡耕地产流产沙特征的影响。结果表明:(1)不同降雨等级下,秸秆覆盖具有较好的水土保持效果,并随覆盖量的增加,产流、产沙总量逐渐减少,但M4与M5处理之间无显著差异(p>0.05);(2)玉米苗期—拔节期各处理产流和产沙量大于后2个阶段,并且各阶段各覆盖处理产流产沙均低于对照;(3)秸秆覆盖对地表产流产沙影响显著,与对照相比,年均径流深减少21.85%~50.46%,土壤侵蚀模数减少50.10%~85.87%;(4)随着秸秆覆盖量的增加,各处理产流、产沙呈下降趋势,但当秸秆覆盖量超过5556 kg/hm^2后,继续增加秸秆至6944 kg/hm^2时,水土保持效果变化不明显。秸秆覆盖是控制喀斯特地区坡耕地水土流失的有效途径,6000 kg/hm^2左右的覆盖量即可起到较好的水土保持效果。     

Burn Effects on Soil Aggregate Stability and Water Repellency of Two Soil Types from East and North Tunisia

To date, evidence of the potential effects of burning practices on soil properties in Tunisia is limited. In order to address this issue, we carried out laboratory investigations of the effects of burning on soil aggregate stability (AS) and water repellency (WR) of a clayey and a sandy loam Fluvisol soils. The treatments included low (100°C, LT), medium (300°C, MT) and high (600°C, HT), heating temperatures. Unburned (0°C, UB) soil samples were used as a control. Two breakdown mechanisms, fast wetting (FW) and mechanical breakdown (MB), were used for the measurement of AS. The latter is expressed by calculating the mean weight diameter (MWD). The water drop penetration time (WDPT) was used to evaluate the soil WR. The results showed that the unburned clayey and sandy loam soils are poorly aggregated. The HT and MT treatments significantly (p < 0.05) increased MWD of both soils, compared to UB samples, following the FW stability test. A lesser increase of MWD was observed with the MB test. The LT treatment did not significantly (p < 0.05) affect the soils AS. For the unburned clayey soil, the FW and MB tests gave significantly (p < 0.05) different MWDs . In contrast, the unburned sandy loam soil had similar MWDs under both tests. As for water repellency, the sandy loam soil was initially wettable and the clayey soil slightly water repellent. Burning treatments did not affect the sandy loam soil behavior but caused a decrease of clayey soil WR.     

How the Soil Moves Upward in the Olive Orchards of NW Syria: Sustainability Analysis of a Local Innovation

This paper analyses a local innovation in response to intense soil degradation in olive orchards of north‐west Syria. Farmers developed a practice consisting of quarrying red clayey soil in valley bottoms and applying this soil to hillslope olive orchards with heavily degraded calcareous soils. A biophysical, economic and social analysis of the practice of soil application identified the opportunities and risks of this innovative soil management technique. On the basis of a pairwise comparison of nine adjacent treated and nontreated orchard plots, soil applications were found to increase soil depth by 36%, soil water availability by 28% and total available soil nutrients: potassium (+45%), nitrogen (+12%) and phosphorus (+6%). Olive yield increased by about 40%. A cost–benefit analysis found this practice to be economically viable within a large geographical area, and farmers scored the practice higher than alternative methods. A socio‐economic analysis revealed its widespread adoption among different farmer types. The positive results of soil applications at the farm level explain its fast adoption. However, potential risks – including the further depletion of soil resources and the transfer of soil‐borne diseases – limit the long‐term sustainability of this locally developed practice. Copyright © 2013 John Wiley & Sons, Ltd.     

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.