张立芸(1981-),女,博士研究生,主要从事坡耕地水土保持研究。E-mail:
水土流失是限制山区坡耕地持续利用的主要问题。为探讨农作物根系固土机理,采用无侧限压缩试验测定了素土、玉米和大豆成熟期根土复合体的抗剪强度和应力应变特性,WinRHIZO(Pro.2019)根系分析系统测定了根系构型特征,分析了根土复合体力学特性与根系特征参数间的关系。结果表明:(1)玉米和大豆根系能显著增强土体抗剪强度(
The area of sloping farmland in central Yunnan accounts for 61.14% of the total arable land area, and its sustainability is affected by serious soil erosion. Thus, it is urgent to study the positive effects of the rational allocation of vegetation on the sloping land space on improving soil erosion and maintaining sustainable agricultural production. About 89.4% of the sloping farmland utilization in the province is for planting crops, and maize and soybean are the main crops in summer. Previous studies have shown that the soil-fixing capacity of vegetation roots plays a significant role in soil and water conservation. This study was conducted to explore the soil-fixing effect of corn and soybean roots and to provide a basis for the calculation of the soil-fixing ability of crop roots.
In this study, a field experiment was designed to have three treatments and a total of 9 experimental plots; i.e. CK (Bare land), MM (mono-maize) and SS (mono-soybean). The unconfined compression tests were used to determine the shear strength and stress-strain characteristics of rootless soil and root-soil composites of maize (
The results indicated that: (1) Compared with rootless soil, the roots of maize and soybean significantly enhance the shear strength of root-soil composite (
The root systems of the two crops could enhance the shear strength of the soil. However, the different root structure types demonstrated different effects on the mechanical properties of the soil. The maize root system with more fine roots and more branches can effectively enhance the strength and restrain the deformation. Thus, fibrous root maize is better than taproot soybeans in holding the surface soil. In the use of sloping farmland, it is possible to prevent soil erosion by rationally arranging fibrous root crops. This study provides a reference for the rational layout of crop planting to prevent soil erosion on slope farmland.
云南省坡耕地面积为472.55万hm2,占耕地面积的69.79%,平均坡度为15.62°,主要土壤类型为红壤、赤红壤和紫色土[
土体抗剪强度的测定方法主要有原位剪切试验[
如前述,现有根系固土研究多集中于林木、灌木和草本植物,对坡耕地上农作物根系固土效应的研究鲜有报道。而云南省滇中地区坡耕地面积占总耕地面积的61.14%,其中坡度为8°~15°的坡耕地在总坡耕地面积中占比最大,约为37.97%,土壤类型以红壤为主[
研究区位于云南省滇中地区昆明市盘龙区松华坝水源保护区内的大摆村,属典型高原山地坡耕地区域,海拔2 234m,年平均气温15℃~16℃,年降水量900~1 100 mm,且主要集中在6—9月,属亚热带季风气候区。夏季主要种植玉米和大豆,两种农作物均以单一种植为主,仅有少量间作;试验小区中心地理坐标为25°2′28.8″N,102°58′39.7″E,平均坡度为10°,土壤类型为山原红壤(质地为粉质黏土),因夏季降雨集中,水土流失严重,无植被覆盖区易形成侵蚀沟,在此进行农作物根系固土研究具有一定代表性。研究区位置示意见
研究区位置图
Location map of the study area
经测定,试验地土壤的机械组成和基本理化性质见
供试土壤的基本特征
Basic physical and chemical properties of the tested soil
颗粒组成 |
土粒比重 |
土壤容重 |
液限 |
塑限 |
塑性指数 |
有机质 |
pH | ||
砂粒/mm |
粉粒/mm |
黏粒/mm |
|||||||
5.03 | 52.42 | 42.55 | 2.75 | 1.35 | 59.70 | 32.90 | 26.80 | 30.15 | 6.29 |
本次试验于2019年开展,数据显示2019年松华坝降雨量1 030 mm,平均气温15.8℃,该年度降雨量、平均气温与多年平均值接近,具有一定代表性。供试作物选用玉米(
试验设3个处理:裸地对照、玉米单作、大豆单作,每个处理3次重复,共9个小区,随机区组排列,各小区规格均为4 m×10 m,坡度为10°。试验小区设置详见
试验小区设置
Layout of experimental plots
按照当地农户常规经验种植模式确定行株距和每穴株数。采用等行距种植模式,玉米行距80 cm,株距25 cm;大豆行距40 cm,株距25 cm;后期间苗,玉米每穴留1株,大豆每穴留5株。
课题组前期研究发现两种作物根系均在成熟期达到最大生物量[
试样制备:用切土器将原状含根土小心切削至要求尺寸:d=61.8 mm,h=125 mm,截面积3 000 mm2,体积375cm3;上下垫滤纸和透水石之后放入内壁涂凡士林的饱和器。切削余土,用烘干法测定样本含水率。
试样饱和:土样放入真空饱和缸饱和10 h以上;饱和后用精度为1/1000的电子天平称重并计算饱和密度、饱和度,试样饱和度大于95%才可进行抗压试验。
采用深圳WANCE试验设备有限公司生产的ETM104B型微机控制电子万能试验机进行试验,试验机选配的载荷传感器最大量程为2KN,试验力示值相对误差为±0.5%,试验力分辨力1/500 000FS,横梁位移示值相对误差为±0.5%,位移分辨力为0.027 μm,电脑自动采集数据。试验过程按照SL237-1999《土工试验规程》进行[
式中,
式中,
含根土与素土黏聚力的差值即为黏聚力增量
无侧限压缩实验结束后,将破坏的土柱放入0.05 mm细筛洗出根系,擦净水分,用精度为1/10 000的电子天平测量鲜重后,将根系样本放置在高透扫描盘中,用EPSON 12000XL扫描仪(光学分辨率2 400×4 800 dpi)在600dpi下进行灰度扫描,用WinRHIZO(Pro.2019)根系分析系统(Regent Instruments,Canada)分析根系长度、平均直径、表面积、体积、根尖数、根分支数、交叉数。将扫描后的根系用烘干法在65 ℃条件下烘干至恒重,称量获得根系生物量。用式(3)~式(6)[
式中,RLD为土柱内根系的根长密度cm·cm–3,表征单位体积土体所含根系长度;RSAD,根表面积密度cm2·cm–3,表征单位体积土体所含根系表面积;RVD,根体积密度cm3·cm–3,表征单位体积土体所含根系体积;RWD,根重密度mg·cm–3,表征单位体积土体所含根系生物量;LEN为土柱内根系总长度,cm;Vs为土柱体积,cm3;
采用Excel 2016处理测定数据并对土体内根系分布特征参数与黏聚力进行回归分析;采用SPSS 23.0软件进行单因素ANOVA方差分析来检验各处理间数据的差异显著性,对各径级根系特征参数、根系构型性状与黏聚力之间进行Pearson相关性分析;使用Origin2018软件作应力应变曲线图。
由
试样自然含水率、干密度、饱和密度、饱和含水率及黏聚力
Water content, dry density, saturation density, saturated water content and cohesion of samples
处理 |
自然含水率 |
干密度 |
饱和含水率Saturated water content/% | 饱和密度Saturated density/(g·cm–3) | 孔隙比 |
饱和度Saturation/% | 黏聚力 |
注:表中数据均为平均值±标准差, |
|||||||
素土 |
26.09±0.77B | 1.37±0.01a | 35.07±0.79b | 1.85±0.04a | 1.005±0.010b | 95.98±1.24 | 12.07±0.86C |
玉米 |
30.31±0.70A | 1.34±0.01b | 36.71±0.49a | 1.83±0.02b | 1.062±0.009a | 95.95±0.49 | 26.27±3.49A |
大豆 |
31.33±0.17A | 1.33±0.00ab | 37.35±0.22a | 1.82±0.01b | 1.075±0.004a | 95.54±0.55 | 20.75±4.74B |
试样中根系特征参数见
不同农作物根系特征与黏聚力增量
Root characteristics of crops and cohesion increment of samples
处理 |
根长密度 |
根表面积密度 |
根体积密度 |
根重密度 |
根系鲜重 |
黏聚力增量Cohesion increment |
玉米Maize | 4.29±1.12 | 0.83±0.18 | 13.26±2.71 | 5.49±1.15 | 17.18±3.52 | 14.21 |
大豆Soybean | 3.89±1.06 | 0.63±0.15 | 7.94±1.84 | 7.64±1.73 | 13.06±2.96 | 10.38 |
为分析黏聚力与根系特征参数之间的相关关系,分别对RLD、RSAD、RVD、RWD与
根系特征参数与黏聚力的回归分析
Regression analysis between cohesion and characteristic parameters of roots
由于根体积与根长、根表面积有密切的函数关系,因此两种根系的RVD与
由
不同径级占根系总长度、总表面积、总体积比例
Ratio of different diameter levels to the total length, total surface area and total volume of the root system
不同径级根系特征、根构型性状与黏聚力的Pearson相关性分析
Pearson correlation analysis between unconfined compressive strength of root-soil complex and root characteristics
根系径级 |
玉米试样黏聚力 |
大豆试样黏聚力 |
||||
根长密度 |
根表面积密度 |
根体积密度 |
根长密度 |
根表面积密度 |
根体积密度 |
|
注:**代表 |
||||||
D≤0.5 mm | 0.819** | 0.814** | 0.310 | 0.775** | 0.793** | 0.802** |
0.5 mm < D≤1 mm | 0.542* | 0.647** | 0.297 | 0.693** | 0.678** | 0.664** |
1 mm < D≤2 mm | 0.435 | 0.416 | 0.371 | 0.350 | 0.332 | 0.304 |
2 mm < D≤3 mm | 0.115 | 0.545* | 0.626* | –0.118 | –0.114 | –0.106 |
3 mm < D≤4 mm | 0.517* | 0.711** | 0.523* | 0.130 | 0.117 | 0.110 |
D > 4 mm | 0.404 | 0.600* | 0.788** | 0.521* | 0.664** | 0.777** |
根尖数Tips | 0.829** | 0.744** | ||||
根分支数Forks | 0.797** | 0.763** | ||||
交叉数Crossings | 0.704** | 0.622* |
根尖数、根分支数、交叉数这3个根构型性状均与黏聚力
为探明两种不同根系结构类型对含根土应力应变关系和强度特性的影响,本文选取了2个不同RWD水平下大豆和玉米根土复合体的应力应变曲线和弹性模量进行分析(
不同土体的应力-应变关系曲线、弹性模量和破坏形态
Stress-strain relationship curve, elastic modulus and failure forms of different samples
曲线RLS表示素土的应力应变关系,S1、M1分别表示大豆和玉米在根系生物量较少时试样的应力应变关系,S1对应RWD为5.19 mg·cm–3、M1的RWD为5.27 mg·cm–3,3个处理均在应变为2%~3%之间达到峰值,其中素土更早达到峰值;此时玉米
在土体破坏形态方面(
为测定农作物根系是否能提高含根土抗剪强度,且不改变土壤的物理性质和使研究结果具有可比性,本文中所有试样全是同一批次在大田获取的原状土。本研究以无侧限压缩试验所得的土体抗剪强度全部以土体黏聚力的形式来体现。结果发现相对于素土,两种作物根系均能显著提高根土复合体的黏聚力
刘定辉和李勇[
除此之外,两种作物的根体积密度、根重密度与土体黏聚力的增长都显著相关(
不同径级的根系在土体中发挥固土效应的机理不同。李建兴等[
根系构型性状在固土机理分析中也尤为重要。嵇晓雷和杨平[
综上,农作物根系增强土体黏聚力的作用机理可以理解为通过根系,尤其是D≤1mm的细根,在土体中延伸、穿插和交织,有效增大了根系与土壤颗粒之间的黏结作用和摩擦阻力,且由于根系具备一定抗拉强度[
为进一步探讨不同根系类型对土体破坏时的固持效应,分析了根系性状与土体应力应变特性之间的关系。Fan和Tsai [
农作物根系可以增强土体的抗剪强度;但不同根系类型对土体强度的增强效应不同,玉米、大豆根土复合体相对素土的黏聚力增量Δc分别为14.21kPa和10.38kPa,须根系玉米对黏聚力的增强效应高于直根系大豆。不同径级的根系在土体中发挥固土效应的机理不同,玉米和大豆根系中D≤1mm的细根对于提高黏聚力c值的贡献最大,其根长和表面积越大,细根在土体中的穿插交织量越大,对土壤颗粒的黏聚串联和拉结作用越显著。根系构型特征显著影响土体的力学特性,玉米根系分支数高于大豆45.44%,且各径级根系分布较均匀,在土体中形成了交错搭接的团状纤维网,受力时更能有效增强土体强度、改善土体的变形特性并约束裂隙扩展。综上,不同农作物中,细根较多、各径级根系分布相对均匀、分支多而密的须根系作物对表层土体的固持能力更强,因此,可利用须根系农作物在空间上的合理配置来减缓水土流失,维持坡耕地可持续利用。
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