刘丹露,赵媛,2,丁树文,邓羽松,王秋霞,吕国安†

(1.华中农业大学资源与环境学院,430070,武汉;2.广西壮族自治区亚热带作物研究所,530004,南宁)

花岗岩剖面土壤崩解特性与初始含水率的关系

刘丹露1,赵媛1,2,丁树文1,邓羽松1,王秋霞1,吕国安1†

(1.华中农业大学资源与环境学院,430070,武汉;2.广西壮族自治区亚热带作物研究所,530004,南宁)

土壤的崩解特性是南方红土区土壤侵蚀的重要指标,也是开展水土保持工作的重要依据。作者选择花岗岩发育的2个土壤剖面,分别采集淋溶层、淀积层、过渡层和母质层原状土壤样品。利用崩解仪进行崩解实验,测定剖面不同层次土壤崩解特性与初始含水率的关系。结果表明:在不同的初始含水率条件下,土壤崩解特性均表现为淋溶层和淀积层土壤崩解缓慢,过渡层和母质层崩解迅速;初始含水率的递变,对淋溶层和淀积层土壤达到崩解完全所需用时影响较大,对过渡层和母质层土壤影响较小;随着初始含水率的减小,淋溶层和淀积层的最大崩解量逐渐增加,风干状态时可趋于完全崩解;过渡层和母质层均可全部达到完全崩解,初始含水率对其影响较小。研究结果可为治理南方崩岗水土流失提供依据,也为探索崩岗的发生机制提供参考。

花岗岩剖面;崩解特性;初始含水率;崩岗

土壤崩解是土体由于浸水而发生结构分散的现象,是不可逆的过程[1]。曲永新等[2]将软岩的崩解分为泥状崩解、碎片泥状、碎片状和完整不崩解4种类型。郑敏洲等[3]对花岗岩残积土崩解性进行室内崩解实验时发现:当土体含水量越大时,崩解速度越快;对不同颗粒组成的土样进行试验,发现粗颗粒质量越多的土体崩解速率越快,崩解量越大;花岗岩残积土的崩解速度大致分为慢速崩解、快速崩解、再次趋于慢速崩解3个阶段。曾鹏[4]将花岗岩残积体制成不同含水量、不同压实度的扰动土样,进行崩解实验时发现:随压实度增大,孔隙、裂隙和渗透性逐渐减小,土体的崩解性也发生相对减弱;当含水量越大时,崩解的速度(完全崩解平均速度)也越快。颜波等[5]通过对花岗岩风化土崩解特性的研究,认为花岗岩风化土在雨水作用下能够发生崩解,致使土体结构遭到破坏;因此,水在崩解中扮演着重要的角色。从初始含水率的角度,对花岗岩剖面土壤崩解特性的研究较少,所以本实验设置6个不同的初始含水率,利用崩解仪探究花岗岩剖面土壤的崩解特性。

花岗岩剖面土壤的崩解特性与花岗岩特有的崩岗现象联系紧密,同时龛又为崩岗形成和发育的重要阶段[6-9];所以,研究不同初始含水率下剖面土壤的崩解特性,与崩岗龛形成的关系,对于防治崩岗侵蚀及开展水土保持工程项目具有一定的科学意义。

1 研究区概况

湖北省通城县地跨E 113°36＇～114°4＇,N 29°2＇～29°24＇,位于鄂、湘、赣3省交界处。该区域属亚热带季风气候,年均温16.2℃,最低气温为-15.2℃,最高气温为39.7℃。年平均降水量1 550 mm,径流总量9.08亿m3。土壤类型以花岗岩发育的红壤为主,结构较为松散。地带性植物以常绿阔叶与落叶阔叶混交林、亚热带针叶林为主。通城县是湖北省有崩岗现象的县市中,数量最多、分布最集中的典型地区,也是全国崩岗防治的重点县。通城县的崩岗主要发生在低丘的花岗岩风化壳上,有完整的崩岗剖面发生层次,具有很强的代表性。

2 材料与方法

2.1分层与采样

本实验选取位于通城县五里镇的具有南方花岗岩崩岗典型特征的五里崩岗BG1(E 113°46＇28″, N 29°19＇55″)和程凤崩岗BG2(E 113°46＇30″,N 29° 20＇3″)为研究对象,进行重复实验。依据花岗岩风化壳的颜色、质地、植物根系等自上而下将剖面土壤划分为4个层次,分别为淋溶层(A层)、淀积层(B层)、过渡层(BC层)和母质层(C层),并逐一进行采样。崩岗剖面土壤的基本性质见表1。

每层土壤通过控制不同的风干时间,依次设置从风干到饱和逐渐递变的6种初始含水率。每一种初始含水率条件下采集3个土样,其中取2个土样运用崩解仪测定崩解量,1个土样用于测定实际水分含量。每层土壤采集18个土样,所以2个崩岗一共采集144个原状土样;并且依据实验结果,分别绘制出不同层次剖面土壤崩解量与初始含水率的关系图。

表1　崩岗剖面土壤基本性质Tab.1 Basic properties of the soil profiles of collapsing gullies

2.2实验方法

通过崩解指标来反映土体的崩解特性,常用的崩解量化指标为崩解量和崩解速率等。笔者选用崩解量来反映土样的崩解特性。

本实验运用崩解仪测定崩解量。依据土力学原理自制实验所需的崩解仪,其主要由3部分构成:浮筒(250mL量筒);网板(10×10 cm2且孔径1 cm2金属方格网);玻璃水槽(25 cm×25 cm×80 cm)。操作时,首先将土样放在网板中央,迅速地将土样浸入装有自来水的水槽中,记录开始时间及浮筒齐水面处刻度的瞬间稳定读数。根据崩解的快慢,灵活记录崩解过程中不同时间段内浮筒齐水面处的刻度读数。当试样完全通过网格落下,或浮筒齐水面处的刻度读数不再有变化时,记下当前时间及刻度读数,结束实验。

张抒等[10]对崩解量的计算公式进行修正:

式中:At为试样在时间t时的崩解率,%;R0为试验开始时浮筒齐水面处刻度的瞬间稳定读数,cm;Rt为试样在时间t时浮筒齐水面处刻度读数,cm;Re为完全崩解后浮筒齐水面处刻度读数,cm。

通过式(1)得到不同初始含水率下花岗岩剖面土壤的崩解特性值,并绘制出不同初始含水率下的崩解折线图。

3 结果与分析

3.1淋溶层土壤崩解量与初始含水率

淋溶层的崩解性较弱,受初始含水率的影响较大(图1)。当初始含水率较大时,崩解量较小,但当初始含水率减小至一定值后,土体在水中的崩解量迅速增加。对设定的6个不同初始含水率下的崩解量进行比较发现:当BG1在含水率＞12.89%时,崩解量很小,当初始含水率下降为6.25%时,BG1的崩解量发生明显增加,稳定时的崩解量达到50%;同样,BG2在初始含水率＜9.90%时,崩解量发生明显增加。当初始含水率减小至4.69%时,BG2达到风干状态,此时崩解量为12.32%,是饱和含水状态下崩解量的6.06倍。

3.2淀积层土壤崩解量与初始含水率

淀积层的崩解情况相似于淋溶层,崩解量与初始含水率成反比(图2)。BG1在初始含水率为35.59%时,崩解量仅为4.35%;当初始含水率降低到2.89%时,崩解量则增至26.32%。同样BG2在初始含水率为31.93%时,崩解量仅为2.78%;但当初始含水率降低到2.20%时,崩解量则能够达到100%。总体来说,BG1、BG2淀积层的崩解量随着初始含水率的增加,崩解量逐渐减小;但除初始含水率接近风干状态时的崩解量较大外,淀积层在各个含水率下的崩解量都较小,均不超过30%。

图1　BG1、BG2淋溶层在不同初始含水率下的崩解量Fig.1 Disintegration amount of eluvial horizon in different initial soilmoisture at BG1 and BG2

图2　BG1、BG2淀积层在不同初始含水率下的崩解量Fig.2 Disintegration amount of illuvial horizon in different initial soilmoisture at BG1 and BG2

3.3过渡层土壤崩解量与初始含水率

由图3可知,过渡层遇水后在较短时间内便可以达到完全崩解;但是由于初始含水率的不同,过渡层达到完全崩解状态所需的时间也不同。在6种初始含水率条件下,BG1的过渡层土体浸水后均在150 s内完全崩解,BG2的过渡层土体则在250 s内完全崩解。BG1、BG2达到完全崩解所需最短时间都为20 s。当BG1、BG2的初始含水率为31.73%和25.81%时,此时的土样均已经接近饱和状态,崩解完全达到第3阶段所用时间都最长。

3.4母质层土壤崩解量与初始含水率

母质层的崩解状况相似于过渡层,但母质层受初始含水率的影响最小(图4)。除饱和含水率处理的情况之外,其他初始含水率条件下的土体都在40 s内达到完全崩解。当初始含水率达到风干状态时,其崩解速率最快,极短时间便可使崩解量达到100%。初始含水率为1.27%时,BG1在10s内可达到第3阶段;初始含水率为1.40%时,BG2完全崩解也只需15 s。

4 讨论

由4层土壤的崩解曲线图,可将崩解过程分为缓慢崩解、快速崩解、稳定阶段[3]3个阶段。第1阶段耗时较短,且从淋溶层到母质层所需时间依次大幅递减;第2阶段占整个崩解实验的主体,所需时间近似与初始含水率成正比,实验进行中可明显观察到有较大土块从原土体崩落;第3阶段即为实验结束。分析3个阶段的崩解过程,4层土壤的崩解特性差异与其土体结构的物化基本性质密切相关[12]。

图3　BG1、BG2过渡层在不同初始含水率下的崩解量Fig.3 Disintegration amount of transitional horizon in different initial soilmoisture at BG1 and BG2

图4　BG1、BG2母质层在不同初始含水率下的崩解量Fig.4 Disintegration amount of parent horizon in different initial soilmoisture at BG1 and BG2

淋溶层砂粒质量分数较大,BG1和BG2分别为36.28%、38.28%;有机质质量分数也较高,分别为17.32和19.50 g/kg;土壤总孔隙度分别为51.70%和53.21%:所以淋溶层的结构较松散,呈团粒结构。淀积层的崩解量小于淋溶层,是因为淀积层的黏粒质量分数较大,分别为40.73%和38.85%。淋溶层的覆盖使得淀积层结构更为紧实,总孔隙度为4层当中最小,分别为47.55%、46.79%。淋溶层土体浸水时,水迅速浸入大孔隙,土体内原有空气无法排出,空气张力增大使得土体的黏聚力减小。当张力大于黏聚力,土体向外剥落,水的楔力作用使土体沿裂隙崩落[9]。同时淋溶层中的胶结物在水中发生溶解,使其有少量土粒散落。崩解释放被挤压的气体,土体内部应力逐渐稳定,崩解速度也逐渐变缓[10-13],趋于第3阶段。淀积层在崩解的第1阶段,空气被挤压而产生的张力对土体影响较小,水分入渗相对均匀,所以土体崩解的程度较小;第2阶段时,当初始含水率＜3%,表面才产生细小的裂隙,使外部土体崩落;部分胶结物溶解使崩解趋于第3阶段。BG2淀积层的崩解状况有所差异,与其土体本身性质不均一有关[14]。土体中有红、黄斑块,使其在风干时收缩不一,发育有较大的裂隙,所以当水分由裂隙浸入土体,土块易发生大量崩塌[15];但是,总体来说BG1、BG2淀积层的崩解量小于淋溶层。

过渡层和母质层遇水后短时间内崩解量可达100%,其崩解完成速度较淋溶层、淀积层快了约30倍。过渡层的黏粒质量分数较前两层相比,下降至16.53%、8.06%,并且其黏聚力几乎为0,而且土壤总孔隙度较淀积层比分别增加至49.43%、49.06%。母质层的黏粒质量分数分别只占3.68%、4.04%,而且颗粒较粗,孔隙度最大,分别为54.34%,55.47%;所以母质层结构较脆弱,基本上为松散堆砌。过渡层和母质层一旦遇水,可迅速软化崩解。初始含水率接近土壤饱和状态时,水分浸入土体速率发生减慢,所以出现短暂的稳定期[4];但随着水的逐步浸入,土体受到外力作用,其结构便瞬间遭到破坏[3]。土体初始含水率越低,水越容易进入土体发生崩解;所以当初始含水率为非饱和状态时,土体浸入水中,水分快速浸入土体,崩解迅速达到第3阶段[16-18]。过渡层和母质层遇水软化性极强,无论是提供可侵蚀的物质量,还是崩解的强度都大幅高于淋溶层和淀积层,这也是崩岗龛出现在这2个层次的关键;因此,只要水分能够进入花岗岩崩岗的下层土体,逐渐侵蚀形成龛后,崩岗侵蚀的速度和侵蚀量就会大大增加。

5 结论

1)在6种初始含水率条件下,花岗岩土壤剖面的崩解特性均表现为淋溶层和淀积层土壤崩解速率较为缓慢,过渡层和母质层崩解速率相对迅速。

2)在不同的初始含水率条件下,淋溶层和淀积层土壤崩解完全所需要的时间受其影响较大,过渡层和母质层崩解完全所需要的时间受初始含水率的影响较小。

3)对4层土壤崩解特性曲线图进行比较,发现崩解量随时间均呈增加趋势,而过渡层和母质层的崩解变化更为明显,近似于“S”形。当初始含水率逐次递减,淋溶层和淀积层的最大崩解量逐次增加,风干状态时接近完全崩解;但过渡层和母质层受初始含水率影响较小,均可全部达到完全崩解。

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Correlation between initial soilmoisture content and the characteristics of collapsing gullies in granite regions

Liu Danlu1,Zhao Yuan1,2,Ding Shuwen1,Deng Yusong1,Wang Qiuxia1,Lyu Guoan1
(1.College of Resources and Environment,Huazhong Agricultural University,430070,Wuhan,China; 2.Guangxi Subtropical Crops Research Institute,530004,Nanning,China)

[Background]The soil disintegrating characteristics is an important indicator of the soil erosion of red soil in southern China,and also the critical basis for conducting the soil conservation. Tongcheng county is one of the typical granite regions in southeastern Hubei Province,with the largest quantity and the most centralized distribution of collapsing gullies.Samples selected in Tongcheng are representative in the subject.[M ethods]The influences of the correlations between the initial soil moisture contentand the disintegrating characteristics of different soil layers on the occurrencemechanism of the collapsing gullies were analyzed.Soil samples of two 3-meter-TVD(true vertical depth)soil profiles were collected in Tongcheng.According to different criteria such as color and texture of weathering crust of granite,the soil profiles can be divided into 4 different layers from top to bottom, including the eluvial horizon,illuvial horizon,transitional horizon,and parenthorizon.To each horizon, this study set6 experimental groups of 6 different levels of initial soilmoisture from dry to saturation,by controlling the length of air-drying duration,and using disintegration tester to conduct disintegrateexperimentwith the soil samples.[Results]The results proved that at different levels of the initial soil moisture content,soil of eluvial horizon and illuvial horizon disintegrated in a slow progress and the soil of transitional horizon and parent horizon did fast.The initial soilmoisture content had a considerable effect on the speed to complete the disintegration of the eluvial horizon and illuvial horizon,while little impact on the easily-disintegrated soil of the other two layers.Themaximum disintegration also was affected by the initial soil moisture content,the maximum disintegration of the eluvial and illuvial horizon significantly increased with the decrease of the initial moisture content,and the disintegration was approximately complete under air-dried condition.The disintegration of the transitional and parenthorizon was fully completed at any level of initial soilmoisture content,so the maximum disintegration was less affected by the initial soil moisture content.Drawing and analyzing the graphical sheets of the disintegration amount of different horizons in different soil moisture,the graphs showed that the disintegration amount presented an increasing trend with the time varying;the influence of disintegrating time on the disintegration amount of the transitional and parent horizon at different levels of initial soil moisture content appeared S curve.[Conclusions]Hence,it can be inferred that in a granite profile, the soil of the transitional horizon and parent horizon disintegrated more easily and formed the niche, which resulted in the head erosion of the collapsing gully.This provided a basis for further research on the collapsing of granite and laid a foundation for the study of the occurrencemechanism of the collapsing gullies.

granite soil profile;disintegration characteristics;initialmoisture content;collapsing gully

S157.1

A

1672-3007(2016)02-0017-06

10.16843/j.sswc.2016.02.003

2015-09-22

2016-02-20

项目名称:国家自然科学基金“花岗岩红壤优先流及其与崩岗侵蚀发育的关系”(41571258);华中农业大学国家级大学生创新创业训练计划“花岗岩崩岗不同层次土壤可蚀性与抗冲性对龛形成的影响”(201510504021);国家科技支撑计划“红壤崩岗侵蚀区农田质量保护与崩岗治理技术与示范”(2011BAD31B04)

刘丹露(1994—),女,本科生。主要研究方向:水土保持。E-mail:liudanlu@webmail.hzau.edu.cn

简介:吕国安(1957—),男,教授,博士生导师。主要研究方向:农业水资源利用与水土保持。E-mail:glu@mail. hzau.edu.cn