耕作侵蚀研究述评

张加琼， 杨明义， 刘章， 张风宝

(西北农林科技大学，中国科学院 水利部 水土保持研究所，黄土高原土壤侵蚀与旱地农业国家重点实验室，712100，陕西杨凌)



耕作侵蚀研究述评

张加琼， 杨明义†， 刘章， 张风宝

(西北农林科技大学，中国科学院 水利部 水土保持研究所，黄土高原土壤侵蚀与旱地农业国家重点实验室，712100，陕西杨凌)

摘要：耕作位移和耕作侵蚀主要是在重力作用下，由耕作工具触发的土壤侵蚀；是造成坡耕地土壤重新分布和坡耕地土壤侵蚀的重要过程之一；对坡面地形演化、土壤性质改变、土壤养分流失与重新分布、土地生产力降低、土壤碳储存变化等都有重要影响。在以往研究的基础上，总结耕作侵蚀的基本过程和机制、研究方法、影响因素和侵蚀速率的研究进展，讨论目前研究中的不足与未来可能的研究方向。不同于风蚀和水蚀，耕作侵蚀发生的动力条件是人为影响，而非自然发生的降水或风力；因而，其侵蚀过程和机制、研究方法、影响因素、侵蚀速率分布等均不同于风蚀和水蚀。耕作侵蚀主要受人为和自然因素的影响，人为因素驱动耕作侵蚀发生，坡面是耕作侵蚀的地形基础。人为因素主要有耕作工具特性、耕作方向、速度和深度等；自然因素主要包括坡面的形状和尺寸、地形、坡度和土壤性质等。强烈的耕作侵蚀主要发生在坡面上部和坡面曲率剧烈变化的部位。耕作侵蚀研究主要通过基于示踪技术的实测方法，结合模型预测开展。由于耕作侵蚀、风蚀和水蚀的研究方法各成体系，通用方法较少，因而，多营力侵蚀研究难度巨大。以137Cs为代表的核素在研究水蚀、风蚀和耕作侵蚀中均表现出独特的优势，为区分多营力侵蚀中各种侵蚀的速率和贡献提供了新的可能。

关键词：耕作侵蚀；耕作位移；侵蚀速率；坡面曲率；多营力侵蚀

耕作侵蚀伴随人类对坡地的耕作而产生，然而人类却在很长时间内对耕作侵蚀鲜有认识。直至20世纪中叶才出现耕作侵蚀研究的相关报道[1]，却并未引起研究者们的重视。随着农耕机械的推广使用，20世纪末耕作侵蚀的影响日益凸显，如直观可见的坡面上部某些地物(如农田防护带)附近出现的显著的“跌坎”式高差、表土流失甚至心土裸露现象等[2]，耕作侵蚀研究逐渐进入到系统性研究阶段[3-5]。现有研究主要采用物理(如标记方块)和化学(如核元素)示踪技术结合模型模拟预测的方法，从耕作侵蚀的过程和原理、影响因素、侵蚀速率、侵蚀力、空间分布、环境效应等方面开展，耕作侵蚀研究从定性描述发展到定量估算和预报，并尝试从多营力侵蚀中区分出耕作侵蚀的贡献[3-9]。中国对耕作侵蚀研究起步较晚，最早的报道[10]见于1993年。1999年开始设立耕作侵蚀研究项目，正式开展中国的耕作侵蚀研究[6,11]。基于国际研究者们的研究成果，结合中国坡面耕作的实际情况，国内的研究者已经在耕作侵蚀的研究方法、过程、空间分布、模拟预报、影响因素、环境意义等方面开展了大量研究[11-14]，对中国的陡坡耕作侵蚀有了新的认识，发展了新的研究方法[14]；然而，目前国内耕作侵蚀研究者仍然很少，中国的耕作侵蚀研究还较薄弱。中国人口众多和耕地紧缺的现状更凸显了加强耕作侵蚀研究和有效防治坡地水土流失的重要性和紧迫性。

1耕作侵蚀

较早的耕作侵蚀研究中，研究者们未严格区分耕作位移(tillage translocation)与耕作侵蚀(tillage erosion)，二者常混用。后来的研究中逐渐明确了二者所表示的内容和意义。耕作位移是耕作造成的一般土壤移动，常用土壤的单宽移动量(kg/m)表示。耕作侵蚀是某个特定方向上的耕作位移，在该特定方向上的土壤位移量大于其他方向，从而造成该方向上的土壤净侵蚀，其常用单位面积的土壤质量变化(t/km2)表示，与土壤水蚀的表示方法类似[2]。G.Govers等[3]提出的耕作位移系数k(kg/m)：假设耕作过程中发生的土壤耕作位移是扩散过程，用耕作深度(m)、土壤密度(kg/m3)、坡度(m/m)与土壤位移之间线性回归的斜率表示土壤位移量。此方法简单而准确地表述了耕作位移而被研究者广泛采用。

弄清耕作侵蚀的过程与基本原理是耕作侵蚀研究的重点和关键。耕作侵蚀过程中，造成土壤位移的主导力是重力，此外还包括耕作工具对土壤的拖曳力。耕作过程中土壤的运动可以分为拖曳、分离、滑动和滚动3个阶段。1)拖曳。土壤与耕作工具直接接触，被耕作工具拖曳移动，该过程耕作工具的拖曳力是唯一起主要作用的力。2)分离。土壤在重力作用下，借助拖曳末期获得的动能从耕作工具上脱离。3)滑动和滚动。在重力、土壤之间的摩擦阻力和脱离耕作工具的瞬间获得的动能共同作用下，土壤或土块通过滑动和滚动重新停留在地表[15]。

耕作侵蚀对坡面地形演化、土壤性质和养分、土地生产力、土壤碳库等均有影响：耕作侵蚀导致坡面由波形向平直发展，土壤肥力和生产能力不同程度地下降，坡面上部和下部甚至丧失生产能力[6,16-17]，可耕种的耕地面积减少和作物减产[6，16-18]，土壤性质改变的同时产生空间异质性。此外，土壤重新分布也导致了土壤中碳的重新分布和流失，因为农田土壤在碳存储中起着重要作用[19]，从而可能与全球碳循环相关的气候变化相关[20]。

2耕作侵蚀的主要影响因素

影响耕作侵蚀的因素包含人为因素和自然因素2方面。人为因素指使用耕作工具进行的耕作管理，是造成耕作侵蚀的根本原因，主要包含耕作方向，速度、深度、次数和耕作工具的特性。自然因素主要包含被耕作坡面的地形、坡度、形状、大小和土壤性质[7,21]。

顺坡和横坡耕作是2种常用的耕作方向，此外还有方向介于二者之间的倾斜耕作。在耕作过程中，土壤沿坡下方向、横坡方向、耕作工具前进方向及其垂直方向移动[22]。一般地，对方向单一的耕作，耕作方向朝向坡下时土壤位移最大，朝向坡上时土壤位移最小[5]。实际耕作中，按照一定方向的往返耕作最为常见[23]。往返耕作时，顺坡耕作的土壤位移较横坡大(表1和图1)，倾斜耕作的土壤位移介于二者之间[22-23]。大量研究均表明耕作速度、深度与土壤位移之间存在正相关关系。耕作深度是影响耕作侵蚀的重要因子，其对土壤位移的贡献显著大于耕作速度(表1和图1)。耕作工具的特性不仅直接影响单次耕作的宽度，速度，深度和方向，甚至还制约着耕作方式。常用的耕作工具有机械、畜力或人力驱动的圆盘犁，凿形犁，铧式犁等[21]。在相同的地形和土壤条件下，畜力耕作产生的土壤位移小于机械耕作，人力耕作时最小，因为畜力和人力耕作的动力强度、深度、速度有限，且耕作工具的特性和效率与机械动力时有明显差异[24-25]。

表1　耕作方向、速度、深度和坡度对耕作位移的影响——以机械铧犁耕作为例[23,26]

注：土壤位移为往返耕作一次的总位移。Note：Soil translocation is the total erosion caused by a round trip tillage.

耕作侵蚀往往发生在坡面上部的凸起部位，沉积发生在下部的下凹部位[1]。可见，坡面是耕作侵蚀存在的基础，坡面地形对耕作过程中的土壤侵蚀分布起着决定作用[3]。坡面曲率(slope curvature)能有效表达复杂地形变化的影响而被广泛应用。前人对顺坡方向的坡面曲率(profile curvature)更为重视；当横坡的坡面地形有明显变化时，沿该方向的坡面曲率(plan curvature)也必须考虑[23,27]。坡度的大小决定了重力沿坡面方向分力的大小，直接影响耕作过程中土壤位移的大小，还影响耕作工具的土壤携带能力[28]。研究发现坡度与耕作位移可呈线性相关[29]，在坡度较大的坡面，土壤耕作侵蚀与坡度可呈指数正相关[30]。然而，部分研究者[31-32]认为制约耕作侵蚀的是坡度的变化程度而非坡度的大小；此外，土壤性质也影响耕作侵蚀，主要包括土壤的结构、组成、密度、水分和石块质量分数，这些性质影响耕作过程中需要的耕作力大小，土块大小，土壤颗粒的分散程度等[33-34]。

图1　以机械铧犁顺坡(ud)和横坡(c)耕作时，耕作深度(D)和速度(V)对土壤位移系数的影响[21]Fig.1　Effects of tillage depth (D) and speed (V) on soil translocation coefficient with mouldboard at the tillage directions of up and down (ud) and contour (c)

3耕作侵蚀速率

耕作侵蚀速率常通过野外试验和模型模拟方法估算。野外试验主要是通过耕作过程中使用的物理和化学示踪物质的位移指示耕作位移，计算土壤侵蚀量[35]。其中效果较好的是核素示踪，运用较成熟的是137Cs[5,9,36]。核素示踪既可以减小使用物理示踪物时的测量误差，也可以避免使用化学示踪剂造成土壤污染的可能。137Cs示踪方法通过对比背景值采样点与研究区采样点土壤中137Cs含量的变化，借助模型将137Cs富集或损失量换算成土壤沉积或侵蚀量。137Cs背景值可以从无侵蚀和沉积发生的土壤中获得[37]。基于137Cs的土壤侵蚀模型主要分为经验和理论模型2类。经验模型常用指数函数表示土壤侵蚀量与137Cs损失量之间的关系[38]，由于经验模型大多基于试验数据建立，试验地块的空间尺度局限性制约了经验模型的适用范围[39]；此外，经验模型未区分耕地与非耕地137Cs在土壤剖面中的分布差异，导致其经常低估耕地的侵蚀量却高估非耕地的侵蚀量[40]。理论模型主要有比例模型和质量平衡模型2类。比例模型假设耕作层内的137Cs与土壤完全均匀混合，土壤侵蚀量依据137Cs损失量直接计算，以D.E.Walling等[40]的模型为代表。质量平衡模型不仅考虑了由土壤侵蚀和大气输入造成的137Cs含量随时间的改变，也考虑了土壤侵蚀和137Cs含量的季节变化，张信宝等的简化质量平衡模型是常用的质量平衡模型之一[41-42]。

137Cs示踪技术除应用于耕作侵蚀研究之外，也广泛应用于土壤水蚀和风蚀研究[43-44]。由于水蚀和风蚀特性与耕作侵蚀差异巨大，应用137Cs等核素示踪土壤侵蚀时还应考虑水蚀和风蚀的特性，如风蚀对土壤颗粒的分选[45]、水蚀的侵蚀形态[46]、土壤的扰动状态等。当水蚀或风蚀发生于耕地时，137Cs含量所指示的是耕作侵蚀与水蚀或风蚀的总量[37,42]。此时，137Cs提供的信息不足以将耕作侵蚀速率与水蚀或风蚀速率区分[47]。

使用模型估算/预测耕作侵蚀是另一重要方法。耕作侵蚀模型的不断发展，不仅极大地促进了耕作侵蚀的定量和预报研究，而且为区分多营力侵蚀条件下的耕作侵蚀与水蚀或风蚀提供了可能。耕作侵蚀模型通常依据耕作侵蚀的主要影响因素，以实测数据为基础，建立适宜的算法表达耕作侵蚀量[29,47]；然而，R.G.Kachanoski等[33]认为未基于耕作过程建立的土壤耕作侵蚀估算模型难以如实反映土壤侵蚀情况，并得到研究者们的认可[3,8,20]，因此，基于过程的模型是侵蚀估算模型的主导发展方向。耕作过程中，土壤侵蚀常被视作扩散过程进行模拟[3]，如土壤侵蚀预报模型(TEP)、耕作土壤再分布模型(SORET)、耕作位移模型(TillTM)等[8,20,28，48]。这些模型虽然在模拟的尺度、耕作工具特性和耕作方式等方面不断改进，但均仅适用于单个坡面耕作侵蚀的模拟，且有各自的不足之处。水蚀与耕作侵蚀模型(WaTEM)能够从小流域尺度模拟水蚀和耕作侵蚀的空间分布，且考虑了土地利用类型和地块边界的影响[31]。总体上，目前模型的建立和发展都基于野外观测数据，同时也在野外应用中不断验证[49-50]。此外，随着新技术在研究中的应用，探地雷达等先进技术也尝试应用于耕作侵蚀研究中[51]，为促进耕作侵蚀的定量研究发展提供了新契机。

4不足与展望

目前对土壤耕作侵蚀速率的研究主要包含实测和模型预测2种方法。现有模型中较先进的是基于过程的模型，然而这些过程模型都不够完善，仅适用于部分耕作工具和耕作情况，对实际耕作中的任意耕作工具、模式、组合不具有一般适用性。在未来的研究中，需要在整合前人研究成果的基础上，建立系统、完善、具有广泛适用性的耕作侵蚀预报模型。

对发生在水蚀或风蚀区域的耕作侵蚀，将耕作侵蚀与水蚀/风蚀的贡献区分开，明确耕作侵蚀与水蚀/风蚀之间的相互影响是目前研究尚未突破的难点。若耕作侵蚀发生于水蚀风蚀交错带，问题就变得更为复杂。目前，研究者[46]常把耕作侵蚀和水蚀/风蚀作为独立的过程对待，分别估算或模拟侵蚀速率。与耕作侵蚀—水蚀的研究相比，耕作侵蚀—风蚀研究非常薄弱，目前仅有少量关于耕作对风蚀状态、风蚀量、地表形状等的影响的探究[49,52-53]，尚无关于二者侵蚀速率与空间分布的相关报道。在未来的研究中，应充分发挥核示踪技术同时适用于耕地侵蚀、水蚀和风蚀的优势[54-55]，选用各种适宜的示踪元素，结合实测和模型方法，为区分多营力侵蚀区域内各种侵蚀的贡献提供可能。该方法既避免了对复杂侵蚀过程的研究，也不必考虑各种侵蚀之间的相互影响，对准确表达多营力侵蚀区各种侵蚀的贡献提供了新思路。

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(责任编辑：程云郭雪芳)

A review of tillage erosion research

Zhang Jiaqiong, Yang Mingyi, Liu Zhang, Zhang Fengbao

(State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Ministry of Water Resources of the Chinese Academy of Sciences, Northwest A&F University, 712100, Yangling, Shaanxi, China)

Abstract:Very few studies have so far focused on tillage translocation and erosion although they have presented since human’s cultivation activity started. Tillage translocation and erosion were discovered in the middle of the 20th century but have attracted limited attention among researchers. Scientists rediscovered these processes by the end of the 20st century when visible changes occurred in slope farmlands and systematic research on these processes started since then. Tillage translocation and erosion are triggered by tillage and are mostly influenced by gravity force. They are the main processes that cause the redistribution of soil in farmlands on hill slopes, and are among the main processes for soil erosion on slopes. They also greatly influence the evolution of slope landform, soil properties and nutrients, land productivity, and carbon pool. Based on the results of previous research, the present study summarized the processes and mechanisms, influencing factors, research methods and techniques, and erosion rates of tillage erosion, and then discussed the weakness of the previous work as well as possible research aspects in the future. Unlike wind and water erosions, tillage erosion is caused by human activity on hill slopes, but not by wind or rainfall. Therefore, tillage erosion is different from wind and water erosions on most aspects. Tillage erosion is influenced by both anthropogenic and natural factors. The anthropogenic factors refer to the tillage management of farmlands, which acts as the dynamical conditions of tillage erosion. Such factors mainly include the characteristics of tillage tool, plough direction, speed, and depth. The natural factors mainly include the shape and size of the tilled area, the terrain and gradient of slope and soil properties, in which slope is the terrain base of tillage erosion. Strong erosion by tillage usually occurs at areas near the top of a slope and areas where slope curvature greatly changes. The study methods for tillage erosion differ from those of water and wind erosions owing to the great difference in their characteristics. Tillage erosion is mainly studied using tracer methods and model simulations. Limited number of reports can be found on the erosion rates of water and tillage erosions, and few reports on wind and tillage erosions so far. Further study on erosions by water, wind and tillage at crisscross regions is important to determine the erosion processes and mechanisms at these multiple-force erosion regions. The main obstacle is the lack of general study methods for these processes. Nevertheless, radionuclide tracing techniques (e.g.137Cs) could be well applied to tillage, water, and wind erosions, providing a possibility to separate the rate and contribution of each erosion processes from the total erosion in crisscross erosions by erosion forces.

Keywords:tillage erosion; tillage translocation; erosion rate; slope curvature; crisscross erosion by multiple forces

收稿日期：2015-03-25修回日期： 2015-10-28

第一作者简介：张加琼(1984—)，女，博士，助理研究员。主要研究方向：土壤侵蚀。E-mail: jqzhang@nwsuaf.edu.cn †通信 杨明义(1972—)，男，博士，研究员。主要研究方向：土壤侵蚀和核素示踪。E-mail：ymyzly@163.com

中图分类号：S157.1

文献标志码：A

文章编号：1672-3007(2016)01-0144-07

DOI:10.16843/j.sswc.2016.01.018

项目名称： 国家自然科学基金青年科学基金项目“黄土高原水蚀风蚀交错带土壤风蚀规律的Be-7示踪研究”(41401314)；国家自然科学基金“黄土坡面细沟侵蚀和细沟间侵蚀贡献率变化规律的研究”(41171228)；西北农林科技大学博士科研启动基金“坡度对坡面风水两相侵蚀的影响研究”(2013BSJJ092)