2. 中国科学院地理科学与资源研究所黄河三角洲现代农业工程实验室, 北京 100101;
3. 根特大学环境系, 比利时 根特 9000
2. Yellow River Delta Modern Agricultural Engineering Laboratory, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China;
3. Department of Environment, Ghent University, Ghent 9000, Belgium
土壤压实是指非饱和土壤中颗粒重新紧密排列,土壤孔隙度降低,土壤容重增加的过程[1]。压实导致土壤大孔隙数量降低,孔隙形态发生改变,土壤功能受到影响,制约着农业的可持续发展。随着农业机械化的不断推进,农机装备的重型化和密集使用加剧压实土壤面积的扩大和压实程度的增加[2]。当前土壤压实问题已经广泛存在。例如欧洲大约三分之一的土壤出现不同程度的土壤压实问题[3],且18%的农田出现中等程度的深层土壤压实[4];59%的比利时弗兰芒区土壤出现极高的压实风险[5],农户调查发现34%的农民在田间管理中遇到土壤压实问题的困扰[6];在荷兰通过容重、孔隙度、有机质含量和黏粒含量来计算相对容重和相对孔隙度从而推求压实土壤面积,结果显示43%的荷兰农田发生压实[7]。在我国关中平原[8]和华北平原[9-11]调查发现土壤压实问题均已存在;东北黑土区由于其较小的容重造成压实风险相对更高[12-14];南方红壤区同样受到土壤压实的胁迫,低产田平均容重为1.38 g·cm–3,高出中产田14%[15]。
为了解国内外研究进程,基于“Web of Science”核心数据库检索了1995—2020年期间土壤学科以“Soil Compaction”为主题的论文发表情况。检索到相关论文共2 717篇,发文量随时间呈逐渐增加趋势(图 1)。按照国家排名依次为美国、巴西、德国、澳大利亚、法国、加拿大和中国。我国近五年的发文数量增加较快,但发文总量、论文引用数和近五年的平均发文量仍然落后于其他国家。这与彭新华等[16]对比整个土壤物理学国内外发展差异不同,表现为土壤压实方向国内外差距更大,主要归因为这方面我国研究起步晚,对这一问题关注度不高,缺少原创性机理研究。为进一步了解1995—2020年间我国土壤压实研究历程,同时在中国知网(CNKI)数据库中以“土壤压实”为主题词检索,根据检索结果与农田土壤压实的相关性,选择了农业基础科学、农艺学、农业工程、农作物、环境科学与资源、林业、园艺、自然地理学和测绘学、水利水电工程和矿业工程共10个学科方向,筛选出期刊论文199篇。结果显示总体与Web of Science中我国学者发文趋势一致,不同的是发文数量快速增加的时间位点前移,从1997年起国内土壤压实关注度开始提升。这说明我国早期研究成果多发表在中文期刊。
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注:“知网”指基于中国知网(CNKI)检索结果,其他国家基于“Web of Science”核心数据库检索结果。 Note: CNKI refers to searching results based on China National Knowledge Infrastructure (CNKI) database and the rest countries are based on the database of the web of science. 图 1 1995—2020年农田土壤压实排名前7国家发文数量变化 Fig. 1 The number of papers on soil compaction published by the top seven countries between 1995 and 2020 |
利用Citespace软件对上述Web of Science核心数据库检索出的2 717篇文献进行关键词共现性分析,结果如图 2所示。土壤压实(soil compaction)、耕作(tillage)、物理性质(physical property)、容重(bulk density)、有机质(organic matter)、作物生长(growth)、农业管理(management)、作物产量(yield)、深层土壤压实(subsoil compaction)和根系生长(root growth)等关键词出现频率最高,反映出该领域主要研究关注点。图中关键词主要可聚合成4个研究领域:土壤压实的检测、危害、缓解和预防措施。其中土壤压实的检测主要以土壤容重、穿透阻力(penetration resistance)、耕作、土壤强度(soil strength)和土壤空间变异(spatial variability)为核心关键词,且土壤含水量(water content)也作为高频关键词出现。土壤压实的危害围绕土壤有机质、土壤结构(soil structure)、土壤功能(infiltration,permeability and nutrition)、土壤质量(soil quality)和作物生长(growth)展开。土壤压实的缓解主要围绕机械耕作和植物根系两方面展开,同时土壤质量也影响土壤恢复力。土壤压实的预防领域除了涉及到对土壤物理性质和作物产量(yield)的评价,同时出现先期固结压力(precompression stress)和机械轴载(axle load)等与土力学和农业机械相关的关键词,说明土壤预防的研究是土壤和农机紧密结合的交叉领域。此外,土壤压实预防领域出现了深层土壤压实(subsoil compaction)高频关键词,说明机械化过程中深层土壤压实同样引起了一定的关注。
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图 2 1995—2020年土壤压实领域基于Web of Science核心数据库论文关键词的共现关系 Fig. 2 The keywords co-occurring spectral feature of publications on soil compaction during 1995—2020 based on Web of Science Core Collection |
本文从农田土壤压实的检测、危害、缓解和预防四个方面来系统阐述该领域最新前沿研究进展和尚存在的问题,以期促进我国农业机械化进程中农田土壤压实问题的研究,为高标准可持续农田建设提供参考。
1 土壤压实检测土壤压实的检测是判定土壤压实程度的依据。本文从室内和田间检测两方面介绍土壤压实检测方法,其中有些方法在室内和田间均适用。
1.1 室内检测法土壤穿透阻力和容重的测定是土壤压实检测应用最为广泛的两种方法。通过采集原状土样测定土壤容重和穿透阻力,并与非压实区土壤容重对比可衡量土壤压实状况。但容重只反映总孔隙度,并不能反映孔隙的形态指标(如连通性和曲率)。基于计算机断层扫描技术(X-ray CT)和土壤功能指标(如水分入渗率、导水率、水分有效性和通气性)同样用来衡量土壤压实程度。例如,Naveed等[17]利用X-ray CT技术测定土壤压实过程中土壤形变量,同时量化孔隙大小、数量和形态指标变化及其与土壤压实过程中应力大小的关系。Alaoui等[18]从水力学角度综述了土壤水力学性质与土壤压实之间的关系,发现水力学指标同样与土壤压实密切相关。Berisso等[19]通过土壤通气性衡量了由于轮胎剪切力造成的土壤压实程度,发现土壤通气性是土壤压实程度的敏感指标。此外除了穿透阻力以外的其他土力学指标也可间接反应土壤压实程度。通过室内土力学试验测定并计算土壤先期固结压力、压缩系数、压缩指数、回弹指数和抗剪强度等指标,这些指标可反映出土壤对压实的敏感性和恢复能力,也可反映出土壤压实状况。例如研究表明土壤压实会增加土体的抗剪强度,且在黏性土壤中更加显著[20]。可见,土壤压实的室内检测可通过多种指标直接或者间接的反应。
1.2 田间检测法压实土壤田间分布不均一,准确获取压实土壤空间分布是精准应对的前提,可有效降低农业和环境投入成本,也是目前精准农业(Precision agriculture)亟待解决的热点和难点问题。相比室内检测方法,田间检测对方法的易操作性要求更高。因此,一系列快速、易行的测定方法不断涌现。这里主要介绍基于近地传感技术的侵入式机械阻力检测法和地球物理检测法。
1.2.1 侵入式机械阻力检测法侵入式机械阻力检测法是利用压力传感器测定其在土体运动过程中受到的土壤阻力大小,进而指示土壤压实程度。最常用的是手持式野外土壤贯穿阻力仪(Penetrometer),其可快速测定一定深度土体剖面(多为80 cm深)土壤穿透阻力,且数据采集密度高(厘米级)。但是穿透阻力和土壤含水量密切相关,因此在衡量土壤压实程度时需要配合土壤含水量数据进行解释。虽然很多学者尝试开发同时测定土壤穿透阻力和土壤含水量的仪器,但是目前市场上难以获取该类型仪器。总体而言,穿透阻力仪与环刀法测定土壤容重相比极大地提高了检测效率,但该方法大多局限于人工采集,且在压实严重的地区,人力大小有限难以获取完整剖面数据,制约了其应用范围。同时手持式穿透阻力法与容重法一样没有完全解决田间采样分辨率问题。为此一系列附带在农机上的压力传感器应用而生[21-22]。具有代表性的是Mouazen等[23]利用农机携带土壤阻力传感器测定特定深度土壤穿透阻力,同时附带土壤含水传感器测定土壤水分含量,最后通过二者与土壤深度数据建立了预测土壤容重模型,并发现基于该模型预测的土壤容重与传统环刀法测定的土壤容重具有高度一致性[24-25]。该法虽然效率得到了很大程度的提高,且排除了土壤水分的影响,但也存在一次只能针对一个深度进行检测,对深层土壤的检测耗能严重和对于富含石粒的土壤不易实施的缺陷。
1.2.2 地球物理检测法地球物理检测法在农业上是指利用定量的物理方法检测土壤性质。本文主要介绍与土壤压实密切相关且应用较广的三种方法:探地雷达法(ground penetrating radar,GPR),大地电导率仪法(electromagnetic induction,EMI)和电阻率层析成像仪法(electrical resistivity tomography,ERT)。这三种方法均因具有快速、原位、无损、高精度和相对廉价的优点而广泛应用到检测土壤性质上,测定基本原理均是通过发射器与接收器之间穿透土体的电磁信号差异来反推土壤性质[26-28]。其中GPR通过雷达波在土壤中的传播速度来反求土壤的性质,雷达波在土壤中的传播速度主要取决与土壤的介电常数,土壤介电常数又主要受土壤水分含量影响。因此,当土壤压实层水分含量不同于非压实层时,雷达波会在两个界面有一个明显的反射[26]。当然,影响土壤水分含量的因素很多,为了排除非压实因素的干扰往往需要多次测定并配合水文气象信息对雷达波进行解译。例如,Keller等[29]在土壤含水量接近的条件下,分别在压实前和压实后通过GPR扫描田间土壤发现雷达波在压实区域信号相比非压实区土壤显著增强。Wang等[30]通过室内和野外实验发现GPR雷达波速度与土壤含水量和土壤压实程度显著相关。EMI是利用电磁感应现象检测一定体积土壤的电导率来反演土壤性质。ERT是利用土壤电阻率来推求土壤性质。EMI与ERT两者电磁信号互为倒数,测定原理上具有相似性。在检测土壤压实问题上的理论基础是根据Archie定律[31],即土壤表观电导率(ECa)或者电阻率与土壤黏粒含量、土壤含水量和含盐量呈正相关。土壤压实增加土壤容重,单位体积内土壤黏粒含量和土壤含水量(非饱和状态)增加。因此,理论上当土壤其他性质(黏粒含量、含水量和含盐量)变异不大时,ECa可以推求土壤压实程度。许多室内外研究证实了这两种方法在检测土壤压实空间分布的可行性[29,32-33]。例如,在土壤剖面尺度上,利用土壤电阻率可区分机械压实区和非压实区[34-35]。在田间尺度,Hoefer等[36]发现ECa与穿透阻力具有较强的相关性,但前提是影响ECa的其他土壤性质相对均一,否则易出现不同因素的交互作用[34]。例如Ren等[37]在砂壤土中发现由于土壤性质的变异,即使压实程度差异较大的田块也未能发现ECa与土壤压实之间较好的相关性。
2 土壤压实危害土壤压实的影响包含正反两个方面,适当的土壤压实可增加土壤小孔隙数量,提高土壤非饱和导水率[18],进而增加表层土壤持水和保肥能力[38]。但过度土壤压实对土壤质量、作物生长和生态环境均会产生危害。过度的土壤压实导致土壤穿透阻力增加、容重增大,土壤通气和导水大孔隙降低,进而限制土壤气体、水分和养分的运移和再分配,阻碍作物根系的生长,从而降低作物地上部生物量和产量[39]。本文重点讨论土壤压实的危害,并根据危害的来源分为耕层和深层(耕层以下土体)两部分进行论述。
2.1 耕层土壤压实危害耕层是指土壤耕作所能扰动的区域,耕层深度随着耕作类型的不同而具有一定的变化范围。例如在传统铧式犁翻耕(mouldplough tillage)条件下,耕层深度多在20~30 cm左右,对于旋耕(rotary tillage)而言,耕层深度一般只有15~20 cm[40]。耕层土壤是土壤水分、养分、动植物和微生物最为丰富和活跃的区域,也是受农业机械影响最大的土层,因此耕层土壤压实显著影响农业生产和环境。研究表明耕层土壤压实显著降低土壤水分入渗速率,增加坡地水土流失风险[41];压实土壤降低空气扩散率,增加厌氧微生物活性,导致温室气体的排放量增加[42];耕层土壤压实抑制作物出苗,限制作物后期根系生长,进而限制作物吸收土壤水分和养分的能力,最终降低作物产量[43]。然而,目前对于压实条件下土壤水分和养分平衡的理解尚不全面[44]。例如,压实土壤对氮素淋溶的影响目前结论上不统一,这是因为压实一方面减少了土壤大孔隙,进而降低土壤导水率,限制水分养分下渗;但是另外一方面压实也会增加土壤优先流的发生,限制作物根系对水分养分的吸收,加剧水肥渗漏[6]。此外,压实对产量的影响,除了压实程度外,也取决于田间管理、作物类型和品种、耕作制度、以及水肥管理等。例如在我国东北黑土区针对不同品种大豆抗压实能力的研究表明,利用John Deer 90马力的拖拉机压实6次,0~30 cm土壤压实程度显著增加,但测试的25个大豆品种产量受土壤压实的影响表现出极大的差异性,大多品种的产量在压实条件下反而出现增加的现象,这可能是大豆品种和当地气候特征共同决定的[14]。
相比土壤压实对土壤理化性质和作物生长影响的研究,压实对土壤微生物区系的影响研究较少[45-47]。随着高通量测序技术的发展,测序深度的增大,压实对土壤微生物影响的认识逐渐增加。物理生境的变化,特别是孔隙大小分布的变化可改变土壤微生物群落。孔隙度增加有利于增加土壤生物的可居住空间和氧气等资源[48]。然而土壤压实可引起土壤孔隙度下降,限制氧气的扩散,增加CO2的积累,使得土壤中的厌氧微生物活动增加,耗氧微生物活动减少[49-50]。土壤压实增加根系可溶性碳的渗出,进而促进微生物生物量[51-52]。真菌的菌丝在聚集和稳定土壤结构上发挥着重要作用[53-54],而压实对外生菌根种类有不利影响,但腐生和寄生真菌在压实土壤中比例增加。此外压实会增加根部微生物的积累,植物更容易感染和患病[45]。在不同的农业生态系统条件下,土壤紧实度与微生物特性之间关系尚不清楚。
2.2 深层土壤压实危害深层土壤压实降低土壤质量,限制植物水分养分利用空间,增加环境污染的风险,降低土壤对极端气候的抵抗能力[55-56]。同耕层土壤一样,深层土壤压实同样造成土壤穿透阻力增加,孔隙度和连通性降低,最后导致土壤导水通气性降低[56-57]。在砂壤土中研究发现,与非压实的深层土壤相比,同一田块中深层压实区域土壤生物孔隙更多,这可能是由于压实层限制了土壤水汽热的运移,土壤动物需要创造更多的生物孔隙来缓解这种限制[6]。此外,生物孔隙在压实土壤中具有更强的抗压能力,从而增加了其存留时间,长期表现出更多的生物大孔隙。深层压实土壤增加生物大孔隙,进而促进土壤优先流的发生,增加了水分养分下漏,最后造成地下水的污染风险增加[58-59]。深层土壤压实对作物的产量影响单季虽然不及耕层土壤压实显著,但长期仍可造成极大的损失。Håkansson和Reeder[60]根据北欧多地多种作物田间试验总结发现,耕层土壤(0~25 cm)压实造成作物产量下降高达15%,深层土壤压实(40 cm以下)对产量的影响只有5%,但这种影响存在时长可达15年之久。随着全球气候变化,极端天气的出现频率增加[61]。然而深层土壤压实降低了农业对极端气候的抵抗能力。首先,深层土壤压实限制了作物根系向下生长,降低作物的抗倒伏能力。例如,现代农业推行密植机械化收割的过程中,密植栽培措施在深层压实土壤中具有极高的倒伏风险,严重制约了密植技术的推广;其次,深层土壤压实会阻断或者降低土壤水分和养分的上下交换,限制作物对深层土壤水分和养分的利用,最终降低深层“土壤水库”和“土壤养库”的生态服务功能,从而降低作物对极端气候的抵抗能力[62]。因此,充分发挥深层土壤的生态服务功能在未来气候变化中显得极其重要。
3 土壤压实缓解农田压实土壤的缓解方法按照来源分为自然、人为和生物法。自然缓解主要指土壤的干湿和冻融交替。人为缓解方式主要指耕作措施,这里以耕层厚度化分为针对耕层和深层的缓解方式。生物缓解又包括植物、动物和微生物缓解,考虑到全文结构、侧重点和篇幅,这里只介绍植物缓解措施。
3.1 自然缓解自然缓解是指土壤被压实后自然恢复至压实前功能状态的过程[63],其恢复能力除了受土壤自身性质影响外还受土壤干湿和冻融过程影响。土壤质地是自身性质中最基本的因素,Page-Dumroese等[64]发现细质土壤压实后自然恢复进程较慢,而粗质土壤的则相对较快。这可能与黏粒含量的增加导致回弹能力降低有关[65],然而黏粒含量影响土壤回弹能力的机理尚不清楚,有待进一步探讨[66]。土壤有机质在稳定土壤团聚体、调节土壤结构、增加土壤恢复力方面发挥重要作用[66-68]。这可能是因为有机质含量影响了土壤颗粒间的结合强度,继而影响土壤形变[65]。但也有研究表明有机质对土壤恢复力并没有显著影响[69],因此Dexter等[70]建议重新评估有机质在调节土壤物理性质方面的作用。土壤容重也是影响恢复能力的因素,有研究表明土壤的恢复力取决于初始土壤容重大小。土壤容重较小时,压实后土壤颗粒间距离变小,在一定程度上吸引力增强,继而更容易保持现状不易恢复。而土壤容重较高的土壤压实后相对容易反弹,因为压实后土体内部颗粒间的排斥力增强[67,71-72]。可见,以上土壤自身性质同样影响土壤自然恢复过程。
干湿和冻融循环是促进压实土壤恢复的主要自然驱动力[67,73-75]。干湿循环过程中湿润会导致双电层膨胀[67],同时毛管水分上升促使土壤团聚体破裂,形成更多孔隙[76],进而改善压实土壤结构;在土壤干燥过程中,颗粒的回缩导致较粗颗粒之间的细颗粒重新定向和差异沉降,继而造成孔隙结构发生变化并增加土壤孔隙度[77]。土壤冻融循环过程中,冰冻使土壤孔隙水凝固增加约9%的土壤体积,土壤基质破裂产生裂缝;解冻后由于土壤内聚力,裂缝不会完全闭合,从而改变土壤结构[78]。有研究表明与低有机质含量土壤相比,具有高有机质含量的土壤对干湿、冻融循环等自然恢复的响应更强[66]。但土壤自身缓解过程是微弱的,且周期长、效果不佳,难以恢复到最初的状态。
3.2 耕作缓解耕作是人为利用工具疏松土壤的过程。耕作作为地球上扰动最大的工程[79],按照耕作深度又可分为浅层和深层耕作。浅层耕作中的翻耕(Mouldboard ploughing)是被广泛使用的耕作方式,其主要通过将下层土壤上翻来疏松土壤,但越来越多的研究指出翻耕的局限性。Horn等[41]指出人为翻耕虽然能提高土壤孔隙度等指标,但土壤大多仍然为块装结构。Ren等[80]在比利时砂壤土中发现,即使在翻耕后压实处理玉米产量仍然较非压实区降低7%,进一步证实了翻耕不能完全恢复土壤结构的事实。此外,Wang等[81]发现翻耕会降低土壤团聚体的稳定性,增加水土流失的风险;Horn[82]通过对比7年的保护性耕作和传统翻耕发现,翻耕降低土壤容重、饱和导水率和通气性,同时降低土壤抗压强度,进而降低土壤抗压实能力。为此,新的耕作方法相继出现,其中条耕(strip tillage)表现出较强的优势。条耕是只针对播种区域进行耕作,不但降低了对土壤的扰动同时可以有效缓解作物播种行的土壤压实状况。然而目前各地对条耕的效果存在不同的看法,例如Temesgen等[83]比较了条耕与传统翻耕时发现,条耕可提高土壤水分含量,降低土壤侵蚀,增加玉米产量。但是Vyn和Raimbault[84]发现条耕处理下在粉壤土和黏壤土中玉米产量均出现降低的现象。Licht和Al-Kaisi[85]与Ren等[86]发现与翻耕相比,条耕对玉米产量没有显著差异。因此,条耕对于不同地区的农业效应尚需进一步的研究。
针对深层土壤压实产生了多种耕作措施。常见的有深翻(deep ploughing)、深旋耕(deep mixing)和深松(subsoiling or chiseling tillage)。其中深翻类似于翻耕,但耕作深度有所增加。深旋耕是将土壤完全打碎,在缓解土壤压实的同时可能对土壤结构有很大的破坏[87-88]。深松类似于条耕,但耕作深度更深,行距更宽且深松行与播种位置不一定重合。深松相比较同等深度的翻耕或者旋耕,具有对土壤扰动小、耕作耗能低、缓解持续时间久,可为作物营造虚实相间的深层土壤环境等优点[89-90]。这种缓解效果在压实严重土壤中更为明显,例如Ren[6]在压实严重的砂壤土中发现,利用间距为90 cm的深松铲进行50 cm的深松作业后,土壤穿透阻力在深松区显著降低,大约两年后相比非深松区深松仍然可显著改善玉米生长。我国自2009年起开始大力推广深松技术,并在2011年发布《全国农机深松整地作业实施规划(2011—2015年)》。规划指出我国当前需要深松农田面积约为7.2亿亩(1公顷=15亩),每3年进行一次深松作业,5年计划深松面积达10.7亿亩。2016年继续推进深松作业并发布《全国农机深松整地作业实施规划(2016—2020年)》。但是也有研究表明,即使是扰动较小的深松也可能造成深层土壤的二次压实[91-93],这主要是由于深松铲和大马力深松机对土壤的压实作用。
针对单一耕作类型长期使用对土壤质量危害的累积效应,土壤轮耕技术被逐渐重视。轮耕是指不同机械耕作类型的组配,同时兼顾不同种植制度的一种耕作系统[94]。目前大量研究发现轮耕相对单一的耕作类型可显著缓解因耕作产生的土壤压实现象[94-96]。然而轮耕研究周期相对更长,目前为止仍然缺少针对缓解压实土壤的合理轮耕制度及其长期影响的评价。
3.3 植物缓解植物缓解土壤压实主要是指植物利用根系生长过程改善土壤结构。与人为的耕作缓解相比,植物缓解法虽然缓慢,但其具有低耗能、环保和存留时间久等优点[97-98]。在过去几十年里,关于植物根系与土壤结构相互作用关系的认识在不断提高。首先,当土壤阻力较弱时,单一生长的根系施加的轴向力会使土壤颗粒重新排列,结合根系分泌物形成稳定的土壤团聚体[99-100]。其次,当土壤阻力继续增加,轴向生长受阻时,根部会呈放射状径向扩张[101-102]。这使得根系选择从已有孔隙进入土壤,从而增加穿透坚硬的土壤的能力[103]。此外,当根系干枯、死亡和被分解后又会形成土壤生物孔隙,进而增加后茬作物根系在压实土壤中的穿透能力[86]。例如,Gaiser等[104]利用田间试验从土壤水分、植物根系以及植物地上部表型数据,多方位综合证实了前茬苜蓿对后茬小麦深层水分吸收利用的促进作用;同样Wahlström等[105]在田间利用根管法原位检测到后茬作物的根系利用前茬作物根系通道生长的过程。由于根系的低弯曲度和高连通性,根系生物孔隙能急剧增加土壤导水性能[106],增加的土壤水分会降低压实土壤的穿透阻力,从而促进根系穿透压实土壤能力。最后,根部吸水会引起土壤含水率和土壤吸力的空间梯度,导致根鞘外的土壤形成裂缝[107]。这种吸水模式受根系结构的控制,从而影响作物不同生长阶段的土壤裂缝形成[108]。根系作用形成的孔隙中,根系大孔隙具有相对较大的直径和稳定性,对于压实土壤中水分、养分和植物生长极其重要,是压实土壤改善的先锋因子[109-110]。根系造成的垂直大孔隙在一定压力范围内具有提高土壤抗压实能力的作用[111-112],即使是在胀缩性土壤下根系产生的生物孔隙仍然具有提高水分入渗能力,而由干湿交替或者土壤动物产生的土壤大孔隙则容易在胀缩性土壤中消失[113]。
植物根系缓解土壤压实能力受植物种类和品种密切影响。研究表明直根系的作物较须根系的作物具有更强的穿透能力和更强的缓解土壤压实的能力[104,114-115]。深根植物类型能够改善土壤大孔隙[116],穿透犁底层[117]。许多作物均有能力通过自身根系和根际土壤特征来改善压实土壤的结构,但作物对压实土壤的缓解能力即便在同一根系类型作物中也随作物品种和压实度的不同而不同。例如,在东北典型黑土区利用染色法研究发现不同品种大豆对压实土壤入渗能力的改善作用随着大豆育种年代的增加而增加[118]。然而,不同植物种类的根系结构类型与土壤结构改良机制仍然有许多不确定性[119-121],比较不同植物对压实土壤的改良能力仍需进一步研究。例如,利用覆盖作物来缓解土壤压实是当前该领域的重要研究方向之一,然而大多数地区存在覆盖作物类型单一的问题,对不同类型作物改善压实土壤能力尚不明确[122-125]。开发利用不同植物资源来缓解土壤压实具有极大潜力。最新研究显示相对于野生型拟南芥和水稻,对乙烯不敏感的突变体拟南芥和水稻具有更强的穿透压实土壤的能力,说明植物根系穿透压实土壤的能力取决于根部乙烯富集浓度的调控作用,而非传统上认为受限于土壤容重[126],这为植物缓解土壤压实提供了新的思路。
4 土壤压实预防土壤压实负面影响多、存留时间久、缓解代价大等问题,土壤压实的预防极其重要。农田土壤压实主要来自农机行走过程中轮胎或者履带的压实和耕作过程中农机具产生的土壤压实,这里主要讨论由前者造成的土壤压实的预防策略。土地高强度不合理利用和农业机械化率的提高与农机装备重型化导致土壤压实的大范围出现:一方面,土地高强度不合理使用造成土壤自我恢复时间被侵占,易出现土壤压实问题。例如,东北一年一熟地区农业机械年进地次数高达10次之多[13],而一年多熟区作业次数更多,作业强度造成的土壤压实风险更大。另一方面,现代农业机械化进程中农机重量持续增加,对土壤施加的压力不断增加。例如,根据丹麦Dronningborg农机提供商数据发现收割机满载质量从1958年的4.3T增加到了2009年的24.9T[56]。同样全球最大的农业机械制造商美国John Deere(Deere & Company,Moine,IL,USA)公司数据显示,收割机后轮载质量从1960年的1.5T增加至2000年的4T[127]。
预防土壤压实的措施主要可分为以下三类:(1)减少农机的行走次数。减少农机的行走次数可有效降低土壤压实出现的面积和程度。农机行走次数直接决定其对土壤产生压实的区域大小(固定行走除外),而在同一区域内土壤压实程度与农机行走次数呈正相关[13]。这是因为一方面土壤压实过程是土壤固、液、气三相的压缩,固相和气相变化相对短时间可以完成,但是液相变化取决于土壤导水率,往往难以在一次压实完成[128];另一方面,土壤弹性形变可增加土壤吸力,进而增加上层压实土壤对下层土壤水分吸收而增加压实层土壤水分含量,从而进一步增大压实风险[129]。当前以无人机为代表的非机械地表作业在农业上的应用可有效降低农机作业次数。同样,免耕和少耕等保护性耕作措施也具有降低农机行走次数进而降低对土壤的压实。但非机械地表作业在降低行走次数中只局限于施肥和喷药环节,对于产生土壤压实风险最大的机械(如收割机)无法取代,即使在保护性耕作措施中也无法避免。(2)降低农机接触面积。固定机械行走车道(Controlled Traffic Farming,CTF)是其中最主要的方法,CTF可有效降低田间土壤压实出现的面积[130]。然而CTF往往受限于不同农机尺寸和作物类型而难以推广,目前全球应使用范围较小。(3)基于土壤压实过程的预防措施。土壤压实过程中土壤作业条件和农机类型是影响土壤压实风险的主要因素。例如土壤水分作为影响压实极为敏感的因素,且在农业管理中易于调控,选择土壤干燥时进地作业可有效降低土壤压实风险[6]。鉴于前两种预防措施在实际生产中的局限性,推广难度较大,因此基于土壤压实过程优化田间土壤作业条件和农机类型来降低土壤压实风险显得尤为重要。以下围绕土壤压实过程的模型模拟和影响因素,讨论其在预防土壤压实中的作用。
4.1 基于土壤压实过程的模型预警基于土壤压实过程建立土壤压实模型可为田间管理提供土壤压实风险预警。土壤压实模型按照大类分为仿真模型和分析模型。仿真模型又根据对土壤介质的不同假设分为有限元(FEM,将土壤体视为连续的介质)和离散元(DEM,将土壤介质视为分散的颗粒)。仿真模型可以模拟更多情景,例如FEM虽然将土壤视为一个整体,模拟多种非线性情形;DEM将土壤看成离散的颗粒状,可以考虑土壤颗粒间的弹缩和黏结性,更加接近真实的土壤结构,对于砂性土模拟结果更好。然而仿真模型由于其参数较多且获得难度较大的问题限制了其广泛使用[131]。相反,分析模型得益于其参数少且易获取而被广泛使用。目前分析模型大多基于应力传递Söhne公式[132]发展而来,Söhne公式是在Boussinesq公式[133]的基础上通过引入集中系数进而考虑土壤弹塑性,同时将接触面积网格化。其表达式如下:
$ {\sigma _{zz}} = \sum\limits_{i = 1}^{i = n} {{{\left( {{\sigma _{zz}}} \right)}_i}} = \sum\limits_{i = 1}^{i = n} {\frac{{v{P_i}}}{{2\pi {z^2}}}{{\cos }^{v + 2}}\psi } $ | (1) |
式中,z为土壤深度,σzz为接触区在深度z处的垂直应力,i代表将接触面积n等分后每个单元面积,ψ为法向载荷矢量和从点载荷到所需点的位置矢量之间的角度,v为集中系数主要受土壤含水量影响,Pi为接触区中第i个载荷单元(应力传感器)的点载荷。
一个典型的分析模型一般包含以下四个部分(图 3):(1)确定上边界条件,即根据机械特性和土壤性质确定机械与土壤接触面积及其压力分布;(2)根据土体应力传递(Söhne公式)确定土体内应力分布;(3)通过测定或土壤转换函数法推求土壤强度;(4)比较土壤应力和强度大小评估土壤压实风险。一旦得到土壤压实风险数据,便可据此调整农机类型或者选择土壤条件开展田间作业。目前基于Söhne应力传递公式发展出的压实模型有Compsoil [134]、SOCOMO[135]、SoilFlex[136]和Terranimo®[137],其中Terranimo®发展相对较为完善,并逐渐应用到欧洲农业生产中,目前已推出国际版、丹麦版、芬兰版、挪威版、法国版和比利时版。在Terranimo®的基础上,Kuhwald等[138]结合土壤、气象、作物和农业机械数据,提出基于区域尺度的时间动态土壤压实模型,可实时预测指定农机作业时的土壤动态压实风险。可见,土壤压实模型的不断发展必然是农业智能化的重要组成部分,结合天空地实时数据可为预防土壤压实提供科学指导。
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图 3 半经验分析模型主要组成 Fig. 3 A typical structure of a semi-empirical analytical soil compaction risk model |
影响土壤压实的土壤因子主要有土壤含水量、土壤有机质、土壤质地、土壤容重、团聚体稳定性和耕作种植制度等。土壤含水量对土壤压实的影响可从土壤压实曲线(Proctor曲线[139])中获得:对于非砂性土壤,土壤可压缩性随土壤含水量呈现出一个抛物线分布。当土壤含水量低于最适含水量时(下塑限附近)土壤可压缩性随含水量的增加而增加,而当高于最适含水量时土壤可压缩性随含水量的增加反而降低。此外,增加土壤含水量可提高土壤应力传递Söhne公式中的集中系数,使得应力垂直分布更集中,从而增加垂直应力传递深度。当土壤含水量和容重一定时,土壤黏粉粒的增加同样具有增加应力传递深度的作用[129]。
土壤压实与有机质关系最早在土木工程领域受到关注,有机质因降低土壤稳定性而被道路工程领域避免。但在农业领域土壤有机质对土壤压实的影响尚有争议,因为有机质虽然可降低土壤容重或增加土壤含水量来降低土壤的抗压实性[140-141],但也有研究表明土壤有机质可增加土壤团聚体稳定性进而增加土壤抗压实性[68,142]。此外,土壤有机质可通过影响土壤颗粒之间和团聚体内部的结合力、弹性、稀释效应、丝状效应、对电荷和摩擦的影响进而影响土壤的压缩性和恢复力[68,143]。可见土壤有机质对土壤压实的影响具有多因素交互性,其交互关系需要根据具体情况进一步量化。
土壤质地是最基本的因素,砂质土壤因其较小的孔隙度使得其压实风险较小,即便发生压实也较其他土壤恢复快。壤质土出现压实的风险较大,而黏质土压实风险受水分影响明显[41]。在土壤低含水量时,土壤可压缩性随着黏粒含量的增加而增加,但是在高含水量条件下土壤可压缩性反而随着黏粒的增加而降低[65]。然而有研究表明这种变化规律只在黏粒含量低于30%时才适用,过高的黏粒含量对土壤压缩指数无影响[144]。容重增加使得土壤颗粒或团聚体之间接触面积和支撑力增大,进而增加土壤抗压实性[71,145-146],但一些田间试验并未发现显著的相关性[142,147-148],这可能归结于田间其他影响因素的交互作用。耕作降低耕层土壤团聚体稳定性,从而降低土壤抗压实能力。大量研究表明保护性耕作条件下土壤水平和垂直形变均显著大于传统耕作[129]。此外,合理的种植制度可利用根系形成的垂直大孔隙在改善土壤结构的同时增加土壤抗压实能力[112]。
4.3 影响压实的农机因素影响土壤压实过程的农机因素主要有轴载和底盘类型。根据应力传递Söhne公式可知:当接触面积一定时,土壤垂直应力随着轴载或者接地压强的增大而传递更深;当轴载或者接地压强一定时,土壤垂直应力随着接触面积的增加而传递更深(图 4a)。因此,当无法降低轴载的情况下,可以通过改变农机底盘类型来增加接地面积进而降低土壤压实风险。当底盘为轮胎时,可通过调整轮胎气压和轮胎类型降低土壤压实。轮胎气压不仅影响轮胎接地面积,同时影响接地压强的空间分布。在低气压时接地压力分布呈现出‘双峰型’,在高气压时呈现出‘单峰型’分布(图 4b)。轮胎类型包括轮胎种类、宽度和花纹类型。轮胎种类目前市场上主要有斜交轮胎和子午线轮胎,子午线轮胎因其可在更低的气压下工作,从而增加接地面积降低土壤压实而被广泛使用。此外,可直接选择不同轮胎宽度来增加接地面积,达到降低土壤压实目的。例如,Ren[6]在不同土壤条件(耕作和非耕作)下利用不同类型农机(播种机和施肥机)研究发现,不同类型子午线轮胎气压和轮胎类型(宽度)均可显著降低其对土壤结构的损害程度,且调整轮胎气压相对于改变农机其他因素具有操作简单和廉价的优点。此外,Peth和Horn[149]指出轮胎花纹类型同样会影响土壤应力分布。当底盘为履带时,履带土壤接触面积显著大于轮胎土壤接触面积,从而有效降低土壤压实风险。然而,履带式农机也存在接触面压力分布不均匀的问题,减小履带张紧力能够在一定程度上提高其应力分布均匀性[150]。
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图 4 a)土壤垂直应力在不同轮胎宽度(R为轮胎半径)和接地压强(P)下的分布(基于Peth和Horn[149]);b)土壤垂直应力在轮胎土壤接触面模拟分布:低轮胎气压(1.0 bar,上图)和高轮胎气压(2.4 bar,下图)(基于Schjønning等[151]) Fig. 4 Simulated normal stress distribution a)under different tyre widths(R = tyre radius)and ground contact pressures(P)(modified from Peth and Horn [149])and b)under low(1.0 bar, upper)and high(2.4 bar, down)tyre inflation pressure at the tyre-soil interface(modified from Schjønning et al.[151]). |
田间土壤压实状况的检测不仅是农田基本属性的重要普查指标,同时也是当前精准农业发展的迫切需要。田间土壤压实状况的检测研究主要集中在新技术和新方法的探索,然而这些方法目前尚不成熟。地球物理法存在受土壤异质性干扰的问题,如何从方法上削弱或者排除空间异质性带来的影响将是以后土壤压实问题田间检测的重要研究方向。
5.2 土壤压实危害针对耕作层土壤压实的危害国内外做了大量的研究,但是由于土壤类型、气候条件和耕作方式的不同,压实的危害与表现形式并不相同,针对压实类型下土壤水分和养分的运移与模拟研究尚不清晰。相比耕作层,深层土壤压实尚未引起足够重视,充分发挥和保护深层土壤的生态服务功能将是应对气候变化背景下维持农业可持续发展的有效措施。此外,随着生物技术手段的普及与对土壤健康的重视,土壤压实对土壤动物和微生物的影响逐渐成为一个新的研究增长点。
5.3 土壤压实缓解耕作在农业生产中发挥着重要作用,但同时给土壤健康和生态环境带来威胁。土壤压实的缓解措施从耕作方面来看国内外发展差距不大,特别是我国在农业农村部的资助下开展了不同类型耕地合理耕层构建专项,同时开展深松技术的推广。这些专项的开展提高了我国在耕作缓解土壤压实方面的认识,然而目前对不同耕作类型下土壤结构的长期动态监测研究较少。国外针对不同作物类型对压实土壤结构的改善已经做了大量研究,我国虽然自古就注重强调轮作的益处,但是主要从植物营养和病虫害防治的角度出发,对作物改善压实土壤结构的能力研究较少。此外,发掘不同类型作物和利用生物育种技术培育抗压实新品种将是应对土壤压实的重要措施。
5.4 土壤压实预防土壤压实是一种非完全可逆的形变过程,一旦发生很难在短期内消除,因此预防尤为重要。欧美发达国家的经验证实了这一结论,这为我国正在加速发展的农业机械化提供了重要借鉴。然而,目前我国农业机械化发展与农田土壤压实之间的关系还未引起足够重视,这主要因为农业机械和土壤学分布在两个不同的学科,制约了两者的交叉研究;此外我国农业生产者对土壤压实的认识尚且不足,简单的认为依靠耕作措施就可以完全消除土壤压实的危害。虽然国内一些农业机械相关的学者也开始关注农业机械化对土壤压实的危害与模拟[152-154],但尚未形成实际应用于指导农业生产的有力工具。通过合理选择机械和田间土壤条件,利用田间压实预警模型指导农事管理是未来有效降低农田压实风险的重要措施。
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