PNAS：现代农业机械像恐龙一样造成底层土壤压实，已经超过了土壤功能的安全机械极限

土壤观察 2022-07-27

The following article is from 农业遥感与作物模型 Author 青山白鹤

重要性：在过去的一个世纪里，机械化改变了农业，大大提高了作物的生产效率。然而，更高的容量增加了农用车的重量。研究表明虽然机械设计保持恒定的表面接触应力，但地下土壤压实的潜在和很大程度上被忽视的威胁已经发展。研究证明，现代车辆引起的高土壤应力现在已经超过了许多耕地土壤的临界机械阈值，导致耕作深度以下的根区土壤慢性压实，并对土壤功能产生不利影响。研究者把现代农具和地球上最重的动物——蜥脚类动物（恐龙）相提并论。研究者假设，这些史前巨兽可能导致了底土的压实，从而出现了支撑它们的土地生产力的悖论。

背景：

文明依赖土壤提供众多的生态系统服务。这些服务始终以维持土壤动物和植物的有利条件为前提。土壤结构作为许多生态、水文和农业功能的中心特征出现，作为生物活动的脆弱脚手架。现代粮食生产的集约化依赖于高效的农业技术措施，这对保持良好的土壤结构带来了越来越大的风险，并对耕地的长期生产力构成威胁。尤其值得关注的是，现代农用车辆的重量不断增加，这可能已经造成了土壤的慢性压实，并可能导致土壤生产力和功能的丧失。与众所周知的(和可见的)土壤表面压实的影响相比，一个潜伏的、在很大程度上被忽视的威胁是每年耕作深度以下的土壤逐渐压实，称为底土压实。来自长期实地研究的证据表明，底土压实很难逆转，并可能损害土壤功能数年至数十年。

该研究分析了联合收割机重量及其相关农用轮胎的历史趋势，表明从20世纪60年代至今，农用车辆的土壤表面应力几乎保持不变，而底层土壤，即植物根部区域的应力则稳步增加。我们的分析表明，大型农用车的设计主要是通过保持恒定的接触面积-重量关系来指导的，这类似于陆地行走动物的足迹大小和身体质量之间的进化尺度关系。然而，由于对植物根系应力扩展的考虑有限，造成了根系土体力学极限的超出，影响了土壤的生态功能。研究者提出了一个底土压实敏感性指数(index of subsoil compaction susceptibility)，以估计的典型土壤应力与土壤强度在0.5 m深度的比值来计算。这一指数的全球地图划定了目前机械化水平可能超过土壤功能安全机械极限的区域。

结果：

根据对机械化水平、平均拖拉机尺寸、土壤质地和气候条件的估计，开发了一个全球地图，用于划定底土压实的敏感性。超过20%的耕地有长期的底土压实风险，这令人震惊的趋势要求我们在设计农业机械时更严格地考虑底土内在的机械限制。由于现代收割机的总重量已经接近在地球上行走的最大动物--恐龙的重量，因此出现了潜在的史前底土压实的矛盾。研究者假设，不受约束的长脚动物的漫游会对土地生产力产生类似于现代农用车的不利影响，这表明减少底土压实的生态策略，包括固定的觅食路径，一定会指导这些史前巨兽。

【文章图表】

Fig. 1. Scaling relationship between footprint size (contact area) and body or vehicle mass. Blue circles represent sauropods (data in SI Appendix, Table S3), and red squares represent agricultural machinery, with tire characteristics given in SI Appendix, Table S2. The orange circles are from Cumming and Cumming, and the light blue line is the scaling law of Michilisens et al., also presented in the inset taken from Clemente et al.. Note the relatively narrow range of surface contact stress that spans the entire range of animals (extant and extinct) and agricultural machinery.

Fig. 2. Subsoil stress but not surface stress is affected by body or vehicle mass. (A) Surface stress and (B) soil stress at a 0.5-m depth as a function of mass per foot or wheel load. Bold black lines are trend lines, and the gray area in A indicates mean value ± SD. Numbers in red indicate the year of a typical combine harvester of that time, blue numbers in italics refer to sauropods (1, Plateosaurus engelhardti; 2 , Morosaurus agilis; 3 , Cetiosaurus sp.; 4, Argyrosaurus superbus; 5 , Prontopodus sp.; 6, Neosodon praecursor; 7, Sauropodichnus giganteus; 8 , Brachiosaurus sp.; 9, cf. Prontopodus birdi; 10, Barosauros lentus; 11, cf. Argentinosaurus).

Fig. 3. Risk of soil compaction has increased over the past six decades in the course of agricultural mechanization. Soil stress is shown in relation to soil strength as a function of soil depth for typical combine harvesters of 1958, 1989, and 2020. The stress/strength profiles for a horse and a sauropod (Prontosaurus birdi) are shown for comparison. Soil compaction is expected for soil stress/soil strength > 1.0. The green area indicates no severe risk of compaction (above the annual tillage depth), and the red area indicates risk of permanent subsoil compaction (below the annual tillage depth). The inset shows the depth until which stress exceeds strength, indicating the progression of critical stress-exceedance depths over time (the horizontal dashed line at a 0.25-m depth indicates the typical tillage

depth), for typical combine harvesters from 1958 to date (red squares), mammals (violet triangles—1, horse; 2, African elephant), and sauropods (blue circles—3, Brachiosaurus sp.; 4, Prontosaurus birdi; 5 , Argentinosaurus).

Fig. 4. Global distribution of subsoil compaction susceptibility expressed as an index relating estimated mean tractor-applied stress to soil strength at a depth of 0.5 m (SCSI). (A) Global map derived from mechanization levels, farm sizes, estimated tractor size, soil texture, and climatic average water content calculated at a 0.1° resolution (SI Appendix, section 7 for details). (B) Fraction of arable land exceeding a value of the SCSI, with SCSI > 1 indicating a likelihood of chronic subsoil compaction. The global fraction of arable land under threat of subsoil compaction exceeds 20% and concentrates in high-income countries with large farms and moist average soil conditions.