耕作对紫色土母（泥）岩成土与侵蚀的对立作用机制

Soil formation and erosion are an opposing process in soil conservation of hillslopes. Although the processes of soil formation and tillage erosion were independently studied previously, direct impacts of tillage on soil formation have not been taken into consideration. This proposal was made based on an understanding of the fact that tillage translocates soil on the slope, and crushes bedrocks and incorporates into the soil layer leading to soil formation in areas with a thin layer of soil. This research will quantify tillage erosion rates by the magnetic tracing and 3D Laser Scanning; determine rock fracturing and soil formation rates by tillage simulation and shear strength methods; and figure out mean annual rates of soil renewal by radionuclides (137Cs and 210Pb) tracing. This project will study rock fracturing and soil formation processes under conditions of soil layer resistance and rock soakage, two major processes in purple soil areas. We will also examine features of soil loss and accumulation in the opposing action of tillage erosion and soil formation due to rock fracturing and explore the mechanism of the offset of rock fracturing for soil loss by tillage. A major innovation of this study will be an integrated understanding of the two processes of soil formation and erosion under the same driving force of tillage. This study will provide the scientific and comprehensive knowledge of soil-forming processes and thus the adoption of effective soil conservation practices in slope landscapes.

成土与土壤侵蚀是坡面土壤保持中两个相反的过程，过去对这两个过程的研究都是独立开展的，并且没有考虑耕作的影响。本项目基于紫色土区耕作既可能导致坡面直接的土壤搬运又可能在浅薄土层区使母岩破碎并成土的认识，在实时田间实验与对长期常规耕作研究的框架下，采用磁性示踪、3D激光扫描法定量确定耕作侵蚀强度；通过模拟耕作、抗剪试验等定量确定母岩破碎与成土速率；并以核素示踪及模型模拟等方法获得多年土壤更新速率。研究长期常规耕作条件下土层厚度对耕作侵蚀的影响；实地研究生产中两种重要的耕作条件—土层阻力与水分浸渍作用下，耕作在母岩破碎及成土中的作用；研究在耕作侵蚀与母岩破碎成土两个对立过程中土壤损失（或累积）特征，揭示耕作对母岩破碎与成土对土壤耕作侵蚀的补偿机制。本项目的主要创新在于探索在耕作的一体化作用下成土与侵蚀的对立作用机制。为科学、全面地认识紫色土成土过程和采取有效土壤保持措施提供新的理论依据。

本项目采用岩土双介质砾石示踪法、母岩钻孔砾石示踪法、基岩染色示踪法、模拟耕作、模型模拟等方法，研究了紫色土坡地耕作在土壤侵蚀与土壤形成两个对立的过程之间平衡机制。源自母岩破碎不同层次的岩屑平均位移距离却随着深度而降低，从上层至下层分别为0.25 m，0.18 m，0.13 m；并且，耕作切入母岩愈深上部层次岩屑顺坡迁移越强烈。对于裸岩耕作，坡度对岩屑传输速率没有显著影响。然而，对于上覆土母岩耕作，随着坡度的增大，岩屑传输速率呈先增大后减小的变化趋势。上覆土层的土壤传输速率Q_sr与覆土厚度呈反相关，这与对纯土耕作的结果相反。. 土壤侵蚀与基质补充平衡机制发生于有两种情景：情景一，当土壤厚度小于锄页深度时，土壤-母岩同步耕作；情景二，首次耕作未触及母岩，但第二次耕作挖掘母岩，即再耕作。对于情景一，为实现坡面土壤侵蚀—补充动态平衡，该过程中母岩破碎产生的岩屑将用于抵消耕作引起的土壤侵蚀与母岩侵蚀，耕作产生的岩屑通量是耕作引起的岩屑传输速率与上覆土壤层的土壤传输速率之和。母岩切入深度d_r与破碎母岩坡长L_r呈反相关，而与上覆土厚度〖 T〗_s呈非线性关系：d_r=(ρ_s·T_s·(-1.399 T_s+0.272))/(ρ_r·(L_r+1.350〖 T〗_s-0.191))，（ ρ_r,ρ_s分别为母岩、土壤干密度）。对于情景二，受首次耕作产生的土坎阻挡，再耕作产生的岩屑与土壤混合物被视为全部用于补充土壤基质。母岩切入深度d_r正相关于首次耕作深度〖 d〗_s与上覆土厚度〖 T〗_s之差，而与破碎母岩坡长L_r无关，即d_r=(ρ_s 〖 (d〗_s-〖 T〗_s))/ρ_r 。. 研究结果的创新性体现在：（1）基于紫色土区强烈耕作的特点，认识了耕作的机械破碎成土过程和机制；（2）基于耕作在成土与侵蚀两个过程中对立作用的研究，揭示了强烈侵蚀下紫色土坡面土壤保持动态平衡的重要机制。耕作通过破碎母岩补充土壤基质，人为加速土壤的形成以之对抗土壤侵蚀，这个过程在阻止或逆转土壤侵蚀中起着积极的作用。在土壤保持措施产生有限效果的情况下，耕作破碎母岩是一个促进土壤形成和维持土壤生产力的主要途径。