耕作对坡耕地紫色土主要元素垂直与顺坡迁移的作用机制

Effects of tillage on the improvement of soil physical properties have been very well documented previously. Yet little is known about machenisms of soil constituent transfer in relation to tillage erosion. This study will start with determining soil translocation and tillage erosion rates on the slope of purple soil regions using radionuclide tracing, magnetic tracing, long-term tillage simulation, etc., based on the fact that tillage not only loosens soil in situ, but also moves soil on the slope in the tillage direction. We will study stratigraphical characteristics of soil profiles using the Cs-137 radionuclide depth-stratigraphy technique to obtain tillage-induced soil deposition rates; meanwhile, we use the C-13 isotope technique to distinguish new till layers from old till layers, or mixture till layers following a selection of hillslopes representive of long-term tillage in practice (normal tillage). By combining normal tillage with simulating tillage, we study the process and mechanism of the vertical and lateral transfer of soil constituents due to soil redistribution by tillage in areas where erosion or deposition occurs, and mechanisms of the lateral transfer of soil constituents in areas where there is no erosion or deposition. Furthermore, we will explore mechanisms of soil constituent variations within and between landscape positions, and establish the models of relationships among the transfer of soil constituens and landscape parameters, tillage erosion rates, etc. What is one of major innovations is that this study will figure out mechanisms of soil constituent movement and transfer due to tillage erosion, not only from the lateral direction, but also from the vertical direction. Finally, this study will provide an important theory for an integrated and scientifical understanding of soil constituent variability and thus for an adoption of effective field management practices in agricultural ecosystems of purple soil.

耕作对于改善土壤物理性质等的作用和机理早已有很好的认识，但是，有关耕作对土壤元素的迁移作用却较缺乏研究。本项目基于耕作不仅产生土壤原位扰动，而且导致坡面土壤再分配的认识，采用模拟耕作，核素示踪、磁性示踪等方法定量确定坡面土壤位移及侵蚀速率；利用长期自然耕作坡面，以环境核素Cs-137深度成层法确定土壤沉积特征，并采用C-13同位素技术鉴别区分新、旧耕层，或混合耕层。通过自然与模拟耕作的结合，研究耕作及其产生的坡面土壤运动在耕作侵蚀区和沉积区的土壤元素垂直迁移过程与机制，及在非侵蚀/沉积区的顺坡传输作用机制；探索景观部位之内和之间土壤元素的变异机理，建立景观参数、耕作侵蚀速率等与土壤元素垂直与顺坡迁移的关系模式。本研究的主要创新在于不仅从顺坡方向，而且从垂直方向揭示耕作对土壤元素再分配的作用机制。本项目为全面、科学地认识紫色土区农耕地土壤的变异性及采取有效田间管理措施提供重要的理论依据。

以紫色土丘陵区的典型坡地为研究对象，采用环境核素137Cs示踪、磁性示踪、模拟耕作、模型预测等方法，研究了紫色土坡地耕作侵蚀对主要土壤成分的顺坡与垂直迁移的作用机制。传统顺坡耕作使土壤沿下坡方向运动，土壤中的主要成分也随之迁移，但土壤成分在水平方向的迁移过程中同时伴随垂直迁移，垂直迁移方向依土壤成分性质而不同：以土壤有机碳（SOC）为代表的非母质来源土壤成分，耕作的混合作用使其从上至下迁移；以碳酸钙（CaCO3）为代表的母质来源土壤成分则表现为自下而上迁移。在水平迁移与垂直迁移的交互作用下，坡地不同的景观位置（坡位）的土壤剖面显示不同的土壤成分垂直分布模式：上坡部位全剖面土壤成分浓度显著下降，并且从上层至下层浓度分布均一；中坡和下坡部位SOC等浓度在耕层（0-15 cm）下降，全剖面从上层至下层呈降低分布模式。长期耕作后，中坡耕层以下土层的SOC等浓度维持相对稳定，但下坡耕层以下土层的SOC等浓度呈增加趋势。这是由于上坡的低SOC浓度土壤传输至中、下坡位置稀释了耕层原始SOC浓度，而下坡由于土壤的堆积抬高了耕层，具有较高SOC浓度的原始耕层部分地变成了心土层，因而新的心土层较原始心土层有较高的SOC浓度。中坡位置则起着类似“传送带”的作用，将上坡搬运至中坡的土壤及其组分传输至下坡，土壤既无侵蚀也无累积。耕作对土壤的原位混合作用导致耕层SOC等浓度的变化对下覆土层的SOC浓度产生显著影响。土壤团聚体、土壤氮、磷等成分在坡面和剖面中的迁移过程和机制类似于SOC。在短坡（<25 m）景观，耕作产生的剖面土壤成分垂直分布模式的显著变化发生在上坡与下坡之间。结果表明，耕作产生的土壤成分的顺坡迁移强烈地作用于垂直迁移，反过来，各个坡位不同模式的垂直迁移又改变耕层土壤成分的浓度，从而影响土壤成分顺坡分布格局。.耕作方式试验结果表明，长期（29年）逆坡耕作和顺坡耕作坡地土壤有机碳储量的平均值分别为3.7 kg m-2和2.9 kg m-2，土壤有机碳储量前者较后者平均增加了27.9%。特别是在坡顶和肩坡部位，逆坡耕作的土壤有机碳储量比顺坡耕作更高。结果表明保护性耕作通过减小耕作侵蚀使原土壤有机碳强烈损失部位的固碳能力显著地提高。因此，生产实践中，采用和推广保护性耕作措施（如逆坡耕作）是增加土壤有机碳储量并由此而提高土壤生产力的有效途径。