黄土丘陵区不同植被下土壤润湿性特征及其与团聚体稳定性的关系

Soil wettability is significantly affected by organic carbon content and composition, moisture content, soil texture and soil process management. In this projective, soil wettability change and its effects on soil aggregates stability are focused under different vegetation. In the field, the WDPT and CST method are used to evaluate the difference of soil wettability and the spatial and temporal variation of wettability under different plant species, this will provide the basis for understanding the distribution of soil water under natural conditions. For the laboratory works, sessile drop and Washburn capillary rise method are used to measure soil wettability from hydrophilic to repellent soil (Soil contact angle (CA) is general more than 10 ° but less than 150 °). The measuring method of soil wettability is optimized according to the interface chemistry principle, especially in that critical wetting soil which is soil contact angle less than 90 °, and the aggregates sizes are determined which could compare with other soils. Fowke’s theory is used to calculate the diffusive and polar free energy of soil aggregates with different sizes, and the properties of soil wettability and soil aggregates are researched from the energy point of view. The infrared spectra of soil organic carbon under different vegetation are measured by Fourier Infrared Transform Spectrometer (FITR) to reveal their chemical composition. The potential wettability index (PWI) are calculated by the characteristics of C-H and C = O band to reveal the relationship between soil organic carbon and wettability. The transfer matrix between dry and wetting soil aggregates is evaluated soil aggregates stability. The relationships among CA, organic carbon content and composition, the surface free energy and aggregate stability are analyzed, this is benificial to understanding the mechanism. Through the project, the results can provide the basis for revealing the soil environment for vegetation restoration.

本项目主要针对不同植被下土壤润湿性变化及润湿性影响团聚体稳定性的机制开展研究。在野外，主要采用WDPT和CST方法评价不同植被下土壤润湿性的差异以及润湿性在时间、空间上的变异。在室内，采用躺滴快速照相（结合Washburn毛管上升法）进行从亲水到斥水土壤全程润湿性研究（土壤接触角（CA）一般在10°-150 °范围）；优化土壤润湿性测定方法、明确土壤测定的团聚体大小，深化对10°<CA<90°的临界润湿土壤认识；用Fowkes理论计算团聚体的色散和极性自由能，从能量角度认识土壤的润湿性与团聚体稳定性。利用FITR测定有机碳的红外光谱，了解其化学组成特点，通过C-H和C=O峰的特征计算潜在润湿能力，揭示土壤有机碳与润湿性的关系。利用干湿团聚体之间的转移矩阵进行团聚体稳定性评价，分析CA、有机碳含量、组成以及团聚体表面自由能、团聚体稳定性之间的关系，为揭示植被恢复对土壤环境过程的影响机制提供依据。

土壤的润湿性反映了土壤颗粒的界面特性，是认识团聚体和土壤与植被关系的关键环节。本项目在野外工作中，研究了不同植被下土壤的斥水性特点，时空变异以及与植被影响关系。在室内工作中，主要研究了润湿性的测定方法、土壤有机碳及其组成的影响，土壤团聚体的孔隙特征以及破裂和稳定性指标。取得的主要研究结果如下：（1）黄土丘陵地区不同植被下土壤的WDPT<60 s，斥水性在轻度及其以下；接触角在53.32~76.28度，属于润湿性土壤。在垂直方向上，土壤的斥水性主要集中于表层的0-6 cm，向下土壤的斥水性锐减。土壤的斥水性存在较强的空间和时间变化，长期干旱之后土壤的斥水性增强，同时存在坡向的差异。（2）随有机质含量增多，土壤接触角变大、润湿性变差（土壤接触角小于90度）。土壤的表面自由能在32.1 -43.5 mJ m-2之间，其中色散自由能为17.0-31.7 mJ m-2，极性自由能为7.7-21.4mJ m-2。随有机质的增加，土壤表面自由能呈降低趋势，其中极性自由能下降显著，而色散力自由能有上升的趋势。（3）通过N2吸附等温线发现不同径级团聚体呈现II型吸附等温线，主要孔径分布于2-200 nm，小于10 nm孔径的孔隙占主导。其中，<0.2 mm 和0.2-0.5 mm团聚体有较高的孔隙体积和相对较大的孔隙直径。（4）土壤团聚体中每个径级在 1740～1600 cm-1 波数范围的亲水性官能团 C=O 吸收峰都较为明显，而 3000～2800 cm-1 波数范围的斥水性官能团 C-H 吸收峰只出现在个别样品的某些径级中。随着土壤中有机质含量的增加，土壤中脂肪族C的含量有少量增加的趋势；对于不同植被其变化趋势不同。（5）快速润湿可以导致表观饱和含水量的降低和残余含水量的升高，产生的空气压缩气爆效应使土壤团聚体崩解随着润湿速率的增加，团聚体稳定性以幂函数的关系衰减。由于团聚体形成等级的不同，0.25~0.5 mm的团具体稳定性最高。由于研究区域的土壤有机碳整体水平较低，有机碳的化学特性对润湿性的影响不显著；但常见的人工林油松、侧柏等林下土壤的斥水等级较高，需要进一步的研究观测研究。