生物质炭添加对土壤斥水性及水分运动过程的影响机理研究

Soil water repellency has significant impacts on soil hydraulic properties and soil hydrological processes, leading to a series of environmental and ecological problems such as soil erosion and land degradation. Moreover, the preferential “finger flow” developing in water repellent soils can provide high-speed pathway for pollutant transportation to the groundwater. Currently, land application of biochar has become a spotlight in soil science study for its potential in long-term soil carbon sequestration and soil fertility improvement. In the previous study, strong hydrophobicity of a type of commercial biochar and the special saturation overshoot characteristics in the soil-biochar mixture profile have been observed. However, there are no clear answers to the questions that how does hydrophobic biochar affect soil water repellency and soil water movement, and what is the underlying physical mechanism. Therefore, the objectives of the proposed project are (1) to investigate the effects of different types of hydrophobic biochar on water repellency of hydrophilic soils; (2) to clarify the biochar effects on soil infiltration and drainage processes via its impact on soil water repellency at both lab and field scales; (3) to discover the underlying physical mechanisms of the special observed water saturation distribution patterns in the soil-biochar mixture profile by using contact angle theory; (4) to accurately simulate the infiltration and drainage processes in the profile of biochar amended soil by using the existed unstable flow models, as well as by establishing a mathematical model containing the soil water repellency parameter (contact angel). The outcomes of the proposed project should be valuable in promoting the understanding of the important role of non-zero contact angle in soil water movement, bridging the gap in the evaluation of biochar effects on soil hydrological processes, as well as improving the risk assessment of pollutant transportation in the biochar amended soils.

土壤斥水性在一些情况下可显著影响土壤水文过程（如“指流”的发生）。具有强斥水性的生物质炭是近些年极受关注的一类土壤改良剂，但添加生物质炭对土壤斥水性及土壤水分运动过程影响的作用机制仍不清楚。本项目拟从生物质炭自身斥水性特征及发生机制入手，通过室内外试验和数值模拟，揭示生物炭添加对土壤斥水性和水力特性的影响机理；探究生物质炭添加对土壤水分入渗及再分布过程的影响；明确“指流”现象在生物质炭改良土壤中的发生条件；以固液接触角理论为基础，揭示“指流”发生的内在物理机制，并构建生物质炭改良土壤中非稳态水流运动的数学模型。研究结果有助于解决生物质炭添加对土壤水文过程影响评估中斥水性考虑不足的问题，对于生物质炭改良农田中污染物迁移风险的评估也有重要参考价值。

本项目在农业废弃物资源化利用和土壤固碳减排背景下，以秸秆生物质热裂解制备的生物质炭产品为研究对象，研究其斥水性特征和对土壤斥水性的影响及作用机理，探究生物质炭引起的斥水性变化对土壤水文过程的影响。本研究选取小麦、水稻和玉米秸秆为生物质炭原料，制备了不同温度（300–700℃）和时长（0.5-5 h）下的生物质炭，通过水滴穿透时间法（Water Drop Penetration Time, WDPT）和固液接触角法（Contact Angle, CA）法确定生物质炭自身斥水性特征，与砂土、壤土、粘土和沙壤土在不同混合比例混合后（1%–5%）的斥水性特征。通过高时空分辨率的影像灰度法，研究了生物质炭–土壤混合体系在不同施用量（1%–5%）、不同点源供水速率（0.5–5 ml/min）下的剖面/孔隙尺度水分入渗和再分配过程中的时空动态；田间尺度上，在定位试验小区内使用电法仪法（Electrical Resistance Tomography）研究漫灌条件下土壤剖面水分入渗和再分布过程中的水分动态。结果表明，传统WDPT法不适于测定生物质炭斥水性，CA法更适于表征其斥水性特征；初始接触角和接触角动态数据表明不同秸秆原料新制备的生物质炭自身均具有较强斥水性，其斥水性特征与温度成反比，炭化时间对斥水性强度影响较小; 生物质炭与不同质地的亲水性土壤混合后均显著提高斥水性，表现为初始接触角大幅提高（亚临界斥水性），且接触角随时间下降速率大幅降低，然而传统斥水性强度分类方法（WDPT > 5s，CA> 90o）不能侦测生物质炭引起的土壤斥水性提高；这种斥水性提高非化学包裹引起，而由斥水性物质在土壤孔隙间形成的的物理隔离机制引起。土壤斥水性的提高显著影响土壤水分运动过程。点源供水条件下，湿润区内入渗过程向非稳态流发展，湿润区边缘出现明显非稳态特征，湿润区内部高含水量积累区下移，表现出指流特征，剖面水分分布不均匀性大大提高。田间ERT试验也发现剖面水分分布不均匀性提高。研究结果表明，生物质炭施很可能引起指流发生，加速物质淋溶和水分分布不均匀性，对农业生产造成负面影响，为综合评价生物质炭还田的生态环境效应提供支持。