基于低场核磁技术研究土壤持水性能与孔隙特征

为研究常规水稻土和设施蔬菜地土壤持水性能与孔隙分布特征,利用低场核磁共振技术,获取田间持水量状态的土壤T_2谱线特征,在此基础上分析了土壤失水时信号幅值、弛豫时间与土壤含水量和孔隙大小分布的关系。结果表明:水稻土和设施蔬菜地土壤中孔隙分布均比较分散,小孔隙居多,大孔隙较少,水稻田转化为设施蔬菜地后,土壤孔隙半径整体变小,土壤结构恶化。在土壤失水过程中,波峰消减与谱线偏移同时发生,小峰优先消失且消减速率大于主峰,表明该过程中孔隙收缩与水分散失同时发生,土壤孔隙由大到小依次不断排水。低场核磁共振作为一种快速、无损的测量工具,能够更直接、更准确地反映出土壤水分迁移过程和孔隙分布规律,从而为设施栽培土壤退化机理和土壤改良提供新的理论和技术支撑。

Study on Water Holding Capacity and Pore Characteristics of Soils Based on LF-NMR

Objective]It is recognized that soil pore size distribution and soil water holding capacity plays an important role in soil aeration and crop root development. This study is to explore changes in water holding capacity and soil pore size distribution induced by the change of land use from paddy field to vegetable field.Method]A new method of using the low field nuclear magnetic resonance(LF-NMR) technology was adopted to acquire features of the T2 spectrum of soil samples collected from the fields full in field moisture capacity,and on such a basis,relationships of amplitude and relaxation time of the signals of the soil samples dehydrating with soil moisture content and pore size distribution were analyzed.Result]Results show that pore size distribution did not differ much between paddy soil and facilitated vegetable soil,being quiet scattered,and composed of more small pores than large ones. After paddy field being turned into facilitated vegetable field,soil pores became smaller in diameter and soil structure worse. In the process of the soil losing its water,peak reduction and spectrum shift took place simultaneously. Small peaks disappeared first,with a reduction rate much higher than that of the main peak,indicating that during the process pore shrinkage and water loss of water occurred simultaneously and the water in big pores drained first and then sequentially that in small ones.Conclusion]All the findings in the study indicated that the method based on LF-NMR can be used to instantly monitor changes in soil moisture content and more directly and accurately determine soil moisture migration processes and pore distribution rules without disturbing the soil, and hence may serve as new theories and technical support for the study on mechanisms of soil degradation and soil amelioration in facilitated vegetable fields.