基于核磁共振技术的孔隙水形态及土壤渗透性分析

干湿循环过程会影响土壤孔隙水储存形态,导致土壤渗透性增大、养分流失。该文基于核磁共振技术,探究不同含水率和经历0~4次干湿循环后土壤孔隙水储存形态的变化规律,研究干湿循环对土壤渗透性的影响。结果表明:依据"饱和-吸力"联测方法可将孔隙水分为束缚水和可动水,二者的横向弛豫时间(T2)阈值为1.96 ms。土壤湿润过程中,孔隙水主要以可动水的形态存在;土壤含水率较低时,束缚水(T2<1.96 ms)和可动水(T2≥1.96 ms)占比接近;当含水率超过13%时,可动水的含量迅速增大,而束缚水量增加较少。经历多次干湿循环后,土壤束缚水含量几乎不变,而可动水含量随着循环次数增加而线性增加;将干湿循环作用引入Coope渗透率模型可知,0~4次循环内,土壤渗透率与循环次数的6次方成正比;干湿循环作用会显著增加农田土渗透性、降低土壤肥力。研究可为农田土水分、肥力保持方案的制定提供理论支持。

Pore-water form determined by using NMR method and its influence on soil permeability

Drying-wetting cycle caused by rainfall and evaporation has an obvious influence on soil pore structure, particle surface, and mineral composition. It seriously affects the soil permeability and threatens the ability of soil to resist nutrient loss.This study was carried out to investigate pore-water form of soil during the wetting process and after experiencing different drying-wetting cycles. Eight soil specimens were prepared. Two of them were used to determine the critical value of the water form, three specimens were used for nuclear magnetic resonance(NMR) tests under treatment of different water content, and the other three specimens were used for NMR tests after 0-4 drying-wetting cycles. A saturation-suction method was used to obtain the critical value of relaxation time. The threshold value between adsorbed and movable water was obtained by comparing the relaxation time curves at the saturated state and the critical suction state. The results showed that the critical suction value of adsorbed and movable water was 71.12 MPa and the critical relaxation time was 1.96 ms. The pore water was adsorbed water when the relaxation time was less than 1.96 ms and it was movable water when the relaxation time was not less than 1.96 ms. When soil water content was 10%-28%, the relaxation time were mostly 0.04-10.72 ms. The relaxation time peaks moved to the right of the curves, which indicated that both forms of water(adsorbed water and the movable water) were increased with increasing water content. When the water content was lower than 19.0%, the proportions of the two forms of water were similar. When the water content was higher than 19.0%, a larger proportion of movable water was observed while the proportion of adsorbed water increased slightly. The relaxation time was mainly 0.03-38.72 ms after 0-4 drying-wetting cycles and the relaxation time curve exceeding 1.96 ms moved to the right as the cycle number increased. After four cycles, the integral area of adsorbed water had a slight fluctuation(about 8%), however, the integral area of movable water increased by about 150%. Drying-wetting cycles enlarged the space among soil particles, and the cycle number had a linear relationship with the movable water and the total water. The movable water, both in the wetting process and after multiple drying-wetting cycles,was mainly in charge of soil permeability because it could lose at a lower hydraulic gradient. The dissolution of solvable cements, the loss of micro-particles, the change of pore wall and the fissures were the factors causing the change of water forms. Based on the Coope model, a simplified model containing only the initial porosity and cycle number was established to estimate the permeability of clay soil that experienced different drying-wetting cycles. Under the condition that the dryingwetting cycles was 0-4 in clay soil, the soil permeability was proportional to the sixth power of the cycle number. The study would provide valuable information for formulating optimal water and fertility schemes in farmland.