北京昌平区农地土壤优先流影响硝态氮运移的试验分析

为了探讨在优先流影响下农地土壤水分与溶质的运移规律,以昌平农地土壤为研究对象,通过原状土取样和分层填充制备实验土柱,模拟存在优先流和平衡入渗2种水分下渗过程,分析优先流对农地土壤中硝态氮运移的影响。结果表明:相较于平衡入渗,存在优先流的土壤中硝态氮运移的速率更快、数量更多,且其穿透曲线表现出拖尾现象。优先流的存在会使土壤的水分出流速率达到平衡入渗过程的1.48～2.69倍,且波动程度较大;受其影响,硝态氮运移表现出快速、大量下渗的特征,原状土柱中NO3-的穿透时间为12 h,此时的孔隙体积为0.36,相较于填充土柱分别减少了57%和27%。此外,原状土柱中以NO3-标记的优先流占水流总量的43.83%,引起的NO3-累计淋出量占总量的97.60%,这表明有限的优先流流量能够引起绝大部分的硝态氮运移。土壤优先流还使得其穿透曲线表现出拖尾现象,这可能是由于优先流和基质流之间下渗速率的不平衡造成的。

Experimental analysis of preferential flow and its effect on nitrate nitrogen migration in soil of farmland at Changping district in Beijing

In order to investigate the principles of the farmland soil water and solute migration under the influence of preferential flow, a comparison was studied between undisturbed and packed soil columns which represented for non-balancing and balancing infiltration respectively. With the methods, the effect of preferential flow on nitrate nitrogen migration was studied in the agricultural land at Changping district. The results showed that nitrate nitrogen migrated in higher rates and in greater numbers under the promotion of soil preferential flow, and tailing was also a vital appearance in the preferential flow＇ s Break Through Curve. The water flow rate with preferential flow in soil appeared larger degree of volatility, and was 1.48 - 2.69 times of that of balancing infiltration. Since the nitrate ions are non-absorptive, the migration of nitrate nitrogen was characterized with higher rates and greater numbers as influenced by preferential flow. The breakthrough time of undisturbed soil column was 12 h with a 57% reductioncompared to the packed one, and the leachate volume of undisturbed soil column was 0. 36 times of Pore Volume when the nitrate ions penetrated which was 0.73 times of that of the packed one. In addition, the preferential flow of NO3- in undisturbed soil column accounted for 43.83% of total flux, but resulted to a 97.60% accumulative lechate mass for NO3- in total mass. These results indicated that the preferential flow with limited ratio in total flux could lead to a large proportion of NO3- migration. Tailing was an important phenomenon in the preferential flow＇ s BTC and was a unique feature of the curve, it may be caused by the disproportion of preferential flow to matrix flow on penetration rate in the infiltration progress.