Combined Effects of Rainfall Patterns and Porous Media Properties on Nitrate Leaching

Abstract

Laboratory experiments were conducted to investigate the effects of water input properties on the movement of nitrate in three different porous media. A series of artificial rainfall amounts from 4.0 to 26 mm were applied every 24 h in 10 min durations over a weighing lysimeter packed with either fine sand, field soil over fine sand, or fine sand with artificial macropores. Movements of nitrate, chloride, and water in the lysimeter were monitored. Significant nitrate leaching occurred with pistonlike flow caused by the highest applications in all porous media. The nitrate movement coincided with that of chloride in a homogeneous sand medium. In a layered medium with soil cover over sand, nitrate/chloride ratios in the field soil decreased with time, suggesting the presence of degradation processes. Some parts of the soil may be serving as spots for denitrification. High nitrate concentration remained in the topsoil layer under light to intermediate rainfall applications in the two‐layered medium, causing large vertical spreading of nitrate under intermediate rains by which infiltration occurs. Double peaks in the concentration profiles were observed in the macroporous medium under 26‐mm applications, indicating that part of the nitrate was transported by preferential flow while the rest was transported by matrix flow. We estimated that heterogeneity of the porous medium resulted in large dispersion under intermediate to heavy rains and denitrification under light rains. Heterogeneity also affected pathways of leaching under heavy rainfalls. We estimated that climate change could significantly increase the chance of nitrate leaching by at least 25% in the Pacific East area of Japan through increases of heavy rain frequency and precipitation intensity and decreased time for denitrification in the surface soil.