Vertical transport of water and chemicals as affected by soil layering: a model study

Abstract

Transport of water and chemicals in soils is controlled by soil properties and processes. Advection, adsorption, diffusion, and dispersion of the chemical are the main processes controlling the extent of transport of a chemical. Soil porosity and pore size distribution are the key factors controlling the water and solute flow by advection and dispersion, soil adsorption phenomena are the main factors controlling the retention of the chemical in soils. All these processes are highly variable by depth due to differences in soil characteristics of different soil horizons. This study was conducted to analyze interactions between soil layering and vertical transport of solutes and water at 2-m wide, 4-m long, and 0.5-m deep lysimeters constructed as field plots. Zero, five, ten, fifteen, and twenty cm thick sand layers (as treatments) were placed over a level alkaline surface, then 30 cm thick nonalkaline soil layers were packed over the sand layers. To represent plant effect, alfalfa was grown at each plot since it has a dense canopy. Changes in pH, EC, and in concentrations of Na, K, HCO3, and B in topsoil were monitored, measuring these variables in water extracts collected by vacuum samplers following rainfall and/or irrigation events. Water content of both repacked topsoil and alkali subsoil were measured in October, 2004 when soil was dry. At the final sampling, a representative profile was open in each research plot and morphological observations were made in these profiles. Soil pH decreased and then increased sharply irrespective to sand layer thickness, and concentrations of HCO3 and B showed a similar behavior. The EC of repacked topsoil decreased continuously probably due to the leaching of salts by application of excess amount of irrigation water, and Na concentration of soil solution increased continuously, which was attributed to sodium transported by capillary rising water from the blow alkali soil. Greater values for water content occurred at final sampling in alkali soil below 5- and 0-cm sand layers, indicating that sand layer with 10 cm thickness obscured percolation of excess water from irrigation and precipitation as observed in layered soil profiles. Roots of alfalfa concentrated in the zone of sand layers, and almost no roots of alfalfa penetrated into the alkali zone in search of water and nutrients. As morphological observations revealed, channels of decayed roots in the alkali soil served as preferential pathways of water and chemical from upper layers.