Characterizing the exudation of water and pollutants from soil into streams during soil-thawing period

Abstract

The objectives of this study were to characterize the exudation of water and pollutants from soil into streams during soil-thawing periods, and to investigate the impacts of hydrological and pollution transport processes in soil, and energy entering soil on the exudation. Field experiments were conducted to measure soil water content and concentrations of ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), and soluble phosphorus (SP) in four closed catchments containing paddy or corn fields, along with the exudation of water and pollutants from soil into streams (i.e., flow rate and pollution fluxes at catchment outlets). The amount of dissolved pollutants from soil particles was the key factor affecting pollution exudation into streams during soil-thawing periods. Water exudation directly affected pollution exudation; when the former was >1.24 × 10−3 m3 s−1 km−2, the pollutant mass in the water reached maximum and changes in its concentration were primarily attributed to water exudation. When water exudation was 1.08 × 10−3–1.24 × 10−3 m3 s−1 km−2, the dissolved pollutant mass from soil decreased with water exudation. The influence of pollutant transport processes on pollution exudation became more significant when water exudation was <1.08 × 10−3 m3 s−1 km−2. Relative concentrations of pollutants in corn-dominated catchments were higher than those in the paddy-dominated catchment, suggesting that pollution exudation was most attributable to water exudation in paddies and to the pollution source in corn fields. Energy entering the soil accelerated the fragmentation of soil aggregates and enhanced the N transformation and SP solubility, further increasing pollution exudation.