Nitrate vertical transport in the main paddy soils of Tai Lake region, China

The influence of texture, bulk density, and organic matter content on the process of nitrate vertical transport in the three main paddy soils (Bai soil, Huangni soil and Wushan soil) of the Tai Lake region were studied in the soil columns. Breakthrough curves (BTC) were obtained separately for each of thirteen horizons. The results were as follows: vertical transport velocity of nitrate decreased, and the BTCs of nitrate were more dispersed, in each horizon from the surface layer to the bottom in every soil profile. Among the three soils, the average flux of Wushan soil was the least and its nitrate BTC was the most dispersed. Under saturated conditions, nitrate penetrated the soil column quickly. The transport of nitrate was affected by clay content. As the clay content increased, nitrate outflow was retarded, and the peak concentration was reduced. Nitrate BTCs rose and fell gently when the nitrate concentration was lower. All nitrate BTCs were asymmetric, and tailing was more obvious when clay content was high. Soil bulk density and the organic matter content also affected the vertical transport of nitrate. Low bulk density and high organic matter content were each associated with faster nitrate transport. An analytical solute transport model (CXTFIT 2.0) was used to estimate the nitrate dispersion coefficient and average pore-water velocity from the observed breakthrough curves. The results showed that the analytical solute transport model was suitable in fitting the observed nitrate transport in these soils. The dispersion coefficient was found to be a function of average pore-water velocity.