| Nitrogen (N) leaching has caused an increasing potential concern over N fertilizer impact on environmental diversity. It is very promising to estimate the leaching of N in soils attributable to the complicated physical, chemical, and biological processes of the chemical not only at point scale but also at field scale. Different theories and models have been applied to describe N transformation and transport at every scale.In the first part of this study, a transfer function model was developed to simulate the outflow concentration of nitrate-nitrogen (NO3-N) at the depth of 2m in the agricultural field, considering the influence of transient water flow, the applied N as input, the initial residual N in the soil, and main N transformations on the NO3-N leaching process. The probability density function (pdf) was assumed to be invariant under transient drainage except for differences in the water storage volume. And then the drainage pdf was corrected for storage volume differences between transient water flow and steady state and can be extended to represent transient outflow flux concentration. On N transformations the study model included immobilization, mineralization, volatilization, plant uptake, and their effects on NO3-N leaching were treated as an input to the initial concentration. Through the simple relationship between N transformations and water storage or evapotranspiration, concentration of source-sink terms was obtained. A weighting factor was introduced to quantify the contributions from the applied and residual N to the leaching process. The root mean square error of leaching concentration with considering source-sink terms was taken as target function to optimize weighting factor, and according to the bi-sectional method the optimized weighting factor was determined. The developed transfer function model was tested based on data of the field experiment conducted for 196 d during the growing periods for winter wheat (Triticum aestivum L.) and summer maize (Zea mays, L.) at Quzhou experiment station of China Agricultural University, Hebei province. Soil water potential and NO3-N concentrations were measured along two 2-m deep soil profiles during the investigation period. After calibrated at steady state, the developed transfer function model was used to simulate NO3-N concentration and leaching amount at the interest depth under transient flow. Four different methods were introduced to compare the effects of water flow and source-sink terms on the estimation accuracy, i.e., 1) considering both the transient water flow condition and the source-sink terms (Method 1), 2) considering the transient water flow condition only (Method 2), 3) considering neither the transient water flow condition nor the source-sink terms (Method 3), 4) considering the source-sink terms only (Method 4). Comparisons between the experimental data and simulated results with the transfer function showed that Method 1 provided the most reasonable prediction of the N leaching process as well as its total amount leached. Results also indicated that considering the transient water condition and N transformations in the transfer function significantly increased the estimation accuracy. Compared with the measured data, relative error of the estimated total amount leached N were 0.99 and 19.62%, respectively, with and without considering the transient water condition and the N transformations. The estimated results using Methods 1 and 4 were similar, which showed thatpractically it may be necessary to consider the source-sink terms only and treat the water flow domain as a steady state when we simulate long-time chemical processes in dry croplands. The leached N resulting from the applied N accounted for 0.42% of the total leached N and 0.10% of the applied N, and the leached N was mainly from the residual N, which accounted for 99.58% of the total leached N. Calculations showed that the N leaching portions during the winter wheat and summer maize seasons, were 0.03 and 0.14% of the applie...
|
PDF Full Text Request