The detrimental impacts of nitrogen (N) losses from agricultural lands on environmental quality have long been recognized. Nitrate−nitrogen (NO3−N) leached from agricultural fields elevates N concentrations in groundwater and surface water bodies, which contaminates drinking water supplies and enhances eutrophication of surface waters, causing hypoxia problems (Gilliam et al., 1999). Gaseous N emitted from agricultural fields may also cause air pollution. Nitrous oxide is a greenhouse gas that contributes to global warming, and ammonia (NH3) contributes to acid rain phenomena. Developing agricultural management practices that reduce off−site environmental impacts of crop production has been the focus of research over many years (Gilliam et al., 1999). This complex task requires understanding of N dynamics in the soil−water−plant system, which is regulated by a large number of interacting and sometimes highly dynamic physical, chemical, and biological processes. The use of N simulation models can lead to a better understanding of N dynamics in agricultural ecosystems and thus improve the process of development and evaluation of best management practices for sustainable agriculture.

 

The nitrogen model, DRAINMOD−N, (Brevé, 1994; Brevé et al., 1997) was originally developed to simulate N fate and transport in artificially drained soils. However, it used a simplified N cycle, which limited its applicability (fig. 1). Brevé (1994) assumed that ammonium−nitrogen (NH4−N) is very short−lived in the soil−water−plant system and thus did not consider it as a mineral N pool. This approach is applicable when the only source of NH4−N is either mineralization of organic nitrogen (ON) or application of slow−release N fertilizers. However, it fails when NH4−N input to the system is large and sudden, as would occur after the application of ammoniacal N fertilizers (Molina and Smith, 1998). It also fails if the nitrification process is impaired either due to unfavorable environmental conditions or by using nitrification inhibitors. Consequently, the model was unable to simulate the application of any NH4 or NH4−forming fertilizers, including urea and anhydrous NH3. Brevé (1994) modeled the interaction between organic and mineral N pools as a one−way net mineralization process with constant potential rate. The model did not simulate temporal changes in the ON pool, but rather considered it as a static source of mineral N. This approach did not adequately describe the interaction between organic and mineral N. It also did not allow the model to simulate soil amendment with ON sources.

 

Figure 1. The nitrogen cycle considered in the original version of DRAINMOD−N.

 

A new version of DRAINMOD−N was developed to simulate N dynamics and turnover in agricultural ecosystems under different management practices and soil and environmental conditions. The new model, referred to as DRAINMOD− N II (Youssef, 2003), considers both NO3−N and NHx−N pools in modeling mineral N and simulates the nitrification process. It includes a comprehensive fertilizer submodel capable of simulating the application of NH4 and NH4−forming fertilizers, including urea and anhydrous NH3, and associated short−term processes such as fertilizer dissolution, urea hydrolysis, temporal change in soil pH, and NH3 volatilization. It simulates OC dynamics in order to better describe N mineralization/immobilization processes and model the application of ON sources. DRAINMOD−N II was successfully tested using a six−year data set from an artificially drained agricultural research site in eastern North Carolina (Youssef, 2003). A summary of the field testing results is available in the proceedings of the 8th International Drainage Symposium (Youssef et al., 2004). This article presents the newly developed model DRAINMOD−N II. It includes a detailed description of different model components and presents a case study to demonstrate how the model can be used to simulate N dynamics for a simple agricultural system. Model parameterization and field testing are beyond the scope of this publication; a manuscript presenting detailed results of field testing is in preparation for submission to a refereed journal.