Essentials of a national nitrate leaching index assessment tool

ABSTRACT: Nitrogen (N) inputs are essential for increasing yields and maintaining the economic viability of farming systems worldwide. Although best irrigation and N management practices have been used, increases in worldwide use of N fertilizers combined with average N use efficiencies of 50 percent have contributed to increased leakage from the N cycle (e.g., higher nitrate-nitrogen (NO3-N) leaching losses). Specific land use patterns have been correlated with higher NO3-N concentrations in underground water resources. There is a critical need to continue improving best management practices to reduce NO3 N leaching losses, increase the economic viability of farming operations, and conserve water quality. To help meet these objectives, this paper recommends the essentials for the development of a national NO3-N leaching assessment tool. The resulting NO3-N leaching index (NLI) should be based on hydrological soil properties and climate, must consider management practices and associated crop rotations, and incorporate off-site effects. Development of the NLI should include the use of simulation models and expert systems; databases for soils, climate, and management; and use of the Internet. The index also needs to allow input of local site-specific information from producers and field personnel. The index needs to be national in scope and yet flexible enough for use in specialized or difficult cases. Routine use of the index needs to be kept simple and quick with minimal input from the user so that field office personnel can apply the tool on a regular basis. Application of the NLI should be linked to the phosphorus (P) index so that management of key nutrients—N and P—can be accomplished simultaneously. We recommend a 3-tier approach to developing the NLI that would provide a uniform index yet allow for refinement of accuracy in the index values as necessary to meet study needs. Tier 1 would involve the initial use of an expert system to separate medium, high, and very high NO3-N leaching potentials from low and very low potential levels by qualitatively screening non-numeric inputs obtained from users. This initial screening technique is simitar to that used to develop the P index, but would be designed specifically for NO3-N leaching. Tier 2 would involve computation of the NO3-N leached (NL) index using application models or databases based on models, followed by introduction of off-site effects and local interpretation and normalization to produce the final NLI. In difficult cases, a tier 3 study involving detailed research models and field data would be needed along with the off-site effects, interpretation, and normalization. The NLI could be used routinely in conjunction with the P index to allow alternative management scenarios that optimize both N and P for maximal economic return while protecting the environment.