Submitted to: Encyclopedia of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2016
Publication Date: 9/15/2016
Citation: Delgado, J.A. 2016. Nitrogen Index. Encyclopedia of Soil Science. Third Edition. CRC Press. doi:10.1081/E-ESS3-120053861.
Interpretive Summary: The Nitrogen Index is a quick assessment tool that can be used to assess the potential risk of nitrate leaching and of surface water and atmospheric impacts (Figure 1). The tool assesses the effects of management practices and source of nitrogen fertilizer (e.g., use of organic manure inputs) by integrating information about management, weather, and site-specific soil properties to assess the potential risk for nitrogen losses via leaching, surface and atmospheric pathways. The tool conducts a nitrogen mass balance to assess the losses via ammonia volatilization, nitrate leaching, denitrification, surface runoff, and nitrous oxide emissions. The tool also estimates crop nitrogen uptake and residual soil nitrate. Several peer-reviewed manuscripts have shown that the Nitrogen Index tool can be used to assess the effects of management practices on nitrogen losses for different cropping systems in the USA, Mexico, South America (Bolivia and Ecuador), Africa (Malawi) and a Mediterranean region of Spain 32,33,34,35,36,37,38,39. Shaffer and Delgado reported on a tiered approach for assessment of nitrogen management practices, describing tiers two and three as having a data-intensive approach, requiring more time to set up the needed information, and being used in coordination with field studies20,23. In contrast, a tier-one tool is easy to use, faster, and requires minimal information. Recently, Saynes et al. discussed the tier zero tool which is even easier to use than a tier-one tool, with just a few screens and minimal information required, and which still provides a robust analysis of nitrogen management practices and a qualitative and quantitative assessment of the potential risk of nitrogen losses37. The Nitrogen Index is easy to use and it takes into consideration the effects of management of fertilizer, manure, crops, irrigation, soils, climate, and off-site factors. The Nitrogen Index also integrates the use of best management practices being applied at the site when assessing the potential risk of nitrogen losses to the environment. The Nitrogen Index accounts for N management practices and estimates the crop N uptake, N dynamics and N sources. It also accounts for crop rotations and rooting depth. It integrates soil hydrology properties and water inputs as well as estimated water leaching. It accounts for precipitation during the growing season and non-growing season and estimates NO3-N leaching losses, direct and indirect N2O emissions, surface nitrogen losses, denitrification, NH3 volatilization, crop N uptake and residual soil NO3-N. The management can be entered for a given crop at a site-specific location and the tool will estimate the potential risk for nitrogen losses as well as the cropping system nitrogen use efficiency. The tool integrates principles from soil management, soil chemical and physical principles, hydrological principles, and ecological engineering principles to assess the potential risk of nitrogen losses and help the user make decisions (Figure 2). The Nitrogen Index can be joined to other nutrient indices and be used to assess multiple indexes in one screen. The Kentucky Nitrogen and Phosphorous Index is one such example40 (Figure 3). The Nitrogen Index can be used as an important tool for conservation management by integrating this approach into nutrient management plans41.The tool is being used in California, and is the official tool in the NRCS 590 nutrient management standards for Kentucky and Wisconsin. It has been transferred to NRCS-South Dakota and to several international institutions such as INIFAP (in Mexico), COLPOS (in Mexico), PROINPA (in Bolivia), INIAP (in Ecuador), University of Brasilia (in Brazil) and the Ministry of Agriculture and Food Security (in Malawi). The tool can be used in English, Spanish and Portuguese. The tool can be used in metric or English units.
Technical Abstract: There is a need to improve the management of nitrogen inputs to agricultural systems because they increase the potential for losses of reactive nitrogen to the environment, resulting in negative impacts to water and air resources. There is a need to reduce nitrate leaching, emissions of N2O from agricultural systems, nitrogen losses to surface waters, and other losses of reactive nitrogen. There is a need to increase nitrogen use efficiencies across agroecosystems. The Nitrogen Index is a quick approach that can provide needed information to nutrient managers to quickly assess the effects of management practices on the potential risk of nitrogen losses via different pathways so nutrient managers could identify situations where risky combinations of management practices and landscape scenarios increase potential for reactive losses of nitrogen (e.g., nitrate leaching, N2O emissions). By quickly identifying these risky management-landscape scenarios, implementation of improved management practices at the site could be developed. The Nitrogen Index can provide important agronomical and environmental information to nutrient managers to help them make improved decisions. It can use a nitrogen balance to develop improved management recommendations. The Nitrogen Index is starting to be applied by USDA NRCS in state 590 nutrient management plans The tool can be used in different languages and is being used in different countries and continents. The Nitrogen Index can be run in English or metric units. Similarly to the P Index that can assess the potential risk of P losses to the environment, the Nitrogen Index can be used to quickly assess the risk of nitrogen losses to the environment to improve nitrogen management, reduce losses of reactive nitrogen, increase nitrogen use efficiencies, and improve economic returns to farmers.