Nitrogen (nitrate leaching) index

Authors: Delgado, Jorge

Submitted to: International Soil and Water Conservation Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2020
Publication Date: 7/29/2020
Citation: Delgado, J.A. 2020. Nitrogen (nitrate leaching) index. International Soil and Water Conservation Research. https://doi.org/10.1201/9781003045045-43.
DOI: https://doi.org/10.1201/9781003045045-43

Interpretive Summary: The Nitrogen Index has been tested and calibrated using data from North America, different regions in Mexico, the Caribbean region and different regions of South America, a European Mediterranean region, and the North China Plain, where personnel from various national and international agencies, universities, and/or research centers have used the tool. The tool has been downloaded from over sixty-five countries and it has been used across thousands of acres to assess management practices. The tool has been used to generate information about nitrogen use efficiencies and management to communicate with thousands of farmers or implement actions at farmers’ fields and educate and train students about nitrogen budgets and management practices and risk of nitrogen losses to the environment. Results of these efforts show that the Nitrogen Index is able to evaluate the effect of management on residual soil nitrate, water leaching, nitrate leaching, emissions of nitrous oxide, crop nitrogen use efficiencies, and crop nitrogen uptake. Additionally, measured groundwater nitrate concentrations have also been correlated with nitrate leaching predicted by the Nitrogen Index, validating the potential of using the tool to assess the risk of nitrate leaching to the environment. Although it is recommended that users get familiar with the capabilities and limitations of the Nitrogen Index, the correlation between predicted and observed values for these different losses of reactive nitrogen suggest that this is a robust tool capable of being used to assess the risk of management practices on potential nitrate leaching and losses via other pathways. These results also support the recommendation of using the Nitrogen Index as part of an ecological engineering approach to soil and water conservation planning. There is potential to use this tool to assess the effects of nitrogen inputs from manure, fertilizer and other organic sources and management practices and to rank the risk of losses of reactive nitrogen, including the nitrate leaching pathways, as very low, low, medium, high, or very high risk. The Nitrogen Index can potentially be used as an indicator to assess the risk of nitrate leaching to the environment, helping conservation practitioners and nutrient managers implement best management practices to increase nutrient use efficiencies while reducing the nitrate leaching risk and losses of reactive nitrogen through different pathways. This new tool is a new approach and is available for download, as well as the code for the tool. The Nitrogen Index shows promise for being used as a quick and effective assessment tool for global sustainability.

Technical Abstract: Nitrogen inputs were a key part of the green revolution in the past century and to this day remain a key tool for achieving national and global food security in the 21st century. Inputs of this key nutrient contribute to increased yields, product quality and profits for farmers and ranchers across the globe. However, when higher rates of nitrogen (N) inputs than needed are applied to agricultural systems, the nitrogen losses across different pathways increase. The concept of the hole-in-the-pipe proposed by Firestone and Davidson in 1989 describing the losses of emissions of greenhouse gases from the soil could be adapted to apply to the nitrogen cycle and the losses of nitrogen from agricultural fields. What Firestone and Davidson described as “holes” in the pipe could instead be thought of as pipes that transport nitrogen from the nitrogen cycle in agricultural systems out of soils. When nitrogen inputs are applied, the flows across the pipes increases the delivery of nutrient to the crop, increasing the yield, quality, and economic value of the product. At the same time, these pipes leak nitrogen from agricultural systems, and when more nitrogen than needed is applied, it can be assumed that the pressure in the system starts to increase, and some or even all of the pipes that leak nitrogen from the system, start to leak more nitrogen to the environment. In this analogy, the main pipes are those that control the gases of nitrogen, and losses of N2O, NOx, NH3 and N2. Another set of pipes consists of those that control leaching of nitrate N (NO3-N), and yet another set of pipes consists of those that control the surface losses of nitrogen via surface runoff. The losses of reactive nitrogen, which flow through these metaphorical pipes to the environment, will potentially impact air and water quality. There are other factors that could affect the flow of nitrogen through these pipes. For example, the type of soil texture, which affects the hydrology, could affect the NO3-N leaching potential, which is higher in coarse-textured soils. Soil organic matter content, slope, and management practices such as addition of manure or other organic amendments and/or tillage (or no tillage) of the soil, could also affect these flows of nitrogen. Crop rotations will also reduce or increase the flow and pressure on the pipes. The good news is that we could use management practices to try to control the pressure on the pipes and the losses of nitrogen from the system, and to try to catch the nitrogen at different points once it leaves the pipes to minimize its transport in the environment. In summary, when more nitrogen than needed is applied, the pressure in the system increases and the losses to the environment increase, especially via movement of nitrates in the soil profile, which is greater for coarser-textured soils, or soils that have tile systems where the tiles contribute to removal of the water from the soil system and also transport available nitrates in solution. Cover crops, denitrification traps and other management practices could potentially be used to reduce the transport of nitrates in the environment. There are also tools such as the Nitrogen Index that could be used to quickly conduct an assessment of cropping systems (and estimate the pressure changes in each of these imaginary pipes) and estimate the potential for nitrogen losses via nitrate leaching and other pathways. A nitrate leaching index is a simple, relatively quick approach (five minutes or less) to conducting a risk assessment of the potential for nitrate leaching. The Nitrogen Index has been shown to be a new ecotechnological approach users can take to quickly assess the risk of nitrate leaching, and the Nitrogen Index has the potential to be used as an effective tool for environmental conservation.