ABSTRACT
Nitrogen inputs were a key part of the green revolution in the 20th century and remain a key tool for achieving national and global food security in the 21st century. Inputs of this important 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 additional pipes that transport nitrogen out of soils. When nitrogen inputs are applied, the flows across the pipes increase the delivery of nutrient to the crop, increasing the yield, quality, and economic value of the product. At the same time, these pipes direct 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 direct nitrogen from the system start to direct more nitrogen to the environment. In this analogy, the main pipes are those that control the gases of nitrogen, and losses of nitrous oxide (N2O), nitrogen oxides (NOx), ammonia (NH3), and dinitrogen (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 flows 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 nitrate 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 nitrate in solution. Cover crops, denitrification traps, and other management practices could potentially be used to reduce the transport of nitrate 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 it has the potential to be used as an effective tool for environmental conservation.
