Control of Nitrogen Transformations to Increase Nitrogen-Use Efficiency and Protect Environmental Quality

Nitrogen (N) is one of the most important nutrients used worldwide to increase and maintain crop production. During the green revolution, N fertilizers contributed to the increase and sustainability of high yields across different agroecosystems. This element has been key in maintaining the sustainability and economic viability of farming systems across the world and in feeding the world population. The industrial production of N fertilizer is driven by the atmospheric dinitrogen (N₂) fixation as a source of different fertilizer materials. The fertilizer industry can produce different sources of N in the chemical form, such as urea, ammonia (NH₃), ammonium (NH₄), nitrate (NO₃), and other forms of fertilizer, that contribute to sustaining viable agricultural production across all regions. Additionally, organic sources of N, such as manures and/or biological N₂ fixation (e.g., the symbiotic association between legumes and Rhizobium spp), are also used across different cropping systems. These inorganic and organic sources of N support agricultural production and impact the N cycle.

Researchers are constantly working on improving agricultural best management practices (BMPs) that can contribute to the improvement of N management. On average, N-use efficiency (NUE) in the United States is reported to be about 50%. Recently, Raun and Johnson (1999) reported that worldwide NUE for cereal production is approximately 33%. For the unaccounted 67%, the economic loss worldwide is equivalent to 15.9 billion US dollars. If we account for the NUE from vegetables, fruits, and other cropping systems, the economic losses worldwide would be multiplied by several factors. There is potential to cut these losses by 50% and save billions of dollars worldwide.

To improve N management practices that can maximize NUE, we need to study and understand the biogeochemistry of the N cycle across different agroecosystems. Soil N is subjected to chemical and biogeochemical transformations, and its dynamic is affected by several factors. This dynamic can change the chemical form of organic N compounds in a relatively quick amount of time, such as changing the proteins in crop residues, to NO₃ and then to the gaseous N₂ form. In this soil dynamic, the soil N can be part of the microbial biomass, roots, or other components of the system. Parton et al. (1987) divided the soil organic carbon and N into three general components according to their dynamics and residence time. The active pool mainly contains live microbes and microbial products and soil organic matter with a short turnover time (one to five years), the slow pool is dominated by the physically protected and/or organic form that are more resistant to decomposition (20-40 years), and the passive pool contains the recalcitrant and slower reactive N (200-1,500 years).

Several authors have described the specific pathways for the transformations of organic N in the soil (Tisdale and Nelson, 1975; Stevenson, 1982). There are several factors that can contribute to the mineralization or immobilization of N. In a simplified general pathway, heterotrophic organisms use organic carbon as a source of energy and drive the aminization and ammonification processes. The autotrophic nitrosomonas convert the NH₄ to NO₂, and nitrobacteria converts the NO₂ to NO₃. In these steps, these autotrophic bacteria obtain energy from the oxidation of these compounds. It is in this nitrification process that nitrous-oxide gases, such as nitric oxide (NO) and nitrous oxide (N₂O), can be formed and emitted from the soil into the atmosphere. Hutchinson (1995) described in detail the controls for these pathways. It is also NO₃, a mobile form of N, that researchers have reported as the main mechanism for movement of N below the rooting zone due to rain and/or irrigation events. Newbould (1989) also reported that contamination of drinking water by NO₃ is becoming a serious problem in many parts of the world. If the soil is ponded and anaerobic conditions develop, significant amounts of NO₃ can also be lost by denitrification or conversion to N₂. This denitrification process can generate emissions of trace gases.

Anthropogenic activities, such as the burning of fuels, and other nonagricultural activities are contributing to the emissions of CO₂ and trace gases, such as N₂O, that contribute to global warming (IPCC, 1994). Cicerone (1989) reported that the long-lived greenhouse gas N₂O is a major source of stratospheric NO, which contributes to ozone depletion in the stratosphere. Agricultural systems that are fertilized with inorganic and organic fertilizer and biological dinitrogen (N₂) fixation also contribute more to the generation of higher N₂O emissions than natural agroecosystems do (Mosier et al., 1996). The addition of organic or inorganic N fertilizer can impact these trace gases and NO₃ leaching mechanistic losses. These are additional reasons why we need to continue developing new technologies and management practices that can increase NUE.

Cropping systems have been traditionally managed with BMPs that contribute to the increasing of NUE and to the recycling of N. Traditional BMPs-such as banding of fertilizers, split applications of N through the growing season, developing new varieties of crops with higher NUE, crop rotations, scavenger crops, fertigations, drip irrigation, and accounting N budgets in soil, crops, and irrigation waters-and others are being used widely across the world. New technologies that can improve the management of N, such as precision farming techniques, remote sensing, quick field tests for insitu analysis of NO₃ concentrations in sap tissue, quick tests for chlorophyll status, use of computer simulation models for evaluation of management practices, and other new tools and technologies are being developed, calibrated, and implemented to continue the improvement in NUE. These new tools will contribute to increasing the average NUE across different agricultural ecosystems to levels much higher than 50%.

There are other new technologies that can contribute to managing the rate of N transformations in the soil system. It has been reported that nitrification inhibitors (NI), slow-release fertilizers, and controlled-release fertilizers (CRF) can be used to increase NUE (Delgado and Mosier, 1996; Engelsjord et al., 1997; Detrick, 1996). Nitrification inhibitors could slow down the nitrification process. Slow-release fertilizers could release N slowly into the soil solution. Controlled-release fertilizers, such as the polymer-coated urea technology, can also reduce the release of N fertilizer from coated pellets into the soil solution.

This polymer technology encapsulates the urea-granule fertilizer. After the pellet is placed in the soil, soil water is absorbed into the encapsulated urea granule. This process initiates partial dissolution of urea, establishing a concentration gradient that is driven by the soil temperatures. The polymer technology can use algorithms and the average soil temperatures to predict the rate of release through the membrane.

This technology can change the properties of the membrane to make it more permeable or less permeable, thus changing the rate of release for a specific pellet design. The technology could also change the thickness of the membrane to increase or decrease the rate of release. The idea is to calculate the rate of release for a specific membrane and to match this rate with the rate of uptake during the growing season for a specific crop. This will minimize the time that the N fertilizer will be available and susceptible to losses, thus increasing NUE and reducing N₂O and NO₃ losses. This polymer technology can also be applied to other sources of N and other nutrients (e.g., NH₄ and potassium, respectively).

The potential use of CRF and NI to increase NUE and reduce mechanistic losses has been studied in Colorado (Figure 1). The polymer technology permits the application of N fertilizer with the seed without the seed and/or germination process receiving damage from high salt concentration.

Field studies show that NI and CRF reduce N₂O emissions from cropping systems (Figure 2). Both NI and CRF slow the transformations of the applied urea fertilizer and maintain higher NH₄ concentrations at 29 days after fertilization (Figure 3). By this time, most of the urea in the fertilized plots has been converted into NO₃.

When CRF was applied at 50% of the rate of traditional farming practices, potato-tuber production was the same as with traditional farming practices that used twice the amount of N fertilizer (Figure 4). NUE with the polymer technology, applied at planting, was 16% higher than with traditional farming practices. The traditional practices contain two split applications: one at planting, one at hill up and 10 fertigations. Although total tuber production was the same, production of tubers greater than 227g with the CRF was lower than with the farming practices. In this one-year study, the CRF was broadcast and incorporated into the hill. A better response would be expected if the potato seed had banded the CRF.

These studies in Colorado show that there is potential to use NI and CRF to increase NUE and to reduce environmental losses of N from the system. Additional research is needed to evaluate their effects on crop quality and how to supply the N at a rate that can better match the N uptake demand of each crop during the growing season. Since initially all the fertilizer is inside the capsule, a starter fertilizer may supply the initial N. If we can cut N inputs by 25-50%, losses of N will be significantly reduced and water and atmospheric quality can be protected. Since, on average, these specific new technologies will have higher prices per unit of fertilizer, economic analyses of the benefits versus the costs need to be taken into consideration.

There is also the potential to use these alternatives with precision farming to further increase the NUE of the cropping systems. Figure 5 shows the results from soil samples that were collected using a grid sampling technique of one random sample per acre with a differential Global Positioning System. The variability of the residual soil NO₃ (ppm) after barley harvest is plotted with a Geographic Information System. The lower values are in general correlated with the sandier and coarser areas. New technology, such as CRF, can be applied to these areas using variable-rate technology to reduce the losses in the coarser and gravelly areas of the field. If N is the limiting factor in these areas, there may be potential to further increase yields from these areas. There is also research being conducted by other scientists on the use of precision farming technologies to manage water more effectively across the fields with center-pivot sprinkler irrigation systems.

The development of such new technologies as precision farming and management of fields in zones will further contribute to better use of nutrients and water, cut N losses by 50%, and increase average NUE to above 75%. These continued improvements in NUE will help minimize the loss of NO₃ and N₂O. Although there is not a simple answer for each cropping system and each field scenario, new technologies that can control the rate of release have the potential to be used as alternatives to increase NUE and cut losses.