Regardless of the source of N, whether derived from N2 fixation, fertilizer, or other exogenous source, N cycles through soil, and the rate and manner in which it cycles matters fundamentally to its availability and loss . Soil has the capacity to not only provide plant-available N through soil organic matter turnover, but also to buffer its supply to plants— whether internal N or exogenous N—and control the loss of unused N to the environment by storing N as organic matter or binding N in the soil mineral matrix. As a result, accounting for soil N is integral to optimizing N use . Perhaps the single most important impact of the soil N cycle on NUE is the soil’s capacity for helping to match the timing of soil N availability with periods of plant N demand. In natural ecosystems, the presence of diverse plant species having different life histories, including perenniality, means that at least some species will be actively demanding N whenever soil conditions permit N release from soil organic matter. The result is a relatively tight N cycle: when N available for loss is instead taken up by plants, loss is at least partially averted. For example, fertilized perennial biofuel crops lose little-to-no nitrate– N to drainage, yet NO− 3 loss in drainage of fertilized corn is no different from unfertilized soybean . This highlights that a lack of synchrony between crop N demand and soil N availability is the primary reason for environmental N losses. How can the agricultural N flow be managed to improve NUE? The various processes depicted in Figure 1 serve as reminders of the major interventions that can be used as levers to adjust the flow of N through the system in such a way as to minimize inputs and losses while maximizing N capture and output. We held a workshop in 2019 to discuss problems associated with N in different agricultural cropping systems and to examine promising research and development avenues to solve these problems. Four broad areas of needed endeavor were identified: soil N cycling, systems agronomy, BNF,aeroponic tower garden system and plant breeding as shown in Figure 2. Strategies in each of these areas were organized into a matrix from low-to-high risk and low-to high reward .

In the sections below, we review briefly the state-of-the art in these areas, consider current frontiers in R&D, and propose an integrated set of strategies to increase by 2050 global cropping system NUE and protein yield by 50%, while simultaneously reducing N losses from crop production by 50%. As discussed above, N is used most efficiently when its availability in soil is synchronized with crop demand . Nitrogen synchrony is rare and difficult to achieve in annual monocultures typical of most high productivity agriculture. Most grain crops, for example, have a 90–100 day growing season and accumulate biomass and N at a significant rate only for 30–40 days mid-season. In the maize example above, N uptake can reach the astonishing rate of 5 kg N ha−1 day−1 . This high rate is sustained for only 3–4 weeks and it falls to nil quickly. Meanwhile, soil microbial processes that cycle N between various organic and inorganic forms are active whenever soils are not too dry or too cold to support biological activity, i.e., much of the rest of the year. This asynchrony between when N is available and when N is needed creates windows of N loss and is a principal cause of low NUE in most cropping systems . Depending upon cropping system and environment, achieving synchrony can be challenging as a result of variable weather, timing of equipment and labor availability, and other limitations and sources of uncertainty. First, plant N demand can be difficult to predict based on the data available at fertilizer decision time points and possible future weather scenarios. Second, estimating the N-supplying power of the system is difficult, particularly in mesic climates where N inputs from mineralization of soil organic matter and N outputs/losses to denitrification and leaching into groundwater exhibit large variation from year-to-year and field-to-field. Addressing these issues inherent to cropping systems remains challenging. The most commonly used N fertilizers readily dissolve into soil solution as mobile N ions that are subject to loss if not acquired by crops or retained by soils. Low NUE characterizes crop systems where transient or permanent N supply exceeds crop demand ; soil has a low capacity for N retention due to low organic matter content, coarse texture, and/or presence of weathered clay minerals with low ion exchange capacity; and climate and agronomic management promote N loss when there is high rainfall, heavy irrigation, or temporary water logging.

The difficulty in synchronizing N supply and demand is exemplified by mechanized sugarcane cropping, where farmers apply all fertilizer early in the crop season because crop height and summer rain prevent field access later. Sugarcane grows over 10 months or longer. Large pools of soluble N, high rainfall and/or irrigation, and an initial low crop N demand drive N losses from sugarcane soils in the first months . To compensate for this risk, sugarcane farmers in the main producer countries apply, on average, twice as much N fertilizer as is required by the crop . The range of N fertilizer use spans from near perfect use of fertilizer N at 100% , 60% to only 10% . Fertilizer timing with most or all fertilizer applied before the cropping season is also common with maize in the US Corn Belt, where crop height and wet fields can also hamper in-season applications. There are many management options available for increasing NUE through matching N supply with crop demand and thereby mitigating loss of N from agroecosystems. Many tools and best management practices are intended to help farmers apply nutrients in a “4R” management framework—using the right N source at the right rate, right time and in the right place. Basic, or low-tech, approaches involve adjusting timing or rates without needing different equipment. Beyond that, a range of precision N tools that detect chlorophyll and other crop vigor-related measures and agronomic techniques combined with weather forecasting are now available to support improved nutrient management. Spatial synchrony can be as important as temporal synchrony for matching soil N availability to plant demand. This is true both at the plant scale, ensuring soil N is most available close to growing plants, as occurs with furrow mulching and fertilizer banding, and at the field scale, where erosion and other geomorphological processes have created sub-field regions of low fertility. Using satellite images with 30 × 30 m sub-field resolution in 8 M cornfields across 30 Mha of the U.S. Midwest, Basso et al. identified that low-yield sub-field areas covering over half of the region had low NUE , in contrast with high-yield, high-NUE areas . The N losses from low-yield areas could explain a major portion of the average annual 1.12 New fertilizer formulations are a target for improving NUE of crop systems, primarily aiming to slow solubilization and the conversion of fertilizer N to more mobile forms while plant N demand is low. Globally, efforts are accelerating to improve N fertilizers, from nanotechnology formulations to achieve targeted release profiles, to supplying N as part of organic matter to slow the N release . Fertilizers and application technologies are being designed to take the physiological needs of crops as an entry point for fertilizer development . Enhanced efficiency fertilizers are formulations with coatings that consist of polymers or other materials that prevent immediate solubilization,dutch buckets for sale or with added inhibitors to temporarily slow the activity of urease enzymes and that of nitrifying microbes.

Several meta-analyses have reported small, but consistent, positive yield responses to N fertilizers treated with urease inhibitors, nitrification inhibitors, or a combination of both . The variability in yield response to these treatments has been attributed to interactions among genetics, environment, and management . EEFs containing urease inhibitors were successful in paddy rice systems, increasing average NUE by 29% and reducing N losses by 41%, while the various types of EEFs in wheat and maize systems are generally less effective , and yield responses may be site-specific . However, meta-analyses have also exposed possible pollution swapping when applying nitrification inhibitors, where a decrease in N2O emissions coincides with an increase in NH3 volatilization . Of additional concern is that some enzyme inhibitors can be transported to surface waters and non-biodegradable polymer coatings can impact soil biota such as earthworms . Alternative slow release fertilizer formulations are being developed, for example with biodegradable polymers that soil microbes can consume . Crop N physiology must be considered in all N fertilizer regimes. While NO− 3 and NH+ 4 are considered the main N sources for crops, all plants that have been examined can use organic N . The exact proportion of inorganic and organic N acquired by plants remains unknown , but the presence of soluble organic N in soils is well-documented. Soluble organic N is associated with reduced losses, which might motivate the development of alternative, organic N-based fertilizers, nutrient management, and crop breeding . Comparing the fluxes of inorganic and organic N forms in differently fertilized sugarcane soil, the estimated root intake rate for amino acids matched soil fluxes, while fluxes of NH+ 4 and NO− 3 exceeded the root intake rate . To maximize NUE, the release rate and forms of N should match the crop’s N acquisition capacity . Organic matter, including recycled organic wastes , has potential as an N source, and is widely used, though not always with the aim to supply nutrients . The overall effects of organic fertilizers are difficult to disentangle because soil physical, chemical, and biological properties are altered. Compared to inorganic fertilizer only, field experimentation often shows benefits when organic and inorganic fertilizers are supplied together due to interacting effects of improved micro-nutrient and soil microbial community status . In a global meta-analysis, Xia et al. found that substituting up to 50% of the mineral fertilizer N with fresh or composted manure increased grain crop yields, crop N uptake, and N use efficiency, but substituting more than 75% of mineral fertilizer N with manure negatively impacted yields. The authors also reported environmental benefits of integrative management, including a reduction in N losses and improvement in soil organic carbon content. On the other hand, regional trends for NUE in the USA are negatively associated with the proportion of total N from livestock excreted N, largely because manure is treated as waste rather than a nutrient . Net anthropogenic N balances for these regions are also high, indicating elevated risk of environmentally-concerning losses . Thus, there is both need and opportunity to repurpose nutrient-rich wastes as fertilizers, which requires formulating suitable nutrient stoichiometry and N release profiles to avoid N over- or under-supply of target crops. Managing the release of N from crop residue and as well from endogenous soil organic matter stores during the growing season is a difficult proposition. Tillage, developed primarily for weed control, has been traditionally used for this purpose but inefficiently so—tillage occurs 6–8 weeks prior to high plant demand for N, leaving a significant intervening window for N loss as accelerated microbial activity mineralizes soil organic N stores. A purported advantage of no-till is to avoid this quick release ofN, and while subsequently slower mineralization rates avoid the early pulses of N associated with tillage, there is little evidence that no-till reduces exogenous N needs and, by inference, improves NUE. No-till does, however, appear to reduce off- season N losses as more of the N immobilized in crop residues persists in accumulating soil organic matter until a new equilibrium is met. Improvements to soil porosity and other physical properties related to soil structure can also keep inorganic N from being easily leached , though beneficial effects on gaseous N losses are less clear . Improving the adsorption capacity of soils, or the ability to bind ions to soil components, is another approach used to control soil N cycling. NUE of maize and rice systems improved substantially when adding clay, such as zeolite, most likely due to enhanced NH+ 4 retention .