Improving Nitrogen Use Efficiency

IMPROVING NITROGEN USE EFFICIENCY IN AGRICULTURE
A PATH TO PROFITABILITY AND SUSTAINABILITY

Nitrogen is an essential nutrient for crop growth however, less than half of the Nitrogen applied to crops is absorbed by the plant. The remainder is lost to the environment, polluting waterways or as greenhouse gas emissions. Nitrogen Use Efficiency (NUE) is a key metric that measures how much Nitrogen is absorbed by plants. NUE helps farmers to optimise fertiliser application by choosing the right Nitrogen source, timing of application, adjusting rates and targeting placement of product. Enhancing NUE can result in improved yields, lower emissions, lower input costs and ultimately greater profit per hectare.

INTRODUCTION

The agricultural industry is at a crossroads, needing to sustain a growing global population while under pressure to minimise environmental harm.

Supporting a larger global population can be achieved through expanding agricultural land and/or improving land productivity. Expanding agricultural land requires emissions-intensive land clearing, negatively impacting ecosystems and the environment. Improved land productivity is therefore a more sustainable option moving forward, driven by improved Nitrogen Use Efficiency (NUE).

Nitrogen is a key driver of agricultural output and a major contributor to pollution. For much of human history, Nitrogen supply was constrained, limiting the productivity of agricultural land and by extension the global population. Today, the availability of Nitrogenous fertilisers supports over half of the world's population [1]. As the global population continues to grow, continued Nitrogen availability will be essential.

NUE cannot be improved by simply adding more Nitrogen. Despite its critical role, Nitrogen uptake by crops is inefficient. Less than 50% of applied Nitrogen is used by crops, with the remainder lost to the environment [2]. Nitrogen losses are directly related to emissions meaning that any attempt to increase food production solely through increased fertiliser use would greatly undermine environmental goals.

Methods for improving NUE are well understood but to date have largely remained costly and operationally cumbersome. However, new technologies and growing evidence for yield improvements are setting the stage for farmers to adopt new practices and deliver value to the agricultural value chain.

Most farmers in Australia apply fertiliser as a granule of Urea directly onto the soil. The granule begins to decompose, releasing the Carbon Dioxide (CO2) in the granule. The Nitrogen in the granule is gradually converted into Ammonium (NH4+) and then into Nitrate (NO3-). As the Nitrogen undergoes this chemical transformation losses can occur. These losses happen via ammonia volatilisation, nitrification and denitrification, leaching, immobilisation and mineralisation. The extent and rate of Nitrogen loss is strongly influenced by the soil type, moisture levels, temperature and environmental conditions.

NITROGEN UPTAKE AND LOSSES

Urea Decomposition

Description: Occurs through a process called hydrolysis, catalysed by urease.

Key Lost Chemicals:

Carbon Dioxide (CO₂)
Ammonia (NH₃)

Volatilisation

Description: A process where NH₃ is lost via air movement, is more susceptible to urea-based fertilisers.

Key Lost Chemicals:

Ammonia (NH₃)

Nitrification

Description: A microbial process, converts Ammonium into Nitrate, which can be leached or absorbed by plants.

Key Lost Chemicals:

Nitrous Oxide (N₂O)
Nitric Oxide (NO)

Denitrification

Description: A microbial process, generates N₂O gas from Nitrate ions.

Key Lost Chemicals:

Nitrous Oxide (N₂O)
Nitric Oxide (NO)
Nitrogen (N₂)

Leaching/Runoff

Description: Rainfall can wash nitrogen away in surface runoff, leading to water pollution.

Key Lost Chemicals:

Nitrate (NO₃⁻)

Immobilisation and Mineralisation

Description: Immobilisation temporarily locks up available Nitrogen in microbial biomass, making it temporarily unavailable to plants.

Key Lost Chemicals:

Ammonium (NH₄⁺)
Nitrate (NO₃⁻)

Table 1: Nitrogen loss pathways for Nitrogen fertilisers

NUE is a key metric of nutrient management that indicates how effectively crops utilise Nitrogen. The aim is for crops to absorb the maximum amount of Nitrogen without oversaturating the soil with Nitrogen [3]. Defined as the ratio of Nitrogen in the plant to the Nitrogen applied to the soil, NUE is a critical metric for productivity. Increasing Nitrogen uptake without increasing input leads to higher yields, better land-use efficiency, reduced environmental impact, and greater profit per hectare [3].

NUE is not a fixed value. It fluctuates depending on crop type, growth stage, weather conditions, and soil characteristics. For example, a wheat crop will absorb Nitrogen at different rates during germination, flowering, and harvest. Rainfall, temperature, and even the time of day can affect Nitrogen absorption. Due to this variability, most farmers still approach fertiliser application with their primary concern being the total amount of Nitrogen applied as it is a simpler and more consistent metric which delivers known outcomes.

Although it is difficult to permanently quantify NUE, there are some proven methods for how it can be improved. The 4R Nutrient Framework highlights the different levers which can be pulled to improve NUE leading to improved productivity. It poses a simple but powerful question: How can Nitrogen be applied using the right source, at the right rate, at the right time, and in the right place? [3, 4]. The improvements each ‘R’ can achieve meaningful overall productivity increases and/or reduced spend on fertiliser. In both cases, farmers can better control their revenue and costs.

NUE is a well-established concept familiar to most in the agriculture sector, yet it is the Nitrogen volume which dominates the conversation. The main challenge with NUE is that it is dynamic and harder to measure compared to recording how much Nitrogen is applied. Further, existing practices of bulk dosing Urea are undoubtedly functional and have delivered sustained success. There has been little incentive to change as steps to improve NUE often require expensive inputs, new infrastructure and behaviour changes. However, as productivity and environmental responsibility become of increasing importance, farmers may need to adopt new methods and behaviours making NUE a more essential consideration moving forward.

UNDERSTANDING & IMPROVING NUE

RIGHT SOURCE

Source is the chemical form in which the Nitrogen is applied. Crops can absorb both Ammonium and Nitrate-based forms of Nitrogen. Nitrate-based fertilisers are generally more efficient as they can be immediately absorbed whereas Ammonium-based fertilisers need to undergo a multi-step transformation [3,5]. One study showed that grain yields in corn could increase by as much as 33% when applying Ammonium Nitrate compared to Urea [6]. In practical terms, this means that farmers can achieve the same – or even better – crop performance using less Nitrogen, simply by switching to a more efficient source.

Although Nitrate-based fertilisers can provide greater efficiency, Urea-based fertilisers are generally cheaper and therefore more common. As an example, the average cost of delivered Urea (on a kg Nitrogen basis) over the last 5 years has been approximately AU$1.95, compared to AU$3.57 when delivered in the form of Ammonium Nitrate. Given that Ammonium Nitrate is approximately 92% more expensive based on current technology, the potential yield gains from using Nitrate-based fertiliser are often outweighed by the cost differential.

RIGHT RATE

Crop growth is limited by the most deficient nutrient. The amount of Nitrogen which is applied determines whether Nitrogen is a limiting factor. When Nitrogen is below the crop’s requirements, yields are limited and when Nitrogen is above the threshold the plant is absorbing as much Nitrogen as it needs. The benefits in increasing the rate at which Nitrogen is applied demonstrates diminishing marginal returns as the crop approaches its threshold. One study in winter wheat showed the diminishing yields as rates increased [7]

The right rate is probably the most commonly and effectively utilised “R” as it is a proxy for Nitrogen content. Applying the right rate requires an in-depth knowledge of a crop’s requirements as well as the level of residual Nitrogen in the soil prior to planting. This is knowledge that many farmers possess and use to their advantage. Farmers can leverage crop rotations to enhance or preserve Nitrogen in the in soil prior to a new rotation and take soil measurements to dictate the amount of Nitrogen required for application.

Figure 2: The effect of increasing rates of Nitrogen fertiliser on wheat yields [7]

RIGHT TIME

Crops require different amounts of Nitrogen at different points in their growth cycle. The study summarised below shows the impact of timing Nitrogen application for oats. Nitrogen was applied at different growth stages (GS) and showed that Nitrogen availability is more important at earlier stages rather than later [8].

Farmers typically apply the bulk of their Nitrogen right at the beginning of a crop cycle with some adding additional Nitrogen ensure availability in the later growth stages. Splitting the application of Nitrogen can deliver even more benefit to farmers but poses some operational complexity. Repeated applications require extra time applying additional fertiliser. In an industry where time is a limited and valuable resource, farmers have not necessarily seen the quantitative benefits of splitting the application of solid fertiliser to justify the extra time spent in repeated field operations. This means that splitting application of Nitrogen has been somewhat limited. However, split applications of Nitrogen are extremely common where farmers are fertilising through irrigation systems (fertigated). Fertigation decreases operational barriers, reduces handling of fertilisers and can improve yields making it a widely accepted practice for those with the infrastructure.

Figure 3: The effect of fertiliser timing on an oat crop [8]

RIGHT PLACE

The placement of fertiliser also has a significant impact on the yield of a crop. Fertiliser can be placed on the surface or at a depth closer to the root zone. One study showed that subsurface placement of Urea increased yields by 27.3% compared to broadcast [9]. This is because crops grow towards nutrient-rich sites, which, if subsurface, encourages healthy root development.

Precision placement of fertiliser requires specific equipment. Many farmers use disc spreaders which are effective at broadcasting solid fertiliser pellets onto the soil surface. This delivers a level a lateral precision which can positively contribute to a higher NUE. Some fertiliser applicators inject or knife pellets into the soil for deeper placement. These vertical precision application requires specialist machines which come at a higher cost reducing the rate of adoption from farmers.

CONCLUSION

The 4R Nutrient Framework is a proven concept to improve NUE, addressing agronomic, environmental, and regulatory demands.

PlasmaLeap’s platform technology offers part of the solution to address current challenges hindering NUE improvements. Our breakthrough approach, eNFix, synthesises a liquid Nitrate-based fertiliser with an ultra-low emission profile. Critically, this fertiliser is cost-competitive with conventional sources, enabling farmers to benefit from the higher yield potential of Nitrate-based products without increasing input costs. Liquid fertilisers also allow for greater flexibility in the rate, timing and placement of application. By leveraging existing infrastructure, cost-effective Nitrate-based liquid fertilisers like that produced by eNFix can deliver, agronomic, environmental and financial benefits – supporting farmers and reducing emissions.