Boosting Nitrogen Use Efficiency in Crops: A Practical Guide

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Nitrogen is a crucial nutrient for crops, but it often limits growth and yield. Farmers and agricultural experts aim to make the most of nitrogen fertilizers, especially considering the costs, environmental concerns, and the role nitrogen plays in global warming. That’s why maximizing nitrogen use efficiency (NUE) is essential. In this guide, we’ll break down NUE, what it means, how it works, and the best ways to improve it.

What is Nitrogen Use Efficiency (NUE)?

At its simplest, Nitrogen Use Efficiency (NUE) can be thought of as how well crops use nitrogen from fertilizers to produce grain or yield. One way to express it is as a formula:

NUE = Yield / Applied Nitrogen

This tells us how much yield we get for every unit of nitrogen fertilizer applied. However, this approach doesn’t dig into what happens to the nitrogen once it’s in the soil or how much of it the plant actually uses. But don’t worry, we’ve got more detailed ways to understand NUE!

Breaking Down the Components of NUE

A more detailed version of NUE takes into account various factors. Here’s the improved formula:

Boosting Nitrogen

NUE = Soil N Available to Plants × Plant N Uptake × Plant N in Grain

Let’s look at these three components:

  1. Soil N Available to Plants:
    • Not all nitrogen applied to the soil is available to plants. Nitrogen can get lost through runoff, leaching, or microbial activity in the soil. In fact, often less than 40% of the nitrogen is available for the crops due to these losses.
  2. Plant N Uptake:
    • Plants are quite efficient in absorbing the available nitrogen from the soil, using high-affinity and low-affinity transport systems. These systems allow plants to take in nitrogen even when the soil has low or fluctuating levels of it.
  3. Plant N in Grain:
    • Not all nitrogen taken up by the plant ends up in the grain. Some of it goes to other parts of the plant like leaves and stems. Roughly 80% of the nitrogen absorbed by the plant can make it into the grain.

The Biggest NUE Challenge: Soil Nitrogen Loss

Soil nitrogen loss is the biggest barrier to improving NUE. Environmental factors such as rain, soil type, and microbial activity can cause nitrogen to escape the soil before plants can use it. Unfortunately, there aren’t many practical solutions to stop these losses at the moment.

However, plants are generally efficient at absorbing the nitrogen that is available. They have complex mechanisms to regulate how much nitrogen they take in based on how much they can actually use at any given time. These internal communication systems make sure plants don’t waste energy taking in nitrogen they can’t process.

Can We Improve NUE?

There have been attempts to genetically modify plants to improve their nitrogen uptake, but most haven’t significantly boosted the amount of nitrogen that ends up in the grain. One promising approach has been to increase plant mass through certain genetic modifications, but the exact reasons for this are still unclear.

Practical Tips for Farmers to Improve NUE:

  1. Time Nitrogen Applications: Apply nitrogen fertilizers at the right time, when the plant needs it most, to reduce nitrogen loss.
  2. Use Slow-Release Fertilizers: These fertilizers reduce the chance of nitrogen being lost to the environment before the plant can use it.
  3. Monitor Soil Conditions: Keep an eye on soil moisture and temperature, as these affect nitrogen availability and loss.
  4. Consider Crop Rotation and Cover Crops: These practices can improve soil health and reduce nitrogen loss over time.

Key Takeaways for Infographics:

  • NUE = Yield / Applied Nitrogen
  • NUE depends on three factors: available soil nitrogen, plant uptake, and nitrogen in the grain.
  • Soil nitrogen loss is the biggest challenge, but plants are good at regulating nitrogen uptake.
  • Timing nitrogen applications and using slow-release fertilizers can help improve NUE.
  • Current genetic modifications for improving NUE have had limited success, but increasing plant mass might be a promising direction.

In summary, improving NUE is essential for maximizing crop yield, minimizing costs, and reducing environmental impact. While there are challenges, especially with soil nitrogen loss, practical steps can be taken to make nitrogen use more efficient in crop production.

How Plants Accumulate and Use Nitrogen for Growth

Nitrogen plays a pivotal role in ensuring high crop yields. But how exactly do plants manage their nitrogen supply, and what can be done to improve it? Let’s dive into how nitrogen is stored and used in plants, especially during the critical growth phases.

Nitrogen Circulation in Plants

Plants have a sophisticated system to manage nitrogen absorption and distribution, primarily through their vascular system. Nitrogen travels between the roots and shoots in the plant via two main routes:

  • Xylem: Transports nutrients, including nitrogen, from the roots to the leaves.
  • Phloem: Helps move the nitrogen back to the plant’s growing parts (like grains) as needed.

This internal “nitrogen cycle” allows plants to balance their nitrogen intake with their growth requirements. When nitrogen levels are high in the leaves or shoots, the plant sends signals to the roots to slow down nitrogen absorption, ensuring no excess is taken in.

Extending Nitrogen Uptake into the Reproductive Phase

One strategy for boosting nitrogen use efficiency (NUE) is to extend the period of nitrogen uptake into the reproductive phase of a plant’s growth, after anthesis (when flowers develop). This can significantly increase the plant’s overall nitrogen content and improve yield.

However, after anthesis, nitrogen uptake usually decreases because:

  • There’s not much nitrogen left in the soil.
  • Root systems lose some capacity for nitrogen assimilation.

Despite this, some crops, like maize and wheat, have been shown to absorb nitrogen after anthesis under certain conditions. To make this happen:

  • Fertilizer management: Providing nitrogen at the right time, especially late in the growing season, is crucial.
  • Slow-release fertilizers: These can help ensure nitrogen is available for longer, minimizing losses due to leaching.

Nitrogen Storage in Plants

Most nitrogen in plants is assimilated into compounds like proteins and stored in various tissues. Leaves, in particular, store large amounts of nitrogen due to the presence of Rubisco, a protein involved in photosynthesis. However, the capacity for nitrogen storage is limited.

Plants store nitrogen early on in growth, but as they mature, stems—where less nitrogen is stored—become a larger part of the plant mass. This results in a slowing down of nitrogen accumulation as the plant grows.

The Role of Nitrogen in Grain Production

As seeds develop, plants rely on the nitrogen stored in their leaves and stems. They break down proteins in these tissues and transport the resulting amino acids to the grains. While this helps nourish the seeds, it also leads to a decline in the plant’s photosynthetic capacity, as leaves begin to age and die off, a process often referred to as plant “self-destruction.”

Can We Improve Nitrogen Efficiency in Plants?

Several studies have explored ways to improve how plants transfer nitrogen to their seeds, particularly by genetically modifying plants. However, these studies have not significantly improved Nitrogen Harvest Index (NHI), which measures how efficiently a plant transfers nitrogen to its seeds. In modern crops, NHI is already very high, often around 80%.

The biggest challenge remains improving how much nitrogen plants can access from the soil. New soil management practices and improved fertilizers may offer the most promise for boosting NUE.

Practical Tips for Farmers to Improve Nitrogen Use in Crops

  1. Apply Fertilizer Strategically: Ensure nitrogen is available during key growth phases, especially the reproductive phase.
  2. Use Slow-Release Fertilizers: This can help reduce nitrogen loss and ensure a steady supply throughout the growing season.
  3. Improve Soil Management: Techniques like cover cropping and crop rotation can help improve soil health and nitrogen retention.
  4. Monitor Plant Growth: Use nitrogen sensors or regular testing to ensure the plant has enough nitrogen at all times.

Key Takeaways for Infographics

  • Nitrogen circulates in plants through the xylem (roots to leaves) and phloem (leaves to seeds).
  • Extending nitrogen uptake into the post-anthesis phase can boost crop yields.
  • Leaves play a major role in storing nitrogen, but as plants grow, nitrogen uptake slows due to stem growth.
  • During grain fill, plants transfer stored nitrogen to seeds, which reduces their photosynthetic ability.
  • Current crops have high Nitrogen Harvest Index (NHI), meaning there’s little room for improvement in nitrogen transfer efficiency.
  • The biggest challenge is improving the amount of soil nitrogen available to plants.

By understanding how plants manage nitrogen and by implementing better fertilizer and soil practices, farmers can boost their crop yield and reduce nitrogen losses. Source

Boosting Nitrogen Use Efficiency in Crops

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