Controlling Nitrogen Fertilization for Crops

Nitrogen deficiency is one of the main reasons for low crop yields. Research is focusing on plant physiology and morphology to make a more accurate estimation of nutrient needs. As studies diversify and become more in-depth, new precision tools useful in the field are needed. Hence, many complicated technologies have been miniaturized into portable and small devices.

Problems Created by Nitrogen Limitation

Industrial agriculture relies heavily on new genetic breeds to improve yield. By themselves, they are not enough. Nitrogen and water availability are just as important. The new strains can provide optimum returns, only if the package of nutrients and water supply they were designed for is available on farms.

Scientists have found that nitrogen is a limiting factor, especially in developing countries.

There is nitrogen in the air and soil, but these are not in forms that the plants can use. Hence, it has to be provided as fertilizers or manure to make up for deficiencies.

However, discriminately increasing the application of nitrogenous fertilizers is not a solution either. Excessive use of nutrients has turned agriculture into a significant polluter of air, water, and land.

The production of nitrogenous fertilizers is one of the main contributors to greenhouse gases, which lead to climate change. Unused fertilizers seep into the soil and destroy beneficial microorganisms, which recycle nutrients and maintain soil structure. Nitrogen is regularly washed off by rainwater into surrounding water bodies and ultimately reaches the oceans to cause nutrient pollution.

In addition, nitrogen fertilization increases the cost of farming.

Hence, it has become necessary to find the minimum quantities of fertilizer that can provide optimum yield.

Earlier fertilizer recommendations were based on crop research that tested different titrations of nutrients and irrigation for a new variety. Research is now far more sophisticated and can track hidden physiology with the help of a new breed of instruments that are available on the market.

A few novel research and technological approaches to reduce nitrogen application are discussed in this article.

Fertilizers Boost Yield by Improving Photosynthesis in Legumes

Figure 1: Symbiotic nitrogen-fixing bacteria are an integral part of the nitrogen cycle, Environmental Protection Agency, U.S. Federal Government. (Image credits: http://tolweb.org/notes/?note_id=3920)

Nitrogen (N) is one of the major nutrients needed by plants, along with potassium and phosphorus. N is necessary to produce chlorophyll that is actively involved in photosynthesis. It is also part of many of the compounds in plants, like amino acids, proteins, and enzymes. It is necessary even to produce the nucleic acids of which genes are composed.

Nearly 95-99 percent of the nitrogen in soils is in the form of stable organic compounds or in microbes that can use nitrogen. Different groups of bacteria fix gaseous nitrogen or break-down organic matter and use the nitrogen in them. Some are free-living, while others live in association with plants, such as legumes, as shown in Figure 1.

Legumes are plant species that form a symbiosis, a mutually beneficial association, with nitrogen-fixing bacteria, such as Rhizobium, Frankia, or Azospirillium. The bacteria live in nodules formed in the roots. Plants give energy to the bacteria and, in return, benefit from the nitrogenous exudates released by the microbes.

Thus, legumes have an advantage that other plants lack, so they are used as companion crops for cereals and other plants. However, sometimes even symbiosis fails to provide legumes enough nitrogen.

In China, cold soil temperatures, water, and pH can retard microbial activity in soyabean, a legume. Moreover, it takes at least nine days before the nodules with microbes can form on the roots of soyabean and fourteen days before nitrogen-fixing can begin.

So, the application of a starter dose of N in the early stages could be a boost for the plants. However, the presence of readily available N can also retard the formation of nodules and nitrogen fixation. Hence, to find out how nitrogen impacted yield, scientists decided to add it as a starter during sowing to check its effect on vital processes, like root activity and photosynthesis.

The experiment was conducted for two years in the fields of Heilongjiang Academy of Agricultural Sciences. Phosphorus and potassium, as well as nitrogen, were supplied. Four doses (0, 25, 50, and 75 kg N ha-1) were supplied in the form of urea. Leaf samples were taken from five plants, which were collected three times at the unrolled leaves stage, full bloom stage, and entire pod stage.

Leaf area was estimated by a leaf area meter, nitrogen content by the Kjeldahl method, and chlorophyll content by a Chlorophyll Meter. Root activity was measured indirectly by evaluating dehydrogenases.

The CI-340 Handheld Photosynthesis System by CID Bio-Science Inc. was used to measure photosynthesis. The CI 340 is a handheld instrument, which can be used with one of ten customized leaf chambers to fit leaves of different sizes. The photosynthetic rate is evaluated as the difference in carbon dioxide (CO2) before and after it enters the leaf chamber. The device can be used with modules that control water, CO2, temperature, and light intensity. It gives non-destructive readings rapidly. It helps experiments by avoiding the need to harvest leaves during repeated data collection in a plant’s life.

Since the instrument measures gas exchanges, it is also used to estimate transpiration and respiration.

The experiment showed that the starter dose of nitrogen increased chlorophyll in leaves and, therefore, photosynthesis in the first two stages. Since, it is the process that produces biomass for the plants, the improved photosynthetic rate increased yield. Root activity, just like photosynthesis, was the best when nitrogen was added at intermediate levels of 50 kg N ha^-1, resulting in improved yields.

The starter dose of fertilizers was beneficial for soyabeans, and it didn’t affect nodule formation or nitrogen-fixation. So, a single dose of fertilizers is needed even for soyabeans.

Data for Precision Farming

Research recommends one set of optimal nitrogen levels for each variety and the number of times the nutrients are to be applied. However, due to local and regional differences in soil and climate, crop response to N fertilizers can still vary. As a result, farmers may not get the best possible yield. Precision farming is one of the ways to ensure that the correct rate of nutrients is supplied. This new approach to agriculture relies on data to locate differences within a farm or a locality in crop performance. Subsequently, nutrients and water are supplied when needed.

Scientists are, therefore, comparing data sources to choose the one that provides more accurate information. To do this, they had to verify the remote imagery with ground-level data. Scientists also needed to check that remote imaging correlates accurately with agronomic measurements of available N in the field and leaf N concentrations.

One of the current, non-invasive methods used to study the effect of fertilizers is imagery, which is collected by airplanes that use remote sensing or the use of small drones. These images measure the greenness of crops. Since nitrogen is needed to produce chlorophyll that makes leaves green, this is a useful estimate of the nutrient absorbed by plants.

An experiment was conducted on maize in Spain to compare the two methods, imagery via drone or airplaine, to see which one could provide accurate information to farmers. The maize was fertilized at five rates varying from 0 to 220 kg N ha^-1.

Multispectral images by drones were collected close to the crop, from a height of eighty meters. Also, hyperspectral images were taken by airplanes three hundred and thirty meters up from the fields.

Chlorophyll indices (Transformed Chlorophyll absorption in reflectance index -TCARI), xanthophyll indices, blue/green/red ratio indices, and fluorescence retrieval were calculated to estimate the crop N status by using the remote imageries.

The canopy of crops were also considered as they are a result of plant growth, and they will also influence field-scale measurements of greenness. Several vegetation indices measured the canopy, like Normalized difference vegetation index (NDVI) and Optimized soil-adjusted vegetation index (OSAVI).

On the same day that the sensors collected data, many ground level indices were also estimated for comparison. This included measuring the chlorophyll of leaves, by a Chlorophyll Meter on the ground and leaf area and N levels.

Leaf area index was calculated by using the CI-203 Handheld Laser Leaf Area Meter produced by CID Bio-Science Inc. with non-destructive readings. The instrument makes rapid measurements in the field, and the inbuilt GPS helps to track the plant and leaves for repeated analysis.

Both ground and airborne sensors were found to be effective in detecting the nitrogen status of plants through chlorophyll estimation. The estimation was more accurate when they were combined with vegetation indices or canopy structures; canopy reading by OSAVI was more useful than NDVI.

The difference in spatial scale between images from drones and airplanes didn’t influence structural indices, but it did alter estimations of chlorophyll. The ground measurements were the best for chlorophyll estimations. Drones were more sensitive to canopy readings and gave more weight to bare soil; therefore, their estimates of leaf N were less accurate compared to airplane images.

Both the remote sensors were more successful at identifying nutrient deficiency than capturing information about N saturation. Here, the ground sensors are more reliable.

Farm-Level Tools

The two studies show how dynamic crop research has become. Investigations are not just more in-depth but are also combined at the same time with data on vast spatial scales. However, it is worth remembering that the importance of ground-level data will never be replaced by remote imagery. If anything, data collected at the farm level will only increase in the future. So, a technology that is small and rapid, yet precise is going to be the order of the day; all stakeholders will need them in the food production industry, from scientists and farmers to government organizations.

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Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Feature image courtesy of CIAT

Sources

Gabriel, J. L., Zarco-Tejada, P. J., López-Herrera, P. J., Pérez-Martín, E., Alonso-Ayuso, M., & Quemada, M. (2017). Airborne and ground level sensors for monitoring nitrogen status in a maize crop. Biosystems Engineering, 160, 124–133. doi: 10.1016/j.biosystemseng.2017.06.003

Gai Z, Zhang J, Li C (2017) Effects of starter nitrogen fertilizer on soybean root activity, leaf photosynthesis and grain yield. PLoS ONE 12(4): e0174841. https://doi.org/10.1371/journal.pone.0174841

Sinclair, T. R., & Rufty, T. W. (2012). Nitrogen and water resources commonly limit crop yield increases, not necessarily plant genetics. Global Food Security, 1(2), 94–98. doi: 10.1016/j.gfs.2012.07.001