Nitrogen fertilization: specifics and applications

Nitrogen fertilization is given the greatest attention since it is a nutrient that affects the yield the most. However, if it is incorrectly applied (at an inappropriate time, form or dose), the natural cycle results in its unnecessary loss. This lowers the efficiency of the fertilization, and the costs of the nitrogen fertilizers and their application will not result in the expected yield.

In order to understand the complexity of the use of nitrogen in plant production and its effects on the ecosystem, it is worth recalling nitrogen transformations in soil. The conditions of nitrogen transformation are influenced by soil properties as well as weather patterns. However, it is difficult to predict the weather in the long term, so it is necessary to take into account the current conditions of the year with regard to the fertilized crops, the dose and form of the applied nitrogen. It is therefore generally not possible to prepare a "guaranteed" fertilization method for nitrogen fertilization, but it is possible to build on the knowledge of the transformations described below.

Nitrogen in soil

During the year, there are significant changes in the content of inorganic nitrogen in the soil (chart 1). In spring, from April to May, the warming of the soil increases the activity of microorganisms and the content of mineral nitrogen reaches its maximum values (i.e. spring maximum). In the course of vegetation, the consumption of nitrogen by plants and the gradual reduction in the intensity of mineralization reduces the content of mineral nitrogen in the soil to a relatively stable value just before and after the harvest (summer minimum). Under favourable moisture and temperature conditions, the N-Min content in the soil in autumn begins to increase by the mineralization of post-harvest residues (autumn maximum) and then drops again before winter because microorganism activity decreases due to the drop in temperatures. This great seasonal variability in mineral nitrogen in the soil must be respected and used in practical plant nutrition when determining nitrogen doses for particular crops before planting as well as during fertilization during vegetation.

Mineralization

Mineralization is the process of decomposition of organic matter in soil, during which nutrients that can be used by plants are released from organic bonds. The mineralization of organic nitrogenous substances is generally understood as the process of ammonification, i.e. the conversion of organic compounds to ammonia. In the conditions in the Czech Republic, 50-90 kg N/ha is released in arable soils per vegetation season during mineralization.

Factors affecting mineralization

• Temperature: The optimum temperature for mineralization is around 30 °C; when the temperature drops by 10 °C, its intensity decreases by 50 %. Mineralization is very low at low temperatures, and it almost stops around 0 °C.
• Moisture: A change in moisture affects mineralization less than the temperature. The mineralization is more intense when drought and wet periods alternate. During mineralization, NH3 is released from organic nitrogenous substances, where it receives a proton in an aqueous environment and is converted to NH4+. In a dry environment, there is a higher risk of loss through N volatilization (see below).
• Aeration: The mineralization process takes place under aerobic and anaerobic conditions of a number of physiologically very different microorganisms and invertebrates.
• pH: The influence of a soil reaction in the range of pH 5–8 is very small.
• Other factors: Mineralization takes place faster in the rhizosphere than in the bulk soil to release more easily degradable substrates. Mineralization is also positively affected by the quantity and quality of organic matter in the soil. Quality is the ratio between C/N. As the C/N ratio increases, the amount of released nitrogen into the soil solution decreases exponentially. The soil type also affects the mineralization process. Mineralization is higher in sandy soils than in clay and argillaceous soils. The addition of nitrogen to the soil also has a positive effect on increased mineralization, i.e. the priming effect.

Recommendations

Under suitable conditions for mineralization, plants have sufficient amounts of mineral nitrogen, often more than with nitrogen fertilizers. Generally speaking, it is also necessary to increase nitrogen fertilizer doses and to use LAV fertilizers in dry weather (as well as in cold weather), and in wet and warm periods it is possible to reduce nitrogen doses, with the exception of very light soils.

Nitrification

Ammonia released by mineralization (ammonification) enters various processes, primarily as the main source of nitrification. Nitrification is a key process in many soils and ecosystems because it transforms a relatively immobile ammonium form into a highly mobile nitrate form of nitrogen. This makes the nitrogen a nutrient that is available to plants, but there is also a risk of its loss by leaching and denitrification.

Factors affecting nitrification

• Temperature: The optimum temperature for nitrification in soils is between 25 and 30 °C; at temperatures below 15 °C the nitrification is limited, and at temperatures lower than 5 °C nitrification only occurs to a minimal extent.
• Moisture: The optimal soil moisture for nitrification is 70 % minimum air capacity. The nitrification stops in dry conditions.
• Aeration: Nitrification is an aerobic process, and it is therefore faster in aerated soils
• pH: Nitrification optimally takes place at a pH range of 6.5–8.5; at a pH under 6.5 nitrification intensity decreases, and it stops below 5.

Recommendations

The rate of nitrification is also affected by the type of fertilizer applied. Nitrogen supplied in ammonium form in ammonium sulfate fertilizers is nitrified slowly, but urea nitrogen is nitrified very quickly. After a relatively short period (5-7 days), its nitrification process is similar to that of the transformations of nitrogen supplied in ammonium nitrate (saltpeter) fertilizers (chart 2). Some types of urea-based fertilizers use nitrification inhibitors, but it is important to emphasize that their effects are also greatly influenced by weather patterns.

In warm and dry weather, these inhibitors may paradoxically reduce the availability of N in the urea, especially when it is applied to the surface layers of the arable land, resulting in ammonia losses by volatilization.

 Denitrification

Unlike nitrification, denitrification is a reduction process where nitrates are reduced to nitrogen oxides and elemental nitrogen in the presence of readily degradable organic substances. In our conditions, denitrification caused by facultatively anaerobic microorganisms, which use the oxygen from nitrates during decomposition, is prevalent.

Factors affecting denitrification

• Temperature: The optimum temperature for denitrification in soils is between 25 and 30 °C; at temperatures below 10 °C, denitrification only occurs to a limited extent.
• Moisture: The highest denitrification values are achieved at a soil saturation of 60 to 100 % minimum air capacity.
• Aeration: Denitrification occurs during insufficient soil aeration (in anaerobic conditions).
• pH: Denitrification occurs at a pH of 6–8; at a pH below 5.5, the intensity of the dentrification decreases.

Recommendations:

It is mostly important that there is not high nitrate content in the soils, especially towards the end of the vegetation period and after the vegetation period when there is an increased risk of high water content in the soil and therefore a limited oxygen content. Free nitrates in the soil can be used by a winter crop in the autumn, but if this crop is followed by a spring crop, it is appropriate to use a green fertilizer to reduce the loss of nitrate nitrogen. This also applies to the reduction of the leaching of N outside the growing season. However, nitrate nitrogen is not used by microorganisms during the plant-back of straw because its intake is energy-consuming, so they mostly use ammonium nitrogen instead. Denitrification losses are higher especially when fertilizing with N nitrate in the autumn. The intensity of denitrification increases as the concentration of NO3– increases in the soil; the soil may therefore lose up to 40% of nitrogen from the applied nitrate fertilizers through denitrification.

Volatilization

Volatilization is the process of nitrogen loss from the soil caused by the vaporization of ammonia from the surface or top layers of the soil. Losses through volatilization are about 5 %, but they may even reach over 25 % of the applied nitrogen depending on the soil-climatic conditions, the dose and the form of the fertilizer, as well as the method and time of application

Factors affecting the process

• Temperature: Increasing temperatures cause a greater release of ammonia.
• Moisture: When the water content in the soil decreases (especially on the surface), ammonia is released in a greater amount.
• Aeration: Greater volatilization of ammonia occurs in anaerobic conditions.
• pH: Ammonia volatilization is predominantly influenced by the pH value; it most often occurs in alkaline soils.
• Other factors: Type of soil - the content of clay particles (as well as stable organic substances) reduces ammonia losses through volatilization.

Recommendations:

Volatilization occurs after the application of fertilizers containing a large amount of N ammonium (such as liquid manure, slurry, sewage sludge and manure) on the surface of the soil (table). The method and speed of the application of the fertilizer is decisive in this case especially in the first hours after application, as documented in Graph 4. Nitrogen is volatilized similarly during the surface application of mineral N fertilizers that contain ammonia or in which ammonia is formed (e.g. urea).

Key information

• The main proportion of nitrogen is in organic nitrogen compounds (microbial biomass, metabolites of organisms living in the soil, plant and animal residues, stable organic compounds, etc.), the nitrogen of which is mostly unavailable to plants.
• Of the total amount of N, only 1–2 % is available to plants in NH4+ and NO3– form, collectively referred to as mineral nitrogen (Nmin).
• Most available nitrogen is generally found in arable land where nitrogen is released by the mineralization of organic substances, but due to leaching, especially outside the growing season,  a greater or smaller amount of nitrogen is gradually shifted - and it is ultimately washed away from the roots of the crops.

The literature used is available from the authors.

Sponsored by the Ministry of Agriculture under Project No. QH 91081

Ing. Jindřich Černý, Ph.D 
Prof. Ing. Václav Vaněk, CSc.
Ing. Ondřej Kozlovský, Ph.D. 
The Czech University of Life Sciences in Prague

Faculty of Agrobiology, Food and Natural Resources
Department of Agroenvironmental Chemistry and Plant Nutrition

SOURCE: ČERNÝ, Jindřich, Václav VANĚK a Ondřej KOZLOVSKÝ. Hnojení dusíkem: specifika a aplikace. Zemědělec.cz [online]. 2011, , 1 [cit. 2018-02-12]. Available at: http://zemedelec.cz/hnojeni-dusikem-specifika-a-aplikace/