The effect of nitrogen fertilizer on nitrate accumulation and produce quality
8 min read
Nitrate behavior in soil and factors of their accumulation in the harvest
Nitrogen fertilizers play a key role in crop yield formation. When applied correctly, they stimulate the growth of not only above-ground biomass but also the root system. Root residues enrich the soil with organic matter, significantly loosen it, and improve its hydro-physical properties. However, unbalanced nitrogen application leads to the degradation of the agroecosystem and a decrease in product quality.
All forms of nitrogen fertilizers in the soil inevitably transform into nitrates. This is the most mobile form of nitrogen, which is not bound by the soil adsorption complex. Nitrate nitrogen moves freely through the soil profile along with capillary and gravitational water, creating risks of leaching and environmental pollution.
Nitrates are a natural element of the nitrogen cycle in nature and a mandatory component of plant nutrition. They are present in the soil even with a complete refusal to use mineral fertilizers; therefore, the agronomist's task is to control their level and prevent the exceeding of maximum permissible concentrations.
Nitrate accumulation in plant tissue occurs due to the inhibition of the biosynthesis of complex organic compounds (primarily proteins). The rate of this process is directly influenced by external conditions and the level of agricultural technology. Factors stimulating nitrate accumulation include:
- lack of light and reduction of daylight duration;
- imbalance of mineral nutrition elements;
- climatic stresses — frosts, drought, or heavy shading of crops;
- excessive application rates of nitrogen fertilizers in unfavorable years.
The share of nitrogen fertilizers among all factors determining nitrate accumulation in plant products is 50%. The remaining volume of risks accounts for application timing, sowing time, and weather conditions of the growing season.
- Permissible dose of nitrates for humans — 5 mg/kg of body weight
- Threshold dose of nitrite ion — 0.05 mg/kg of body weight
- Maximum daily nitrate intake (WHO) — 200 mg/day
- Maximum daily nitrite intake (WHO) — 10 mg/day
- Maximum permissible concentration of nitrates in water (temperate latitudes) — 22 mg/l
- Maximum permissible concentration of nitrates in water (tropics) — 10 mg/l
Risks to human health, maximum permissible concentration standards, and nitrogen losses from leaching
When entering the human and livestock animal body through food or drinking water, nitrates are reduced to nitrites, nitrosoamines, and nitrosoamides. These compounds oxidize the ferrous iron of hemoglobin into ferric iron, forming methemoglobin, which is incapable of carrying oxygen. Hypoxia develops: a person feels suffocated and lethargic, skin turns bluish, and breathing and pulse rate increase.
Replacing 20% of hemoglobin with methemoglobin leads to anemia, and 80% to a lethal outcome. In cattle, excess nitrates in feed or water cause abortions, a sharp decline in milk productivity, lameness, and immobility.
To prevent poisoning and control product quality, strict standards for maximum permissible concentrations (MPC) of nitrates in crops have been established. The accumulation level varies significantly depending on the biological characteristics of the crop and growing conditions.
| Product | Open field | Greenhouse |
|---|---|---|
| Potatoes | 250 | — |
| Early head cabbage | 900 | — |
| Late head cabbage | 500 | — |
| Tomatoes | 150 | 300 |
| Cucumbers | 150 | 400 |
| Table beet | 1400 | — |
| Early carrots | 400 | 400 |
| Late carrots | 250 | — |
| Onion | 80 | — |
| Green onion | 600 | 800 |
| Leafy vegetables | 2000 | 3000 |
| Sweet pepper | 200 | 400 |
| Squash | 400 | 400 |
| Apples, pears | 60 | — |
| Melons | 90 | — |
| Watermelons | 60 | — |
| Grapes | 60 | — |
Nitrogen losses from arable soils resulting from leaching prevail in zones with excessive moisture and are less pronounced in soils with periodic leaching regimes. When excessive doses of fertilizers are applied, not only is the active ingredient lost, but the environmental burden on water sources also increases sharply.
| Region / Zone | Moisture coefficient (MC = P/E0) | Data on leaching and nitrogen losses |
|---|---|---|
| Territory average | (2.85) | 3156 | 90 | 131267 | 0.7 |
| Excessive moisture zone | (5.75) | 1270 | 73 | 30789 | 2.4 |
| North-Western region | ≥1.33 | 6 | 30 | 64 | 36 | 14.6 | 85 | 12.4 | 210 | 26 | 3501 | 7.4 |
| Central region | 1.00–1.33 | 5 | 25 | 83 | 17 | 8.4 | 60 | 5.0 | 625 | 31 | 14571 | 2.1 |
| Volga-Vyatka region | — | 4.7 | 23.5 | 87 | 13 | 7.1 | 55 | 3.9 | 286 | 11 | 7571 | 1.5 |
| Central Black Earth region | (1.13) | 362 | 4.1 | 10693 | 0.4 |
| Black Earth district 1 | 0.77–1.15 | 4.3 | 21.5 | 98 | 2 | 4.6 | 45 | 2.1 | 71 | 1.5 | 1968 | 0.8 |
| Black Earth district 2 | 0.77–1.15 | 4.3 | 21.5 | 98 | 3 | 4.6 | 45 | 2.1 | 51 | 1.1 | 1647 | 0.7 |
| Black Earth district 3 | 0.77–1.15 | 4.3 | 21.5 | 98 | 2 | 4.6 | 45 | 2.1 | 69 | 1.5 | 2265 | 0.7 |
| Volga region | (0.89) | 462 | 4.1 | 29439 | 0.1 |
| Volga district 1 | 0.77–1.15 | 4.3 | 21.5 | 97 | 3 | 4.8 | 45 | 2.2 | 60 | 1.3 | 2528 | 0.5 |
| Volga district 2 | 0.77–1.15 | 4.3 | 21.5 | 84 | 16 | 7.1 | 45 | 3.2 | 89 | 2.8 | 3752 | 0.8 |
| Ural region | (2.03) | 237 | 4.8 | 17572 | 0.3 |
| Ural district 1 | 1.00–1.33 | 5 | 25 | 95 | 5 | 6.0 | 60 | 3.6 | 47 | 1.7 | 2061 | 0.8 |
| Ural district 2 | 1.00–1.33 | 5 | 25 | 98 | 2 | 5.4 | 60 | 3.2 | 57 | 1.8 | 1531 | 1.2 |
| Ural district 3 | 0.77–1.33 | 4.5 | 22.4 | 90 | 10 | 6.3 | 45 | 2.8 | 46 | 1.3 | 1555 | 0.8 |
| West Siberian region | (0.95) | 190 | 1.8 | 19456 | 0.1 |
| West Siberian district 1 | 1.00–1.33 | 5 | 25 | 99 | 1 | 5.2 | 60 | 3.1 | 21 | 0.7 | 671 | 1.0 |
| West Siberian district 2 | 0.66–1.33 | 4.3 | 21.5 | 93 | 7 | 5.5 | 45 | 2.5 | 43 | 1.1 | 1719 | 0.6 |
| Far Eastern region | 1.33 | 6 | 30 | 98 | 2 | 6.5 | 85 | 5.5 | 128 | 7 | 3071 | 2.3 |
Fertilizer application enhances the intensity of denitrification processes and leads to nitrogen volatilization. Losses of fertilizer nitrogen in the form of nitrous oxide can reach 50%.
Nitrous oxide, formed as a result of denitrification, is capable of destroying the atmospheric ozone layer, which protects the Earth's surface from direct exposure to ultraviolet rays that are harmful to all living things. By being oxidized by ozone, nitrous oxide binds a water molecule and forms nitric and nitrous acids, which fall with atmospheric precipitation onto land and the surface of the World Ocean.
When applying increased rates of nitrogen fertilizer, especially physiologically acidic ones, the migration of humic and fulvic acids, as well as calcium and magnesium cations, through the soil profile increases, and plant potassium nutrition is disrupted. Losses of Ca and Mg and their migration also increase when applying fertilizers containing nitrates and chlorides. These anions are not retained by the soil adsorption complex; their leaching is accompanied by the removal of an equivalent amount of calcium, magnesium, and other elements necessary for plants from the arable soil layer.
High application rates of nitrogen fertilizer reduce the use of soil nitrogen by suppressing nitrogen fixation by free-living soil microorganisms and nodule bacteria. When applying high application rates of nitrogen fertilizer, the mineralization of natural reserves of organic nitrogen compounds is significantly intensified, which may result in unused excess amounts of mineral nitrogen formed as a result of organic matter mineralization.
It has been established that up to 75% of the nitrogen applied with fertilizers can be lost from the soil in gaseous form. Crops that have an unbalanced level of nitrogen nutrition are generally prone to lodging and are more severely affected by diseases and pests. In addition, it should be noted that nitrogen fertilizers introduce a certain amount of ballast substances into the soil, including heavy metals such as lead, cadmium, copper, and zinc. Thus, the one-sided application of nitrogen fertilizers poses a great danger to the environment.
A comprehensive assessment of nitrate accumulation processes in plants, conducted by V.M. Nazaryuk (2004), showed that their intensity is determined by the interaction of a number of natural and anthropogenic factors, which can be represented as a block diagram.
"The indicators characterizing the processes of intensive nitrate accumulation in produce," writes V.M. Nazaryuk (2004), "are interrelated and interdependent."
- Natural impact is associated with hydrothermal conditions and the level of soil fertility.
- Anthropogenic influence, causing an excess of nitrates in plants, is due to:
- deviations in complying with the requirements for agrotechnical measures;
- insufficiently correct selection of a cultivar genotype for specific soil and climatic conditions;
- failure to account for the biological characteristics of the crop;
- disturbances in nitrogen and ash element nutrition.
Nitrate accumulation indicators adequately reflect both the intensity of the impact of an individual factor and the contribution of the main blocks (soil, fertilizer, physiologically active substances and sorbents, plant genotype) to the regulation of nitrate accumulation processes.
Heavy metals – 1) a group of chemical elements with a density of more than 5 g/cm3;
2) metals with a relative atomic mass of more than 40.
Fig. 117. Factors affecting nitrate accumulation in plants. Changes in nitrate content are given in parentheses. Almost all the anthropogenic factors presented in the diagram associated with excessive nitrate accumulation in produce can be used to a significant extent to regulate nitrogen nutrition and, as a result, to manage nitrogen metabolism in plants so as to prevent the deterioration of harvest quality. The strongest impact on nitrate accumulation in produce is exerted by excessive doses of nitrogen fertilizer, cultivar genotype, and biological characteristics of the crop. While the crop and cultivar can only be selected before sowing, nitrogen nutrition can be regulated throughout the entire growing season. This allows for prompt intervention in the plant production process and thereby regulating nitrate accumulation in produce."
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