Nitrogen cycle and balance in agriculture

Last updated: June 21, 2025

Mineral nitrogen compounds in soil are mobile. Their reserve and transformation are the result of numerous, often interdependent physical, physicochemical, and biological processes of the nitrogen cycle.

In virgin soils of natural biocenoses, a closed cycle of nitrogen and other biogenic elements occurs. It includes input items:

nitrogen input with plant litter and root residues;
excrements and remains of animals;
biological fixation of atmospheric nitrogen by microorganisms;
input with atmospheric precipitation (formation of NO₃^- and NH₄⁺ under the action of lightning discharges and industrial emissions).

Expenditure items:

nitrogen consumption by plants;
infiltration;
denitrification;
losses from water and wind erosion;
immobilization of nitrogen — transformation of mineral nitrogen into organic, (in this case nitrogen is not lost but passes into a form inaccessible to plants).

Natural biocenoses are usually characterized by a balanced balance of nitrogen and other biogenic elements. Losses from leaching and denitrification are compensated by its input with precipitation and biological nitrogen fixation.

As a result of soil plowing, the nitrogen regime undergoes changes: expenditure parts of the balance sharply increase. A large amount of nitrogen is removed with agricultural products. Intensive soil tillage leads to increased mineralization of organic matter, increased losses from infiltration, denitrification, and erosion. Under conditions of sufficient and excessive moisture, irrigation, especially on light soils, nitrate consumption for leaching from the root layer into drainage water increases. Loamy and clayey soils rich in humus retain water better, so nitrate leaching is insignificant — up to 3-5 kg/ha of nitrate nitrogen per year. On soils light in granulometric composition, especially in a fallow field, nitrate nitrogen losses from leaching reach 20-30 kg/ha per year.

Losses of gaseous nitrogen forms from denitrification are also significant. The denitrification process occurs under anaerobic conditions. Plowing eliminates these conditions; however, loosened soil consists of separate structural aggregates (clods), inside which anaerobic conditions are created. The denitrification process is the more intensive, the more nitrates not absorbed by plants accumulate.

Nitrogen balance

A feature of the nitrogen balance in the system soil — fertilizer — plant is the mobility of its compounds. Nitrogen is a biogenic element that has natural sources of replenishing reserves in soil: biological fixation by symbiotic and free-living microorganisms. Under any fertilizer system, a deficit-free nitrogen balance is a condition for obtaining a high yield.

Nitrogen balance is calculated by the equation:

Б_N = (N_у + N_с + N_б + N_{ро} + N_о) - (N_в + N_{ро} + N_п),

where

Б_n: — nitrogen balance, kg/ha N.

Input items of the balance:

N_у: — input with fertilizers N_у = (N_м + N_op), where N_м — input with mineral fertilizers;
N_op: — input with organic fertilizers;
N_с: — input with seeds (sowing material);
N_б: — input from biological fixation, N_б = (N_сим + N_нсим);
N_сим: — input from symbiotic fixation;
N_нсим: — input from fixation by free-living microorganisms;
N_ро: — input with plant residues;
N_о: — input with atmospheric precipitation.

Expenditure items:

N_ву: — removal with the harvest of main and by-products;
N_ро: — removal with plant residues;
N_п: — losses from soil N_п = (N_пв + N_пэ), where N_пв — losses from leaching;
N_пэ: — losses from erosion processes.

Nitrogen entering with seed material and removal with main and by-products are determined analytically or from reference data. Nitrogen input with seed material depends on the crop, seeding rates, nitrogen content in seeds, and crop rotation. Thus, when sowing cereals, 4-6 kg/ha of nitrogen enters with seeds; for grain legumes, 8-15 kg/ha; for potatoes, 9-12 kg/ha. Depending on the type and specialization of the crop rotation, over a rotation 20-50 kg/ha of nitrogen is applied with sowing material.

According to E. P. Trepachev, nitrogen input from leguminous crops is calculated by the formula:

N_{об} = \lbrack \lparen 2.5 \cdot M_{пк} \rparen \%N + \lparen M_{пу} \%N \rparen \rbrack К_ф - N_у \lparen 1 - К_ф \rparen ,

N_{об} = N_б - N_в

where

N_oб: — enrichment of soil with biological nitrogen, kg/ha;
N_б: — biological nitrogen of plant residues (kg/ha);
2.5: — correction coefficient for completeness of organic matter accounting;
М_пк: — mass of dry stubble-root residues (dt/ha);
М_пу: — mass of yield losses for all cuttings (dt/ha);
K_ф: — nitrogen fixation coefficient, i.e., the ratio of the amount of fixed nitrogen to the total;
N_y: — total nitrogen in the hay yield (kg/ha) for all periods of legume cultivation;
N_в: — nitrogen removal by the legume harvest (kg/ha).

Biological nitrogen of plant residues (N_б) is determined by the formula:

N_б = \lbrack \lparen 2.5 \cdot М_{пк} \rparen \%N + \lparen M_{пy} \%N \rparen \rbrack К_ф.

Nitrogen removal by leguminous crops:

N_в = N_у \lparen 1 - К_ф \rparen.

Example.

The hay yield of clover over 3 years is 129.7 dt/ha with a weighted average nitrogen content of 2.7%. The total nitrogen consumption by clover will be:

129.7 \cdot 2.7 = 350.2 \text{ kg/ha}.

Stubble-root residues (М_пк) after the third year of clover life are 71.8 dt/ha of dry matter with a total nitrogen content of 2.3%; the mass of yield losses for all cuttings (М_пу) is 4.1 dt/ha of dry matter with a total nitrogen content of 2.8%; the averaged nitrogen fixation coefficient (K_ф) is 0.74. Thus:

N_{oб} = \lbrack \lparen 71.8 \cdot 2.5 \rparen \cdot 2.3 + \lparen 4.1 \cdot 2.8 \rparen \rbrack \cdot 0.74 - 350.2 \cdot \lparen 1 - 0.74 \rparen ,

the enrichment of soil with nitrogen will be N_oб = 222.9 kg/ha.

Nitrogen fixation coefficient (K_ф) according to research data for clover, lupine, sainfoin is 0.7, for alfalfa — 0.8, for peas and vetch — 0.6. The nitrogen fixation coefficient of stubble residues of grain legumes is 0.3-0.4; of stubble-root residues of leguminous grasses — 0.5-0.7. Conventionally, the nitrogen content in stubble-root residues can be taken as equal to half its content in the aboveground mass.

A source of nitrogen is nitrogen fixation by free-living heterotrophic and saprophytic microorganisms. In different soil-climatic conditions, non-symbiotic nitrogen fixation binds: in the northern taiga and tundra zone — a few kilograms per 1 ha of nitrogen, in sod-podzolic and gray forest soils — 15-20 kg/ha, in chernozems — 30-40, in the tropics and subtropics — up to 80 kg/ha.

Losses of nitrogen from soil erosion, subsurface runoff, and infiltration into deep soil layers are determined from reference data.

Table. Approximate amounts of soil nitrogen losses depending on slope steepness and crops (Trepachev et al., 1976).

Slope steepness, degrees	Crop	Nitrogen losses, kg/ha
less than 1°	Row crops	5-10
	Spring cereals	3-5
	Winter cereals	2-3
	Perennial grasses	0
1-2°	Row crops	10-15
	Spring cereals	5-8
	Winter cereals	3-5
	Perennial grasses	2-3
2-4°	Row crops	15-30
	Spring cereals	8-15
	Winter cereals	5-10
	Perennial grasses	3-5

Minimum nitrogen losses under different granulometric composition from infiltration are equal (% of applied): heavy loamy — 0-0.5%; medium loamy — 0.5-1.5%; sandy loam — 2.0-4.0%; sandy — 5.0-8.0%.

Sources of nitrogen input and its transformation in soil

Natural sources of nitrogen input are: the activity of free-living and nodule nitrogen-fixing bacteria and input with atmospheric precipitation.

With atmospheric precipitation, annually in the form of ammonia and nitrates formed under the action of lightning discharges, 2-11 kg of nitrogen per 1 ha enters. Natural sources of nitrogen are of practical interest, but their amount is significantly less than the amount of nitrogen removed with the harvest.

An important role in providing plants with nitrogen is played by humus reserves, which contain about 5% nitrogen. Mineral nitrogen forms account for about 1-3%. The arable layer contains the largest share of humus, which is enriched with microflora and from which the main part of mineralized nitrogen for plant nutrition comes.

Ammonification is a microbiological process of transforming nitrogen of organic matter into ammonium compounds. Ammonium salts are oxidized as a result of the vital activity of nitrifying bacteria (Nitrosomonas spp. and Nitrobacter spp.) into nitrates and nitrites. For normal vital activity of these groups of bacteria, optimal conditions are required: temperature 25-32 °C, sufficient oxygen and water, soil acidity close to neutral. This is achieved by loosening the soil, applying organic fertilizers, and liming acidic soils. These practices allow activating the processes of nitrogen transformation from organic matter and reducing its losses. Violation of these requirements leads to the opposite effect — transition of nitrogen compounds into gaseous ammonia and nitrogen, i.e., activates denitrification processes.

Another technique for regulating nitrogen balance in soil is the use of bacterial preparations (rhizotorfin).

Nitrogen losses

The content of nitrogen in mineral form is very dynamic and depends on the activity of soil microflora, moisture, and plant development phase.

Nitrogen losses consist of:

immobilization, i.e., uptake of nitrogen by soil microflora;
leaching — washing out of nitrogen, mainly nitrate forms, into groundwater;
volatilization as ammonia into the atmosphere;
fixation of ammonium forms by soil or non-exchangeable absorption.

The immobilization process is especially intensive when applying organic fertilizers with a wide carbon-to-nitrogen ratio — 20-25:1. Microbial biomass contains a significantly higher amount of nitrogen (10:1), as a result of which nitrogen consumption by microflora occurs at the expense of organic matter and mineral soil reserves. To compensate for the effect of nitrogen immobilization by microorganisms, when plowing in straw or other plant residues rich in cellulose, about 1% mineral nitrogen is additionally added before sowing subsequent crops.

Immobilization of nitrogen can be positive on light soils with sufficient moisture, due to the fixation of mobile nitrogen forms under conditions of strong leaching. Subsequently, during decomposition of microbial residues, part of the fixed nitrogen is bound by humus compounds, and another part passes into mineral forms.

Leaching of mobile nitrogen forms, mainly nitrates, is relevant on soils light in granulometric composition with a low level of organic matter under conditions of sufficient, excessive moisture, and irrigation. Continuous sowing crops reduce this effect due to intensive nitrogen uptake, whereas in fallow fields the leaching effect is enhanced.

Nitrogen losses in the form of gaseous substances occur due to denitrification, i.e., reduction of nitrate nitrogen to ammonia and gaseous nitrogen as a result of the activity of denitrifying microorganisms. The activity of denitrifiers is activated by anaerobic conditions, when microbes are forced to use oxygen contained in nitrate form for respiration, reducing nitrogen to a free form. The denitrification process is also stimulated by an alkaline reaction of the medium, excessive content of organic matter with a high content of glucose and fiber, and high soil moisture.

Another pathway of nitrogen loss in the form of gaseous forms (nitrogen dioxide and monoxide) is the decomposition of nitrites at soil acidity below 6.

Total nitrogen losses can reach 50%. During decomposition of 1 t of humus, 50 kg/ha of nitrogen is formed; however, part of it is lost to the atmosphere in the form of gaseous ammonia. This is especially relevant when the technology of storage and application of manure, slurry, and other organic fertilizers is not observed; losses reach 30-40%.

Part of the nitrogen is consumed by weeds, and this amount can exceed consumption by cultivated plants.

To prevent nitrogen losses, and consequently, reduce irrational costs, it is necessary to optimize doses, forms, and timing of application of nitrogen fertilizers for each crop in the rotation, uniformly distribute and incorporate them into the soil.

Nitrogen fixation by soil

Part of nitrogen can be absorbed by some minerals from the hydromica group. In a moistened state, the crystal lattice of these minerals exchangeably absorbs ammonium nitrogen, but upon drying it binds it, making it poorly available to plants and microflora. According to A.V. Peterburgsky and V.N. Kudeyarov, the arable layer contains from 130 to 350 kg/ha of fixed nitrogen depending on the type and variety of soil. The upper layer contains 2-7% of fixed ammonium of the total amount; in the subsoil its share increases to 30-35%. This is explained by a decrease in humus content in deep layers, and consequently, nitrogen in organic matter.

The ability of soils to non-exchangeably bind ammonium is influenced by the type of clay minerals, environmental temperature, humus content, soil solution reaction, microbiological activity, and moisture. Ammonium fixation increases with increasing temperature, pH (maximum on solonetz soils), and humus content (chemical binding). Nitrogen binding is influenced by the content of clay minerals with a three-layer crystal lattice, primarily vermiculite.

Fixed ammonium can be displaced back into the soil under certain conditions, for example, by introducing calcium, magnesium, and sodium cations into the crystal lattice.

References

Bazdyrev G. I., Loshakov V. G., Puponin A. I. et al. (2000). Agriculture. Textbook for universities. Moscow: Kolos Publishing House.
Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. B. A. Yagodin. Moscow: Kolos.