Nutrient cycling
The intensity of the biological cycle is the amount of chemical elements contained in the growth of phytocenosis per unit area over an interval of time.
One of the tasks of agrochemistry is to assess the direction of the cycle of biogenic elements and the degree of intensity of the anthropogenic impact on the soil-plant system in the balance of nutrients in the agrocenosis, as well as creating conditions for the rational cycle of nutrients in agriculture and ensuring their positive balance, to optimize crop nutrition by using science-based fertilizer system in the crop rotation.
In 1825 for the first time in Russia professor of Moscow University M.G.Pavlov published a scientific work “Crop Farming Chemistry”, in which he substantiates the problem of increasing the soil fertility by increasing the nutrients in the soil or at least by returning what is taken by plants. The development of nutrient balance research in agrochemistry began with the publication of J. Libich’s work “Chemistry in Application to Farming and Physiology” (1840) and the doctrine of complete return to the soil of minerals taken from it by the harvest of plants.
D.N. Pryanishnikov, the founder of the national agrochemistry, gave attention to the problem of the circulation of substances in agriculture and their balance. He wrote that the development of chemical industry was one of the most important material prerequisites for regulating the cycle of substances in agriculture, their exchange between man and nature. He noted that if depletion of soils as a result of disturbance of the exchange of substances between man and the earth disturbs “the natural condition of constant soil fertility”, then wide application of fertilizers based on the chemical industry is one of the powerful factors not only maintaining at a constant level (as J. Libich thought) but also further increasing the soil fertility, as it can be seen from the historical example of yields growth in Western European countries with increasing level of chemicalization.
Human economic activity, including the intensification of agricultural production and chemicalization, leads to changes in the processes of transformation of substances and energy in nature. Thus, there are changes in the nitrogen cycle in the biosphere during the transition from the natural state of the soil to the state under intensive cultivation. In soils of natural biocenoses, losses of nitrogen from volatilization and denitrification are balanced by its inflow with atmospheric precipitation and biological fixation.
When a land plot is developed for intensive agricultural production, the nitrogen cycle undergoes changes. At the same time, losses of nitrogen from the system exceed its inflow, which leads to soil depletion of this element. With agricultural development of lands, the number of ways of nitrogen loss from the system increases: along with gaseous losses of nitrogen from the soil, leaching of nitrates increases. Nitrogen is also removed from the system by burning crop residues. Significant amounts are alienated when agricultural products are used for industrial and other needs, and are absorbed by weeds.
The natural inflow of nitrogen into the cycle is due to biological fixation and with precipitation. Only by applying nitrogen fertilizers and manure it is possible to eliminate the deficit in the nitrogen balance and create conditions for maintaining and increasing fertility. Losses of nitrogen and other nutrients cause eutrophication of water bodies, pollute groundwater and cause a number of undesirable phenomena in the environment.
No matter in what form nitrogen is applied to the soil – as part of organic or mineral fertilizers, nitrate, ammonia, amide or molecular nitrogen fixed by legumes – in the end, only the reduced ammonium form of nitrogen (NH4+) takes part in the synthesis of amino acids and proteins in plants. All other forms are reduced to ammonium through a chemical or biological conversion process in the soil or in plants.
Organic and mineral fertilizers as sources of nutrients are equal. However, organic fertilizers are preferred because they are less concentrated. For example, in terms of nitrogen, 0.1 ton of urea is equivalent to 10 tons of manure. Violations in the technology of mineral fertilizers leads to high concentrations of nutrients in the soil, which, entering in excessive quantities in plants, degrade the quality of production or cause ammonia poisoning of plants. Mineralization of organic fertilizers occurs slowly and does not create high concentrations of mineral salts.
Balance of soil nutrients
The balance of soil nutrients is a quantitative expression of the content of nutrients in the soil of a particular area, taking into account all items of input and output during a certain period of time.
Sources of nutrient inputs:
- mineral fertilizers;
- organic fertilizers;
- crop residues;
- seeds;
- biological fixation of nitrogen;
- atmospheric precipitation.
The expenditure part consists of:
- removal with the crop of the main and by-products;
- removal with crop residues;
- leaching to ground waters and flushing from the surface;
- losses from erosion processes;
- gaseous losses.
For research purposes, quantitative values of balance items are taken on the basis of experimental data, for practical purposes – reference data.
To carry out theoretical studies considering all balance sheet items, a method using a lysimeter is used. This method allows us to determine the patterns of changes in the balance items and give a scientific explanation. In these experiments, fertilizers with labeled atoms are used. Thus, according to the results of lysimeter studies conducted in All-Russian Institute of Fertilizers and Agro-soil Science on sod-podzolic soils of Non-Black Soil Region with a stable nitrogen isotope 15N, it was found that 30-60% of nitrogen brought with fertilizers is used by plants, 15-30% – accumulated in the soil, 10-30% – is lost due to volatilization into the atmosphere and 1-5% – washed away by water.
For practical purposes, data on biological, farm and external balances are used.
The biological balance covers all items of inputs and outputs of nutrients involved in the cycle. It is used to assess the system of fertilization of crops and specialized crop rotations.
Farm balance takes into account only the removal of nutrients from the soil with the main and by-products and inputs through the application of fertilizers. Calculation of the farm balance gives a fairly objective agro-economic assessment of the fertilizer system.
The external balance takes into account the alienation of nutrients with marketable products outside the farm and their receipt with mineral fertilizers. This type of balance is important for the distribution of fertilizers and is determined by the specialization of the farm. With agribusiness specialization in the production of marketable products, the balance is more scarce than in enterprises livestock specialization, as part of the nutrients alienated from the fields in the form of forage, returns back in the form of manure.
Studies of balance of soil nutrients in long-term stationary experiments with fertilizers allow to take into account the multi-year application by rotation of nutrient elements and their removal with crops. Experiments are carried out under close to production conditions, so the data obtained are applicable for scientific and practical purposes.
The main expense item is the removal of nutrients with the crop. The amount of removal in relation to specific crops, varieties and soil and climatic conditions may differ from the reference data.
Table. Nutrient removal from the soil in kg per ton of main agricultural products, including by-products[1]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. - M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. - 854 с.
| Winter wheat | |||
| Winter rye | |||
| Barley | |||
| Oats | |||
| Rice | |||
| Corn on grain | |||
| Peas | |||
| Sunflower | |||
| Flax fiber | |||
| Sugar beet | |||
| Potatoes | |||
| Corn for silage | |||
| Forage root crops | |||
| Perennial grasses (hay) | |||
| Annual grasses (hay) |
The greatest losses of nutrients occur under bare fallows, less under row crops, then in descending order: cereals, perennial grasses, hayfields and pastures.
Nitrogen balance
The peculiarity of nitrogen balance in the system soil – fertilizer – plant is the mobility of its compounds. Nitrogen is a biogenic element that has natural sources of replenishment in the soil.
The source of nitrogen replenishment is biological fixation by symbiotic and free-living microorganisms.
With any fertilizer system, a deficit-free nitrogen balance is a prerequisite for high yields.
The nitrogen balance is calculated using the equation:
BN = (Nf + Ns + Nfix + Nin/res + Nprec) – (Nmain + Nout/res + Nl),
where BN is nitrogen balance, kg/ha N.
Input items in the balance: Nf – inflow with fertilizers Nf = (Nmineral + Norganic), where Nmineral – inflow with mineral fertilizers; Norganic – inflow with organic fertilizers; Ns – inflow with seeds (seeds); Nfix – inflow from biological fixation; Nfix = (Nsymb + Nfree); Nsymb – ingress from symbiotic fixation; Nfree – ingress from fixation by free-living microorganisms; Nin/res – ingress with plant residues; Nprec – ingress with atmospheric precipitation.
Expenditure items: Nmain – removal with crop of main and by-products; Nout/res – removal with crop residues; Nl – losses from soil Nl = (Nleach + Nerosion), where Nleach – losses from leaching; Nerosion – losses from erosion processes.
Nitrogen coming in with seeds and removal with the main and by-products are determined by analytical method or by reference data. Nitrogen intake with seeds depends on crops, seeding rates, nitrogen content in seeds, and crop rotation. Thus, when sowing cereals with seeds, 4-6 kg/ha of nitrogen comes in, grain legumes – 8-15 kg/ha, potatoes – 9-12 kg/ha. Depending on the type and specialization of crop rotation, 20-50 kg/ha of nitrogen is applied per rotation with seeds.
According to E.P. Trepachev, nitrogen intake into the soil from legume crops is calculated by the formula:
Nenr = [(Mc/r ⋅ 2.5) ⋅ %N + (Mh ⋅ %N)] ⋅ Kfix – Nhay ⋅ (1 – Kf), or
Nenr = Nres – Nc,
where Nenr – soil enrichment with biological nitrogen, kg/ha; Nres – biological nitrogen of crop residues (kg/ha); 2.5 – correction factor for completeness of organic matter; Mc/r – mass of dry crop-root residues (100 kg/ha); Mh – mass of harvest losses for all haying (100 kg/ha); Kfix – nitrogen fixation factor, that is the ratio of fixed nitrogen to total; Nhay – total nitrogen in the hay crop (kg/ha) for all periods of legume growing; Nc – nitrogen removal with the legume crop (kg/ha).
Biological nitrogen of crop residues (Nres) is determined by the formula:
Nres = [(Mc/r ⋅ 2.5) %N + (Mh ⋅ %N)] ⋅ Kfix.
Nitrogen removal by legume crops:
Nc = Nhay ⋅ (1 – Kfix).
Example. Yield capacity of clover hay in 3 years is 12970 kg/ha with the average weighted nitrogen content of 2,7%. Total nitrogen consumption by clover will be
12970 ⋅ 2,7 / 100% = 350,2 kg/ha.
Root and crop residues (Mc/r) after the third year of clover is 7180 kg/ha of dry matter with a total nitrogen content of 2.3%, the mass of yield losses for all mows (Mh) – 410 kg/ha of dry matter and total nitrogen content of 2.8%, the average nitrogen fixation factor (Kfix) – 0.74. Thus:
Nenr = [(7180 х 2,5) х 2,3 + (410 х 2,8)] х 0,74 / 100% – 350,2 х (1 – 0,74),
soil enrichment with nitrogen will be Nenr = 222.9 kg/ha.
Nitrogen fixation coefficient (Kfix) according to research data for clover, lupine, sainfoin is 0,7, for alfalfa – 0,8, for pea and vetch – 0,6. Nitrogen fixation coefficient for grain legume residues is 0.3-0.4; for grain legume grasses – 0.5-0.7. Conventionally, we can take the nitrogen content in crop and root residues equal to half of its content in the above-ground mass.
The source of nitrogen is nitrogen fixation by free-living heterotrophic and saprophytic microorganisms. In the zone of northern taiga and tundra due to non-symbiotic nitrogen fixation several kilograms per 1 ha of nitrogen is fixed, 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.
Nitrogen losses from soil erosion, subsurface runoff and infiltration into deep soil layers are determined by reference data.
Table. Approximate sizes of soil nitrogen losses depending on slope steepness and crops (Trepachev et al., 1976)
| less than 1° | ||
| 1-2° | ||
| 2-4° | ||
Minimum losses of nitrogen from infiltration at different granulometric composition are equal (% of applied): heavy loamy – 0-0.5%; medium loamy – 0.5-1.5%; loamy – 2.0-4.0%; sandy – 5.0-8.0%.
Phosphorus balance
Phosphorus has no natural sources of recharge in soils. Its replenishment in agro-ecosystems occurs only through the application of phosphorus and organic fertilizers.
In the atmosphere, phosphorus may be contained in the form of dust in small amounts (0.5-1 kg/ha per year). Soil, water and plants are involved in the phosphorus cycle in ecosystems. However, its availability to plants depends on many factors.
Losses of phosphorus occur from soil erosion in the composition of fine-grained soil and liquid runoff. Leaching of phosphorus from medium and heavy soils on granulometric composition as a rule does not exceed 1 kg/ha, on light and peaty soils – up to 3-5 kg/ha.
Phosphorus balance (Bp) is determined by the equation which takes into account the difference between entering into the soil with fertilizer, seeds and precipitation and alienation with the crop and losses from leaching and erosion:
Bp = (Pf + Ps + Pin/res + Pprec) – (Pmain + Pout/res + Pl),
where Bp – phosphorus balance, kg/ha P2O5.
Input balance items: Pf – input with fertilizers, Pf = (Pmineral + Porganic), Pmineral – input with mineral fertilizers; Porganic – input with organic fertilizers; Ps – input with seeds (seeds); Pres – input with crop residues; Pprec – input with precipitation.
Expenditure items of the balance: Pmain – removal with yield of main and by-products; Pout/res – removal with crop residues; Pl – losses from soil Pl = (Plech + Perosion), where Pleach – losses from leaching; Perosion – losses from erosion processes.
Phosphorus intake with fertilizers and seeds is determined by chemical composition and seeding rates. Value of inflow from precipitation for phosphorus does not exceed 0.5 kg/ha. Phosphorus removal with the crop (Pmain) is determined by the chemical composition and the amount of yield of the main and by-products.
Phosphorus losses from erosion (Perosion) according to averaged data are: for phosphorus – 1.5-2 kg/ha. Phosphorus losses from leaching depend on the granulometric composition of the soil, rainfall, fertilizer doses and crops. Phosphorus losses (Pl) for loamy soils averages up to 0.1 kg/ha, for sandy and sandy loam soils – 1.2 kg/ha.
Potassium balance
The balance of potassium (BK) is determined by the formula:
BK = (Kf + Ks + Kin/res + Kprec) – (Kmain + Kout/res + Kl),
where BK – the balance of potassium, kg / ha K2O.
Input balance items: Kf – input with fertilizers Kf = (Kmineral + Korganic), where Kmineral – input with mineral fertilizers; Korganic – input with organic fertilizers; Ks – input with seeds; Kin/res – input with crop residues; Kprec – input with atmospheric precipitation.
Expenditure items of the balance: Kmain – removal with yield of main and by-products; Kout/res – removal with crop residues; Kl – losses from soil Kl = (Kleach + Kerosion), where Kleach – losses from leaching; Kerosion – losses from erosion processes.
Potassium intake with fertilizers and seeds is determined by chemical composition and seeding rates. Value of intake with atmospheric precipitation for potassium varies in the range of 2-6 kg/ha. Potassium removal with the yield (Kmain) is determined by the chemical composition and the amount of the main and by-products.
Potassium losses from erosion (Kerosion) on the average data makes 3-5 kg/ha. Potassium losses from leaching depend on the granulometric composition of soil, amount of precipitation, doses of fertilizers and crops.
Potassium losses from fertilizer leaching (Kleach) are: for light soils – 5%, for heavy soils – 2% of the applied amount.
Sources
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov, etc., ed. by V.G. Mineev. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.