Evaluation methods and indicators of soil biological activity in agrochemistry

Biological activity of soil is the sum of biochemical reactions occurring in the soil, which lead to the renewal of the supply of used or decomposed constituent substances, due to which it possesses a unique homeostasis. The biological activity of soil depends on the number and species composition of living microorganisms inhabiting it, the soil type, time of year, climate, and crop cultivation practices. Therefore, when discussing the biological activity of soil, it is necessary to consider the entire set of conditions and factors under which the given soil was formed.

Indicators of biological activity include: carbon dioxide emission, i.e., soil respiration; the ability of the soil to ammonify and accumulate nitrates; the rate of cellulose decomposition; the activity of nitrogen and phosphorus metabolism enzymes, as well as enzymes that catalyze oxidation processes; and the absolute number of microorganisms, especially Azotobacter, epiphytic bacteria, and non-spore-forming soil bacteria. Indicators of soil biological activity are necessary for its characterization as a biological system and for assessing the degree of change under the influence of anthropogenic impact, particularly from damage by toxicants and technogenic overloads.

No single indicator of biological activity reflects the complexity of the biological and biochemical changes occurring in the soil. For a comparative assessment of soil biological activity based on a set of parameters, a scale of indicators has been proposed (Table 32; Kiryushin V.I., 1996).

Table 32 – Scale for comparative assessment of soil biological activity Activity Indicator very very low low medium high high

 CO2 emission, CO2/10 g day        <5       5–10      10–15       15–25      >25
 Catalase, O2, cm3/g/min           <1        1–3       3–10       10–30      >30
 Dehydrogenase by TTH* reduction,
 mcl H2 g/day                      <3       3–7           7–15    15–22      >22
 Phosphatase, mg P2O2/10 g/h       <5      0.5–1.5    1.5–5.0      5–15      >15
 Protease, mg album/10 g/h        <0.5      0.5–1       1–2         2–3       >3
 Invertase, mg glucose/g/day       <5       5–15       15–20      50–150     >150

Note. TTH – 2,3,5-triphenyltetrazolium chloride.

Organic fertilizers, and especially manure, have the greatest influence on the biological activity of soil. With manure, a large amount of nutrients for soil microflora and a mass of microorganisms are introduced into the soil, for which, however, there are no conditions for development in the soil. For example, Escherichia coli, the number of which in 1 g of manure reaches 10,000 cells, is found in the soil after a short time in amounts of up to 100 cells per 1 g. Only a portion of manure bacteria survives in the soil and becomes involved in its metabolism. These are microorganisms that decompose cellulose and pectin, as well as ammonifiers. Green fertilizers and composts have a similar positive effect on the biological activity of soil, as they provide microorganisms with assimilable carbon and nitrogen compounds.

The effect of mineral fertilizers on the biological activity of soil depends on the type, form, timing, method, and application rate. It is appropriate to recall here that microorganisms not only use the fertilizers applied to the soil as a source of one or another nutrient element but also actively participate in converting them into forms available to plants.

For example, urea in the soil undergoes hydrolysis to ammonia under the influence of urease — an enzyme secreted by the microflora. Urea itself is practically not absorbed by plants, but ammonium ions are an excellent source of nitrogen for them. Enzymatic hydrolysis of urea is carried out according to the following scheme:

CO(NH 2) 2  H 2 O   CO2  2NH3

Ammonia interacts in the soil with mineral acids, forming salts that dissociate in an aqueous medium into ions.

Some fertilizers have a detrimental effect on soil microorganisms. An example is calcium cyanamide, which sterilizes the soil after application. However, after a short time, this compound is decomposed under the action of a number of bacteria and fungi and turns into water-soluble non-toxic ammonium carbonate, which is used by microorganisms and plants as a nitrogen source.

Potassium, calcium, magnesium, sulfur, and iron fertilizers increase the biological activity of soil. The same applies to micronutrients. In general, it can be said that mineral and organic fertilizers used in agriculture increase the biological activity of soil, i.e., they contribute to:

• an increase in the number and group composition of microorganisms;
• an enhancement of soil respiration;
• an increase in its enzymatic activity.

However, the application of high rates of mineral fertilizers causes a disruption of the natural cycle of major organogenic elements in nature, which negatively affects the vital activity of soil microorganisms. The depressive effect of high rates of mineral fertilizers on the development of active microflora is associated with:

• an increase in the concentration of the soil solution;
• the presence of large amounts of mobile nitrogen forms in it, especially N-NO3;
• a change in soil pH;
• the accumulation of nitrates, which have inhibitory properties in relation to plants and microorganisms.

The main reserves of soil elements are concentrated in its solid part. The aqueous phase holds only an insignificant fraction of even the exchangeable forms of ions. This is especially evident in soils with a well-developed soil adsorbing complex.

Composition and properties of the solid phase of the soil

The solid phase forms the organo-mineral framework of the soil, which inherits the composition and properties of the parent material. This basis is the most stable over time and in terms of its volume. By composition, the solid phase is divided into mineral and organic parts.

All ash elements are concentrated in the mineral part of the soil. Nitrogen, carbon, oxygen, phosphorus, and sulfur are distributed between the mineral and organic parts. Understanding this balance helps to more accurately plan the system of plant nutrition.

The solid phase directly affects the physical state of the tillage area and the water regime. For practical field assessment, a complex of its physicochemical characteristics is important. These include the following parameters:

• granulometric, mineralogical, and chemical composition;
• constitution and structure;
• porosity.