Mycological composition of soils in Kuban rice agrocenoses as a biotic component for their health

. Studies of the water-physical, physical-chemical and biological properties of meadow-chernozem soils were carried out on a rice irrigation system located in the present-day delta of the Kuban River. Rice meadow-chernozem soils have good water and general physical properties of humus layers. An increase in compaction is observed in the lower horizons of these soils. Meadow-chernozem soils are distinguished by low content of organic matter in arable horizons (3,2-3,4 %), neutral reaction of soil solution (рН 6,8-6,9) and high indices of cation exchange capacity (38,7-39,1 mmol/100g). The mycological composition of the studied soils in the autumn-spring periods was identified and identified. The soil was more saturated with representatives of the suppressive group at the beginning of rice vegetation, the opportunistic group was not detected and the antiphytopathogenic potential was 1:2. The spring-fed rice crop rotations soils are prone to soil stresses that are oversaturated with suppressors. In autumn, the ratio of conditionally suppressive to saprotrophic and pathogenic mycota was 1.5:4.5:7. The growth of colonies of the micromycete genus Trichoderma is traced, which confirms the beginning of a long process and the slow recovery of soil degradation processes.


Introduction
The state of soil cover in the modern world and the attitude of mankind toward it is more and more appealing. Many domestic and foreign scientists propose to interpret the concept of "soil health" as its ability to perform ecological functions; this concept combines knowledge and experience of agriculture, agrochemistry, agronomy, soil science, soil microbiology, microorganism ecology, and general ecology [1,3].
Soil health is mainly formed by the native soil biota, which by its balanced diversity contributes to the continuous self-purification of the soil from pollutants and provides suppressiveness against pathogenic or parasitic and phytopathogenic biota. A healthy agrocenosis soil is safe for humans, characterized by the production of normatively clean bioproducts, uncontaminated water and air environments, and sustainable yields [4,5].
Instrumental and biological modern methods of ecological monitoring make it possible to objectively estimate the soil microbial diversity and potential self-purification capacity, but it is much more difficult to quantify the degree of soil suppressiveness, its suppressive capacity with respect to parasitic microbiota. Therefore, the suppressive activity of the soils of agrocenoses is still estimated only semi-quantitatively in labor-intensive field experiments [8].
Periodic flooding of rice checks contributed to a significant change in the redox, air and thermal regimes of soils. This, in turn, affected their water-physical, physicochemical, and biological properties. These transformations were reflected in morphological features: colouring, new formations, structure, profile structure and even granulometric composition [6,11]. At the same time, taking into account the specificity and radical changes that occur in the soils of rice agrocenoses, the term 'rice type' is added to the name of genetic type at the regional level [1].
Rice, which has a rather high degree of resistance to the unfavourable influence of flooded soil, imposes requirements on soil conditions, basically coinciding with the requirements of cultivated grains in general: soil should be structural, have good water and air permeability, contain in assimilable forms nitrogen, phosphorus, potassium, iron and also should not be acidic or alkaline (pH in limits, close to neutral reaction) [6].
The aim of our research was to study properties of meadow-chernozem soil of rice agrocenosis, to give ecological assessment of quality and quantity of mycological composition of soil for further use in diagnostics of biological condition and change of soil fertility.

Materials and Methods
The study object was rice meadow-chernozem soils of the Krasnoarmeisky district of the Krasnodar Territory.
Full-profile soil sections were laid in the surveyed area, with subsequent sampling from genetic horizons for morphological description, study of water-physical, physical-chemical and biological properties of the soil.
From the soil samples taken in the laboratory of FSBI CAC "Krasnodar" the following types of analyses were carried out [3]: soil density by cutting ring method; solid phase density by pycnometric method; total porosity by calculation method; field moisture by weight method; organic matter by Tyurin method in CIAS modification (GOST 26213-91); exchangeable calcium and exchangeable (mobile) magnesium by GOST 26487-85; pH of water extract by potentiometric method (GOST 26423-85); to detect mycological composition of soil micromycetes we applied the method of N. A. Naumova [7].

Discussion and Results
In meadow-chernozem soils formed on loess-like sediments, when they are used for rice, there is a significant and irreversible change of water physical properties, porosity and water permeability decrease sharply, soil density increases. In the underlying loess-like sediments, the processes of overmoistening, suffosion, and their degradation intensify. Soils and transformed loess-like sediments tend to swell when overwatered and to crack when dried. These are indirect signs of changes in mineralogical composition of soils, increase in their composition of minerals of montmorillonite group, which give the properties mentioned above to soils and underlying rocks. These changes are most intense in the 4 to 6 year period of rice soil use [9,11].
The area of Krasnoarmeisky district includes the following geomorphological formations: 1) an alluvial-alluvial plain of the right bank terraces of the Kuban River; 2) an ancient deltaic alluvial alluvial plain; 3) a modern deltaic alluvial-accumulative plain. In terms of age, the ancient delta occupies an intermediate position between the oldest alluvial-algal plain and the youngest modern delta [6].
It should be noted the clear attachment of soil types to geomorphological formations. On the II and III terraces of the Kuban, as the most ancient surfaces, zonal soilschernozems were formed. Semi-hydromorphic analogues of chernozem -meadowchernozem soils are widespread on I and partly on II terraces [4].
The geomorphological conditions in the study area determined the genesis of the granulometric composition of the soil-forming and underlying rocks. On the I, II and III terraces of the Kuban, the soil-forming and underlying rocks are loess-like loamy, rarely heavy loamy sediments. Their thickness on the III terrace is 8-11 m, on the II terrace 3-5 m, on the I terrace 1-3 m. The loess-like sediments are underlain by the ancient Lower Quaternary alluvial sediments [9,12].
The granulometric composition of the meadow-chernozem soil is clay. The water-physical properties of soils are characterized by a decrease in total porosity, an increase in density, and a decrease in water permeability and water yield (Table 1). Solid phase density depends on the nature and proportion of the minerals and organic matter in the soil. It varies within a fairly narrow range and changes slightly over time (2.64 to 2.75 g/cm 3 ).
Rice meadow-chernozem soils are characterised by weakly to medium compacted compaction. Within the humus horizon, the density of folding ranges from 1.30 to 1.53 g/cm 3 .
Total porosity decreases down the soil profile to 38.0 %. In humus horizons, it is 44.4-50.8 %, which provides quite sufficient aeration and favourable conditions for soil water permeability.
From the agronomic point of view, such indicators of density and porosity of the upper soil horizons can be considered satisfactory for rice growth and development. Compaction of horizons in the middle and lower parts of the soil profile makes it difficult to aerate the soil during the non-growing season.
The most important components of soil fertility are its physical and chemical properties, which characterise the state of the soil absorbing complex (SAC), as well as the content of humus and mobile forms of nutrients (Table 2).
The highest values of the total amount of absorbed bases are noted in the arable and subsoil horizons of the soil. With the depth of the soil profile their amount significantly decreases, which is associated primarily with a decrease in humus content.
Rice meadow-chernozem soils refer to low humus content (3.36 %) and to thick horizons (A+AB) -to thick ones (110 cm). Nitrogen in the soil is 97-98 % represented by organic compounds and 1-3 % by mineral compounds. The content of humus and total nitrogen are characterized by high correlation direct dependence.
Thus, the physicochemical properties of the upper horizons of the rice meadowchernozem soil are characterized by low humus content (3.2-3.4 %), neutral reaction of the soil solution (pH 6.8-6.9) and rather high indicators of cation exchange capacity (38.7-39.1 %).
To determine the diversity of micromycetes in soils different nutrient media are prepared, quantitative and qualitative determination of the species composition of pathogens is carried out by isolation of fungi from the soil suspension. The ratio and species composition of fungi and their location in soil horizons depend on different soil properties as well as the presence of vegetation. Information on the species and quantitative composition of soil fungi in various agrocenoses, the ratio of pathogenic and suppressive species, makes it possible to determine the value of antiphytopathogenic potential, which will make it possible to predict the ability of soil to resist various manifestations of pathogenic fungi and highlight its suppressive qualities [10].
Soil-dwelling micromycetes are divided into three groups: conditionally pathogenic, saprotrophs, and conditionally suppressive [7]. The conditionally pathogenic fungi (hemibiotrophs) are fungi of the genera Alternaria Ness, Fusarium Link, Trichothecium Link, Verticillium, Cladosporium, Cephallosporium and others [6]. The pathogens of the genus Fusarium are the most dangerous of these fungi.
In soil samples, obligate saprotrophs are almost always present, the role of which for soil well-being can hardly be overestimated. They actively participate in decomposition of plant residues after harvesting of preceding crops in crop rotation, which are considered as accumulators of diverse phytopathogenic mycoplasma. Fungi from the genera Humicola spp., Gliocladium spp. can be examples of micromycetes that contribute to the decomposition and recycling of plant residues [3,7,10].
Conditionally suppressive micromycetes are fungal objects whose metabolic products are toxins and antibiotic substances capable of suppressing soil pathogenic mycoflora. Representatives of this group include fungi of the genera Trichoderma spp., Penicillium spp., Aspergillus spp. The prevalence of the last two genera of m ould fungi with high toxin-producing properties in the soil indicates the presence of signs of soil fatigue. Fungi in the genus Trichoderma are capable of forming several antibiotic toxic to phytopathogens, enzymes capable of hydrolyzing the cell structure of pathogen fungi. These fungi have the ability to directly parasitize pathogen mycelia and play a special function in increasing the antiphytopathogenic potential of the soil.
For a comparative assessment of the species composition of micromycetes, the mycological composition of rice meadow-chernozem soils in spring before crop sowing and after rice harvesting was isolated and identified in our studies. Soil suspension was applied on starvation agar to count fungal colonies. Chapek's medium was used to identify and clarify the species composition.
As a result of these experiments, the representatives of the saprotrophic group were identified in rice meadow-chernozem soils in the spring period: Mucor and Rhizopus. The number of colonies in 1 g of absolutely dry soil did not exceed 1.5 and 2.0 CFU, respectively. Penicillium spp. and Aspergillus spp. constituted the core of the conditionally suppressive group (4.5 and 3.0 in 1 g of absolutely dry soil). Consequently, the soil of rice checks in spring can be characterized as prone to soil stress, oversaturated with suppressors ( Table 3).
Studies on determination of mycological composition after rice harvesting showed that not only quantitative but also qualitative composition of soil fungi changed. Thus, the ratio of conditionally suppressive to saprotrophic mycota shifted towards suppressors, and colonies of positive micromycete p. Trichoderma. The number of its colonies was -4.5 CFU/1 g of absorbed dry soil. Micromycetes p. Verticillium, Trichothecium Link, Fusarium Link, Alternaria were also detected. Thus, as a result of the experiments conducted, it was found that during the spring period, rice meadow-chernozem soils were saturated with representatives of the suppressive group, while the opportunistic group was absent at all; the value of the antiphytopathogenic potential was 1:2 (the ratio of saprotrophs to suppressors), which undoubtedly indicates a tendency for soil to soil fatigue.
On the contrary, the situation was significantly different after rice harvesting. Samples were taken 30 days after harvesting, as the analysis of samples taken immediately after harvesting did not show the growth dynamics of various micromycetes (the presence of significant overwatering prevented almost all fungi from developing).
We observed colony growth of representatives of different soil groups: Verticillium, Trichothecium Link, Fusarium Link, Alternaria. The ratio of conditionally suppressive to saprotrophic and pathogenic mycota shifted towards pathogens (1:2.5:4), indicating the nature of the infection load in the soil studied, despite this there was a trend towards growth of colonies of the positive suppressor -Trichothecium microfungi. This positive trend suggests the possibility of the beginning of a long-term process of slow recovery of soil degradation processes.

Conclusions
Rice meadow-chernozem slightly humus thick light clayey soils formed on degraded loesslike clayey sediments are characterized by favorable water-physical properties of the upper horizons, where soil density is 1.30-1.35 g/cm3 and total porosity is 49.6-50.8 %. The lower horizons are characterised by increased compaction and decreased porosity. The physicochemical properties of the top soil of rice meadows and chernozem are characterised by a low humus content (3.2-3.4 %), neutral reaction of the soil solution (рН 6. 9), rather high indicators of the capacity to exchange ions (38.7-39.1 mmol/100 g of soil).
The rice meadow-chernozem soil was saturated in the spring period with representatives of the suppressive group of fungi, the opportunistic group was absent, the antiphytopathogenic potential value was 1:2. The soil was prone to soil fatigue and was oversaturated with suppressors.
Colony growth of representatives of different soil groups was established in autumn: Verticillium, Trichothecium Link, Fusarium Link, Alternaria. The ratio of conditionally suppressive to saprotrophic and pathogenic mycota was (1.5:4.5:7). There was an increasing trend in the colonies of the micromycete genus Trichoderma. This positive dynamic indicates the possibility of starting a long-term process of slow recovery of soil degradation processes.
To improve rice agrocenose soils, it is recommended to use the ratio of pathogenic and suppressive fungi species, as well as the value of antiphytopathogenic potential to predict the ability of the soil to resist manifestations of pathogenic fungi and to allocate its suppressive qualities. Further research is required to develop the estimation scale.