1. Introduction

The soil is the main environment for the life of living organisms, in which various biogeochemical processes take place. Its ecological and biological state directly affects food security, crop yields, and the efficient use of land resources [2,21].

The biological properties of the soil are largely determined by the content of organic substances in it, microbiological activity and enzymatic activity [3]. At the same time, soil fertility directly depends on the degree of degradation and salinity. On irrigated sierozem-meadow soils in the Mirzachul region, degradation processes and the degree of salinity differ, which affects their biological activity and ecological condition.

For the last several years, much attention has been paid to the use of biological indicators and integrated methods for assessing the ecological and biological state of soils. Increased salinization and degradation processes, especially on agricultural irrigated lands, negatively affect crop yields, which leads to a decrease in the biological stability of the soil [4]. It is characterized by a significant decrease in the number of microorganisms in saline and degraded soils, their enzymatic activity, and respiratory rate [20]. Therefore, when determining the ecological and biological state of degraded soils and assessing their general condition, it is advisable to use diagnostic methods.

In biodiagnostics, the use of humus, the number of microorganisms, enzymatic activity, respiratory intensity and biological integral indicators is used as an effective method for determining the general biological state of the soil. This method allows timely detection of degradation processes, assessment of the biological stability of the soil and effective use of bioindicators [10,11,12].

When assessing the biological activity of the soil, the main biological indicator is the number and activity of microorganisms. They play an important role in the decomposition of organic substances, the formation of humus and the circulation of biogenic elements [13,16,18]. Scientific sources note that under conditions of degradation and salinization, the number and activity of microorganisms sharply decreases, and the biological stability of the soil decreases [4,6].

Enzymes are the main driving force of the processes of organic matter circulation, mineralization and humus formation occurring in the soil [8,9,15,19]. They activate biochemical reactions in the soil, ensure a stable flow of the cycle of substances, and also determine the level of biotic potential. The decrease in enzyme activity is explained by a violation of the biological balance of the soil ecosystem, a decrease in organic matter reserves, and a decrease in microbiological activity. In this context, the analysis of enzymatic activity is recommended as a reliable method of biodiagnostics for assessing the ecological condition of the soil and maintaining its fertility [1].

One of the important biological indicators in assessing the ecological and biological state of the soil is the process of its respiration. It provides reliable information about the activity of microorganisms and the degree of mineralization of organic substances, as well as about energy metabolism [5,17]. However, the soil microbiome and its structure vary depending on the type of land use; in conditions of high degradation and salinity, the volume and activity of the microbiome decrease, and the circulation of organic matter slows down, and the biotic potential of the soil decreases [7].

Together, humus content, microbial populations, enzyme activities and soil respiration act as reliable bioindicators for evaluating the overall biological state of saline and degraded soils [14].

The humus content is a key indicator in assessing soil fertility, structure, water retention capacity, and biological stability. It also plays an important role in the transformation of organic substances, microbial activity, and enzymatic reactions.

Due to the high sensitivity of microorganisms to changes in environmental conditions, they are the most sensitive and reliable indicator in the biological diagnosis of soil conditions. In addition, microorganisms play a key role in the processing of organic substances and enzymatic reactions, which makes them an important element in assessing the biological stability and fertility of the soil.

Enzymatic activity is the most important indicator in assessing biochemical processes and transformations of organic substances in the soil. The amount and activity of catalase, peroxidase, polyphenol oxidase and other enzymes play an important role in determining the degree of decomposition of organic substances in the soil, the formation of humus and metabolic processes.

The intensity of soil respiration makes it possible to assess the physiological activity of heterotrophic microorganisms in the soil and plays an important role in determining the intensity of transformation of organic matter, the energy cycle, and biological processes.

The integrated indicator of the ecological and biological state of soils (IIEBSS) has been developed by many researchers, which is effectively used in various studies, in particular when assessing the state of degraded and saline soils. IIEBSS allows you to compare them with each other, taking into account their relative values, even if different indicators are expressed in different units. For example, the number of microorganisms, enzymatic activity and humus content are included in a complex integral indicator, which allows you to obtain accurate information about the degree of soil degradation, as well as the state of salinity. This method serves as the scientific basis for planning agrotechnical measures, preserving yields and sustainable use of soil resources.

2. Materials and Methods

Field and laboratory investigations have been conducted on the basis of generally accepted standard methods. The research was carried out on irrigated sierozem-meadow soils of the Khavast district of the Syrdarya region. Soil sampling from genetic strata, observations, chemical and biological analysis were carried out on the basis of the “Methodological manual for conducting field experiments” developed by UzPITI scientists, and the calculation of the integral indicator of the ecological and biological state of soils (IIEBSS) has been carried out according to the manual of Kazeev et al. [3] “Biological diagnostics and indication of soils: methods and research methods”.

The study was conducted on irrigated sierozem-meadow soils classified by salinity level as non-saline, lightly saline, medium saline, and highly saline.

To compare the obtained data comprehensively, the integrated indicator of the ecological-biological state of the soil (IIEBSS) was calculated as:

Where B _avg. is the average assessment score of all indicators; B _{avg. маx.} is the maximum assessment score of all indicators [3].

The maximum value of each of the indicators was taken as 100%, and the relative value of the same indicator in the remaining samples was expressed as percentage:

where B is the relative indicator score, B_x is the indicator score for the sample, B _max is the maximum indicator score in the general sample.

Then, several relative indicators (humus content, microbial populations, enzyme activities and soil respiration) are summarized, and their average value is calculated.

where B mean is the arithmetical mean score of the studied indicators, N is the number of indicators [3].

3. Obtained Results and Their Analysis

In the biodiagnostics of the soils of the studied territory, such parameters as the most informative indicators of biological activity were used - the humus content, the species and quantitative composition of microorganisms, enzyme activity, and the intensity of soil respiration.

Based on the data obtained during the research, a comprehensive assessment was carried out based on such indicators as the enzymatic activity (catalase, peroxidase, polyphenol oxidase) observed in soils, the number of microorganisms (ammonifiers, oligonitrophils, actinomycetes, fungi, and etc.), humus, as well as the intensity of soil respiration.

According to the results, increasing soil salinity causes a consistent decline in all biological indicators of irrigated sierozem-meadow soils. The content of humus, enzyme activities (catalase, peroxidase, polyphenol oxidase), soil respiration rate, and the abundance of key microbial groups significantly decreased with salinity.

In non-saline soils, all studied indicators were taken as 100%, representing optimal biological functioning. While lightly saline soils maintained relatively high biological activity (97% of the control), showing that mild salinity has only a slight effect on microbial and enzymatic processes.

In medium-saline soils, however, most parameters sharply declined. Humus content dropped to 68,6%, catalase and peroxidase activities to 63,2-71,5%, and the number of microorganisms such as actinomycetes, fungi, and phosphorus decomposers decreased drastically - some up to 35% of the control. The IIEBSS value in this group was 57%, indicating significant biological degradation.

On highly saline soils, biological activity indicators decreased significantly. Humus decreased to 56.2%, catalase to 47.4%, and soil respiration to 47.4%. The number of ammonifiers, actinomycetes, and nitrogen-fixing microorganisms declined by more than 80–95% some indicators (for example, spore-forming agents 2,4%, actinomycetes 4.8%) fell to almost a minimum. The integral ecological-biological indicator (IIEBSS) was only 33%, From this it can be seen that a high level of salinity has a great impact on the microbiological and enzymatic activity of the soil, leading to a slowdown in biological processes (Table 1). The highest biological activity was recorded in the non-saline irrigated sierozem-meadow soil.

Table 1. Тotal relative biological activity (RBA) of the irrigated sierozem-meadow soils distributed in the Khavast district (in % relative to the maximum) (Reviewed in Kazeev et al., 2003)

All indicators were evaluated relative to the non-saline irrigated sierozem-meadow soil, which was taken as the control (100%). The obtained values represent relative (normalized) data, allowing a comparative assessment of the biological activity of soils under different salinity levels. Statistical processing of the data was carried out, and the mean values were used for comparison (Figure 1).

Figure 1. IIEBSS values of irrigated sierozem-meadow soils in the investigated area (%). 1 – non-saline; 2 – lightly saline; 3 – medium saline; 4 – highly saline

The level of total biological activity of the studied soils was estimated by IIEBSS value (Figure 1). Тhe results demonstrated a clear dependence of soil biological activity on the degree of salinity. Non-saline and lightly saline irrigated sierozem-meadow soils exhibited “very high” and “high” biological activity levels (81–100%), reflecting favorable ecological conditions that enhance microbial development, enzyme activity, and organic matter transformation processes.

In contrast, medium-saline soils showed a “moderate” level (41–60%), while highly saline soils were characterized by a “low” level of biological activity (21–40%). These results indicate that increasing soil salinity significantly suppresses microbial biomass, enzymatic reactions, and soil respiration intensity, leading to the overall reduction of soil fertility and biological potential.

All measurements were performed in triplicate (n = 3), and mean values were calculated. The non-saline irrigated sierozem-meadow soil served as the control sample, against which other soils with varying salinity levels were compared. Although absolute mean values are presented in the table, statistical analysis using standard deviation (SD) and percentage comparison methods was conducted to ensure the accuracy, reliability, and reproducibility of the experimental data.

4. Conclusions

The results of the study showed that the ecological and biological state of the Mirzachul irrigated sierozem-meadow soils varies dramatically depending on the degree of salinity. According to the data obtained on the basis of integral indicators (IIEBSS), all indicators of biological activity - humus content, number of microorganisms, enzyme activity and respiration rate - were at the highest level in saline non-soils. While biological activity indicators were maintained at a relatively high level on slightly saline soils, a sharp decrease in enzymatic activity, the number of microorganisms, and respiratory rate was noted on soils with medium to high salinity. As the salinity level increases, the microbiological and enzymatic activity of the soil weakens, and the circulation of organic matter and energy metabolism slow down. This condition is explained by the intensification of degradation processes, a decrease in biological stability and soil fertility.

According to IIEBSS, non-saline and lightly saline soils exhibited very high and high biological activity (81-100%), while medium saline soils showed moderate activity (41-60%) and highly saline soils displayed low activity (21-40%).

Biodiagnostics of soils using IIEBSS allowed us to determine the degree of degradation, conduct a comprehensive assessment of biological activity and scientifically analyze the impact of salinity on the ecosystem. It also allows you to compare the levels of total biological activity of the soils studied using the IIEBSS value with each other and with different types of soils.