Biological degradation of chernozems under irrigation

We studied the changes in the state of microbial cenosis of Ukraine’s chernozems under irrigation. Considerable part of Ukraine’s chernozems is located in the areas where humidification is insufficient and unstable. Irrigation is a soil-reclamation measure for chernozems of Ukrainian Forest-steppe and Steppe which enables getting the assured yield, especially vegetable and fodder crops. At the same time, irrigation is a powerful anthropogenic factor that affects the soil, causes a significant transformation of many of its properties and regimes including biological ones. Often these changes are negative. The purpose of our investigation was to identify changes in the state of microbial cenoses of chernozem soils under irrigation which depend on such factors as the quality of irrigation water, the duration and intensity of irrigation, the initial properties of soil, the structure of crop rotation, usage of fertilizing systems and agroameliorative techniques. We identified direction and evaluated a degree of changes in biological properties of chernozems under influence of irrigation in different agro-irrigational and soil-climatic conditions. In the long-term stationary field experiments we identified the following biological indices of irrigated soils and their non-irrigated analogues: a number of microorganisms which belong to main ecological-trophic groups, activity of soil enzymes (dehydrogenase, invertase, phenol oxidase), soil phytotoxic activity, cellulose destroying capacity of soil, indices of oligotrophy and mineralization, summary biological index (SBI) and index of biological degradation (BDI). Results of researches showed that irrigation unbalanced the soil ecosystem and stipulated the forming of microbial cenosis with new parameters. Long-term intensive irrigation of typical chernozem (Kharkiv Region) with fresh water under condition of 4-fields vegetable crop rotation led to the degradation changes of its microbial cenosis such as reduction the number of microorganisms and the diversity of fungi species, repression of cellulose destroying capacity, decrease invertase activity and the rate of humification, intensifying mineralization processes and soil toxicity increasing. Long-term irrigation of ordinary chernozem (Kharkiv Region) with fresh water in moderate regime under 7-field crop rotation including alfalfa caused no disturbances of microbial cenosis. In this case parameters of biological indices did not deviate from the level of its non-irrigated analogue. Irrigation with saline water causes more profound negative changes of microbial cenosis of chernozem, which not always can be corrected using agroameliorative techniques. Intensive irrigation with saline water with total mineralization from 1.2 to 2.2 g/l of ordinary chernozem (Odesa Region) for 13 years has led to a significant degradation changes in the structure and functioning of its microbial cenosis, its radical alteration such as oppression of microflora, decrease in the number of its main groups by 30 – 40 %, intensification of its mineralization function. Application of agroameliorative techniques (such as annually use of phosphogypsum 3 t/ha) or /and complex measures (phosphogypsum 3 t/ha annually + N150P90K60 + manure 18 t/ha of crop rotation)) enable to regulate of soil biodynamic processes and partially or completely eliminate the phenomena of biological degradation. It was stated that after the cessation of irrigation the degradation changes of ordinary chernozem’s biological properties caused by irrigation with saline water were gradually restored.