Evaluation of the assimilation capacity and self-cleaning ability of biogeoecosystems along the Caspian Sea coastal zone

The purpose of this study was to determine the level of ability of the Caspian coastal soils to neutralise anthropogenic load based on the assessment of their assimilation and self-purification characteristics. The research methodology was based on a comparative analysis of the soils of the Absheron-Khizi and Guba-Khachmaz regions of Azerbaijan, where samples were taken from the 0-20 cm horizon, after which different levels of oil load were reproduced under model conditions and microbiological, biochemical, and bioindicative parameters were assessed to determine the self-purification capacity of ecosystems. The greatest resistance was demonstrated by meadow-forest soils, which had a neutral reaction of the environment (pH 7.1-7.4), high humus reserves (70-240 t/ha) and increased biological activity, which was confirmed by the carbon dioxide content of 0.65%. Their ability to decompose hydrocarbons was 1.12 ± 0.08 mg per gram of soil per day, and the mineralisation factor reached 0.52 ± 0.03. Meadow soils with humus of 35-60 t/ha and a share of oil-degrading microorganisms of 18-22% had lower biodegradation – 0.78 ± 0.05 mg/g×day with a mineralisation factor of 0.31 ± 0.02. The most vulnerable soils were grey-brown soils with a minimum organic matter stock (24-38 t/ha) and a low proportion of destructors (8-12%); the assimilation capacity did not exceed 0.34 ± 0.04 mg/g×day with a mineralisation factor of 0.12 ± 0.01. Bioindication tests showed a drop-in germination of watercress and alfalfa to 40-45% and a 50% reduction in biomass under conditions of 10% oil contamination. The results confirmed that meadow-forest soils were the most resistant, meadow soils were in an intermediate position, while grey-brown soils were the most vulnerable to oil pollution. The practical significance of the findings lies in their potential to be used by environmental services and agricultural research centres to monitor soil conditions and develop bioremediation strategies in the Caspian Sea regions