Activity in post-pyrogenic soils in the Utrish State Nature Reserve (Russia) in the early succession stages

Due to the increasing frequency of fires, the study of the influence of the pyrogenic factor on the state of forest ecosystems is becoming an increasingly relevant research topic for scientists around the world. Of particular interest in the study is not only the direct wildfire impact, but also its indirect influence, which manifests itself in the early stages of succession. The study has been conducted in the Utrish State Nature Reserve, located on the Abrau Peninsula, northwestern part of the Black Sea coast of the Western Caucasus. The area of the State Nature Reserve «Utrish» is similar to the territory of the entire zone of the Mediterranean climate. The wildfire of 2020 has destroyed 4800 trees, and considerably changed biological properties of brown soils that led to a violation of their ecological functions. Changes in soil properties in the first few years after the wildfire impact are studied insufficiently compared to long-term changes. This study was aimed to investigate the activity of soil enzymes from the oxidase and hydrolase classes in the Utrish State Nature Reserve, immediately after the fire impact (2020), one year later (2021), and two years later (2022). During the field surveys, nine monitoring plots were established, i.e. three plots on each site under weak, medium, and strong degrees of the fire-induced damage. During the study, 81 soil samples were collected, i.e. three soil samples from each monitoring plot. Such enzymes as catalase, peroxidase, invertase, urease, and phosphatase were investigated. To assess the ecological condition of soils, we calculated the Integral Indicator of Biological State (IIBS), and determined the content of organic carbon. The results obtained were compared with the control values, typical for the background area of the forest. A correlation and factor analysis have been carried out. The Mann-Whitney U-test was used to assess the reliability of the data obtained. The enzyme response depends on the type of enzyme and the degree of fire-induced damage. Immediately after the fire impact (2020), a significant decrease (by 53% in average) in catalase, urease, and invertase activity was found on the sites under weak, medium, and strong degrees of damage. Phosphatase activity did not change significantly, while peroxidase activity increased on average by 47%. In 2020, the ecological condition of soils was determined as unfavourable, with the IIBS equal to 82–90%. The activity of catalase, invertase, urease, and phosphatase increased by an average of 31% for soils with a weak degree of fire-induced damage. In 2021, a decrease in activity of the studied enzymes was noted in post-pyrogenic soils under medium and strong damage degree, but to a lesser extent than in 2020. The activity of catalase and urease decreased for all investigated impacts by 47%. The activity of peroxidase and phosphatase was higher in relation to the control values by 55%. For the weak degree of fire-induced damage, a decrease in phosphatase activity by 16% was found. In 2022 (two years after the fire impact), the ecological condition of soils was still recognised as unfavourable, with the IIBS decreased to 89%. However, it was demonstrated that the activity of enzymes from the class of hydrolases two years after the fire impact was closer to the control values than the activity of oxidases. Understanding the relationships between the nature of damage and the response of ecosystem components, such as soil, can allow us to predict the response of ecosystems after forest fires.