The work objective was to assess the ecological state of soils by changing the residual oil content and restoring catalase activity after remediation. The soils were selected in various ecosystems: a steppe of the Rostov Region (Haplic Chernozem), beech-hornbeam forests in the Republic of Adygea (Haplic Cambisols), and semi-desert of the Caspian province of the Republic of Kalmykia (Eutric Cambisols). Soil samples were polluted with oil at a concentration of 5% of the soil mass. After that, ameliorants (biochar, nitroammophoska, sodium humate, and Baikal EM-1) were introduced into the oil-contaminated soil. The catalase activity of Haplic Cambisols was stimulated only with the introduction of D₂ biochar by 11% relative to the control, and in Haplic Chernozem, catalase was most stimulated with the addition of nitroammophoska D_0.5 and D₁ by 65% and 57% of the control, respectively. Nitroammophoska in all doses significantly stimulated the enzymatic activity, in Eutric Cambisols by four to six times compared to the control. The range of soil stability determined by catalase activity: Eutric Cambisols > Haplic Chernozem > Haplic Cambisols. Thus, it is most effective to apply biochar in doses of D and D₂ and D_0.5 and D nitroammophoska during the remediation of oil-contaminated Haplic Chernozem. For the remediation of Haplic Cambisols, it is effective to introduce biochar in dose of D₂, and Eutric Cambisols—biochar and sodium humate in dose of D_0.5 and nitroammophoska (all doses). The results of the study allow using catalase activity as a very informative and statistically significant diagnostical indicator of the health of oil-contaminated soils after remediation.

Soil pollution with oil as a result of accidents at oil pipelines and oil refineries is a frequent occurrence in the south of Russia. To restore such polluted lands, it is necessary to carry out soil remediation measures. This work aimed to evaluate the use of ameliorants of various natures (biochar, sodium humate, and microbial preparation Baikal EM-1) to restore the ecological state of oil-contaminated soils with different properties (Haplic Chernozem, Haplic Arenosols, Haplic Cambisols). To assess the ecological state of soils, the following physicochemical and biological indicators were studied: residual oil content, redox potential, and medium reaction (pH). Changes in enzymatic activity were also studied, including catalase, dehydrogenases, invertase, urease, and phosphatase. The greatest decomposition of oil in Haplic Chernozem and Haplic Cambisols was provided by Baikal EM-1 (56 and 26%), and in Haplic Arenosols, this was provided by biochar (94%) and sodium humate (93%). In oil-contaminated Haplic Cambisols, the content of easily soluble salts with the addition of biochar and Baikal EM-1 increased by 83 and 58%, respectively. The introduction of biochar caused an increase in pH from 5.3 (Haplic Cambisols) to 8.2 (Haplic Arenosols). The introduction of oil-contaminated Haplic Arenosols of biochar, humate, and Baikal stimulated the activity of catalase and dehydrogenases by 52–245%. The activity of invertase was stimulated in the Haplic Chernozem after the introduction of ameliorants by 15–50%. The activity of urease was stimulated after the introduction of ameliorants into borax and Arenosol by 15–250%. The most effective ameliorant for restoring the ecological state of Haplic Cambisols after oil pollution was biochar. For Haplic Arenosols, this was sodium humate, and for Haplic Chernozem, the effectiveness of biochar and sodium humate did not differ. The most informative indicator for the remediation of Haplic Chernozem and Haplic Cambisols was the activity of dehydrogenases, and for Haplic Arenosols, this was the activity of phosphatase. The results of the study should be used to biomonitor the ecological state of oil-contaminated soils after bioremediation.

One of the most modern and effective methods of soil restoration after pollution is the use of bioremediation. In recent years, special attention has been paid to the use of biochar. However, the ecological state after the introduction of biochar into petroleum-hydrocarbons-contaminated soils at different levels of pollution has been little studied. The work objective was to study the effect of biochar (10% of the soil mass) on the ecological statement of Haplic Chernozem contaminated with different levels of oil, from 0.1 to 50% of the soil mass. To assess the state of Haplic Chernozem after remediation with biochar, indicators of the biological activity of soils were studied. The maximum information content in the case of oil pollution and remediation with biochar was established by the activity of dehydrogenases (r = −0.90) and the total number of bacteria (r = −0.98). When applying biochar, the maximum stimulations of the integral indicator of the biological state (IIBS) relative to the background, by 62, 76, and 72%, were noted in samples with oil at concentrations of 0.5, 25, and 50%, respectively. The results of the study should be used for biodiagnostics and monitoring of the state of oil-contaminated Haplic Chernozem at different levels of contamination after remediation with biochar.

Bacteria are a sensitive marker of the state of soils contaminated with oil and petroleum products. Introduction of ameliorants leads to the changes in the