The Impact of Fires on the Properties of Steppe Soils in the Trans-Ural Region

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The impact of fires that happened in different times (1 week, 1.5 and 2 months, 1 and 2 years ago) on the properties of southern (textural-carbonate) chernozems was studied in the Trans-Ural steppe (Bashkortostan Republic and Chelyabinsk oblast, Russia). The topsoil horizons (0–5, 5–10, and 10–20 cm) were sampled in summer 2018. After the fires, the carbon content in the layer 0–5 cm increased by 8.7–12.0% relative to the initial content, and the contents of major nutrients (nitrogen, phosphorus, and potassium) sharply increased and remained at this level for two years. The pH values in the humus horizon after the fire increased by 1.5 pH units. The contents of exchangeable calcium, potassium, and sodium correlated with pH (r = 0.57, 0.58, and 0.65, respectively, at p < 0.001). The content of exchangeable sodium increased from 0.17 ± 0.01 to 0.23 ± 0.01 cmol(+)/kg of soil. The TDS of water extracts from the soil of background plots averaged 0.11 ± 0.01% increasing to 0.17 ± 0.02% in the soils after fires; on some plots, the soils became slightly or moderately saline with the TDS of up to 0.56%; the composition of major anions remained stable and was characterized by a predominance of sulfates and bicarbonates. The activities of dehydrogenase, urease, and protease in a week after the fire decreased by 8–11%; the activity of invertase decreased by 18%. However, in 1.5 months after the fire, the activities of dehydrogenase, invertase, and protease were even higher than those in the background soil. The urease activity was restored to the background level in a year after the fire. The soil acidity approached the background level after two years. Fires are an important factor of pedogenesis in steppe ecosystems that have positive and negative consequences.