Restoration of biogeochemical indicators in post-pyrogenic bogs

Wildfires have profound impacts on biodiversity, greenhouse gas emissions and other environmental components [Gajendiran et al., 2024; Li et al., 2025]. Bogs, as peat deposits, are particularly vulnerable to fire [Rybina et al., 2015]. The areas most frequently affected are those disturbed by human activity, and in such mires the area affected by fire reaches several square kilometers [Sinyutkina et al., 2024]. Fires lead to complex physical and biogeochemical transformations that affect all components of the ecosystem [Granged et al., 2011]. When exposed to high temperatures and burning of organic material during a fire in a mire, a charred layer is formed in place of the vegetation cover, which leads to an increase in the hydrophobicity of the peat and an increase in the level of runoff [Leonard et al., 2017; Wu et al., 2020]. All the main processes of change in the chemical composition of the peat deposit in response to the impact on the mire are most indicative in the high peat layer [Stepanova, Pokrovsky, 2011]. The transformation of the main processes of leaching and accumulation of elements occurs in the upper layer of peat [Dymov et al., 2022; Gashkova, 2022]. Atmospheric and hydrological transfer of elements during a fire occurs intensively therefore changes affect not only the burned sites but the impact of the fire also affects adjacent territories [Kala, 2023; Kuzmina et al., 2022; Ortíz-Rodríguez et al., 2019]. Biogeochemical parameters of the ecosystem are restored over a long period [Belkova et al., 2016]. Therefore, the processes of change in the chemical composition of the peat deposit occur continuously over many years. In the first years after a fire, the concentration of most elements and the pH of the soil increase during the process of restoring acidity and vegetation cover, the ratio of elements continues to change [Stavrova et al., 2019]. The most indicative change in the elemental composition of peat is tracked through the ratio of the element content in peat to its content in the underlying rock [Efremova et al., 2003]. This article presents the results of monitoring the elemental composition of peat and underlying rock in raised bogs 6–8 years after a fire.

The objects of the study were mires located on the West Siberian Plain. We explored three mires. Two mires are located in the taiga zone, in the interfluve of the Bakchar and Iksa rivers (areas BB and BF), and on the terrace of the Bakchar river (UBB and UBF). The third mire (palsa) is located in the forest-tundra zone, between the Pur and Nadym rivers (PB и PF). The fire in the BF and PF sites occurred in 2016, and in the UBF site in 2014. We conducted research from 2022 to 2024, in the burned sites and in sites immediately adjacent to the burned sites, but not disturbed by fire. Before the fire, both sites from each of the bogs were similar in vegetation, depth, and peat deposit structure. We previously published a detailed description of the studied bogs [Sinyutkina et al., 2024]. In each of the 10 m² test plots, we collected samples of plants (Chamaedaphne calyculata (L.) Moench), peat from the root zone and the rock underlying the peat. In the palsa between the Pur and Nadym rivers, we measured the depth of frozen rocks. We analyzed the selected samples for botanical composition and element content: Na, Mg, P, K, Ca, Mn, Fe, Cu, Zn, Cd и Pb mass spectrometric method (ICP-MS) at the “Plasma” chemical-analytical center (PerkinElmer, США). Sample preparation was performed using a Speedwave microwave digestion method (Berghof, Germany) after preliminary acid digestion. To interpret the data, we calculated concentration coefficients (KK) – the ratio of the element's concentration in the topsoil to its concentration in the underlying rock. We processed the data using Microsoft Excel.

This work is part of a comprehensive monitoring of post-pyrogenic restoration of bogs, conducted since 2017. Between 2022 and 2024, some changes in the content of elements were identified in both post-pyrogenic and adjacent sites.

The underlying rock in the studied areas varies in granulometric composition. In the palsa sandy deposits characteristic of this area lie beneath the peat layer [Voronova, Grebenyuk, 2018]. In sites of bogs in the taiga zone, blue-gray gleyed loams typical of organogenic acidic soils [Karavaeva, 1978, p. 71-74] represent the underlying rocks. The depth of the frozen layer on the palsa varied between 40 and 60cm. The content of elements in the underlying rock varies slightly; no changes in the concentration of elements were noted over three years of observation. No differences were found between samples from burnt and pristine sites within the same mire. In addition, the content of K and Na does not differ significantly across all sites. At the same time, in the mire between the Pur and Nadym rivers, the content of elements P, Mg, Ca, Mn, Fe, Cu, Zn and Pb is significantly lower than in other mires (Table 1).

Comparing the KK in post-pyrogenic and adjacent sites it was found that in most post-pyrogenic sites, compared to unburned sites, the KK of the elements Ca, Fe, Cu, Zn and Pb was increased. However, in most sites the KK does not reach 1. The exception is the sites of the palsa, where KK > 1 was found for the elements P, Ca, Mn, Zn and Cd (Table 1).

The peat deposit in the areas of the palsa was 50-75 cm in the botanical composition, Sphagnum balticum (Russ.) Russ.ex C.Jens. co-dominated together with Sph. fuscum (Schimp.) H. Klinggr. In areas in the taiga zone, the thickness of the peat deposit varied from 250 to 300 cm; the top layer of the deposit (0-20 cm) at all points was represented by high-moor sphagnum fuscum peat with a small admixture of shrubs and cotton grass. We compared the elemental composition of peat collected over three years and found changes in the concentration of some of the elements examined. To determine trends in element content over time, a linear approximation method was used. A trend with an approximation coefficient greater than 0.8 was considered significant. In the root-inhabited peat layer in all post-pyrogenic sites from 2022 to 2024, a trend towards an increase in the concentration of Mg, K, Mn and Ca and a decrease in the concentration of Na, Zn, Pb and Cd is observed (Figure 1).

In sites located adjacent to burnt areas from 2022 to 2024, no increase in the content of elements was observed in the upper peat layer, but a trend towards a decrease in the content of Zn, Pb and Cd was recorded (Figure 2).

The vegetation cover in sites located adjacent to burnt areas did not change after the fire; in post-pyrogenic sites, restoration of the vegetation cover began already in the first year after the fire with the active restoration of the shrub layer. [Sinyutkina et al., 2024]. In all studied sites, Chamaedaphne calyculata had fully recovered to its pre-fire abundance by 2022. Therefore, during the period considered in this article, the dwarf shrubs made a major contribution to the change in the chemical composition of the upper part of the peat deposit. As in peat, a downward trend in Na, Zn, Pb, and Cd content was observed in all sites from 2022 to 2024, and the concentration of Fe and Cu in leaves decreased. In post-pyrogenic sites, as in peat, an increase in the concentration of K, Mg, Ca, and Mn was observed over the three years (Figure 3).

In sites of bogs not affected by fire, but located next to burnt ones, no increase in the concentration of elements in the leaves of Ch. calyculata was observed, but, as in peat, an increase in the concentration of K, Mg, Ca and Mn was found (Figure 4).

The change in the elemental composition of the upper peat layer that we discovered during the restoration of the bog is natural, since the upper part of the peat deposit reacts most sensitively to the post-pyrogenic transformation of the bog, which was noted earlier [Stepanova, Pokrovsky, 2011; Dymov et al., 2022]. Post-pyrogenic decrease in the concentration of Zn, Pb and Cd over three years indicates a gradual leaching of elements mineralized and condensed from smoke particles during the fire [Gray, Dighton, 2006; Alves et al., 2010]. This reduction occurs because the acidity of the peat, which decreased after the fire, begins to increase in subsequent years during the process of bog restoration, increasing the mobility of heavy metals [Lipatov et al., 2016; Colin et al., 2024]. When studying the soils of the interfluve of the Pur and Nadym rivers, researchers noted a lower content of elements compared to the taiga zone [Romanenko et al., 2020], which we also noted, in particular, as a lower content of P, Mg, Ca, Mn, Fe, Cu, Zn and Pb in the underlying rock in sites in the forest-tundra.

The content of elements in peat is directly related to the chemical composition of plants, the litter of which begins to influence the biogeochemical situation in post-pyrogenic areas, due to the rapid restoration of the shrub layer, which began already in the first year after the fire [Sinyutkina, Gashkova, 2025]. In the leaves of plants in post-pyrogenic areas, as well as in peat, over the course of three years, we found an accumulation of the elements Mg, K, Mn and Ca, which, as noted (Stavrova et al., 2019), are actively absorbed by plants from ash in the first years after a fire. In addition, ash and charred remains continue to enter the soil for several years after the fire, as a result of the destruction of the charred forest stand [Ukraintsev et al., 2016].

The high levels of CC that we noted for all forest-tundra areas are explained by both the low levels of elements in the underlying rock, which are characteristic of this region [Romanenko et al., 2020], and airborne pollution associated with the activities of oil and gas production enterprises, which affects the elemental composition of peat [Voronova, Grebenyuk, 2018]. The higher KK of the elements Ca, Fe, Cu, Zn and Pb in post-pyrogenic areas, compared to unburned ones, indicates that the process of restoring the elemental composition of peat has not yet ended; according to some authors, such a process can drag on for several decades [Leonard et al., 2017; Mergelov, 2015].

In the upper layer of peat, 6-8 years after the fire, in post-pyrogenic and adjacent areas, processes of restoration of the elemental composition continue to occur. The most noticeable changes are reflected in the fact that over the course of three years, the concentrations of Zn, Pb and Cd have gradually decreased both in post-pyrogenic sites and in adjacent sites. Concentration coefficients show the residual impact of post-pyrogenic changes and the specific characteristics of certain sites. Regional characteristics are expressed in the low element content in the underlying rock of bogs located in the forest-tundra zone compared to those in the taiga zone.