INTRODUCTION

In recent years, wildfires occur more and more often causing the expansion of areas covered by postpyrogenic soils. These circumstances worsen the natural rehabilitation of damaged areas.

Combustion, thermal effects and smoke are the main phenomena that accompany fires. Combustion is a process of thermal destruction of ecosystem components, heat release and glow of combustible materials [4]. Smoke is the release of gaseous substances due to ignition of products that have undergone heat treatment.

Fires exert a negative effect on flora and fauna as well as on the quality of soils by changing their properties (physical, chemical and biological). The pyrogenic impact on soil differs depending on the type of fire (top, ground, or underground fire), its intensity (weak, medium, strong), duration (from several minutes to several hours) and recurrence [9, 12, 17, 28]. A fire front can advance for many kilometers, being able to transform the terrain instantly. As a result, the fire-affected areas lose their protective, water controlling and other properties, fauna disappears, engineering structures and sometimes settlements are destroyed. These points to significant transformations in ecosystems, therefore, the pyrogenic impact causes serious concern for the ecological status of the environment.

Flame exerts the strongest impact of all fire-accompanying phenomena. As is known, the presence of combustible materials is one of the main conditions for spreading fire, their amount increasing with the biomass volume [13]. However, in addition to flame, smoke also impacts severely the soil quality, as well as living beings. Combustion products formed due to the pyrogenic factor differ in their moisture content; the higher it is, the less polluting gases enter first the air, and next the soil. CO, HNC, hydrocarbons, etc. are released upon incomplete combustion of materials. For example, a process may be observed often in forests that is accompanied by the release of carbon monoxide [2, 28]. A bulk of particles formed during forest fires (more than 90%) are less than 10 μm in diameter. They can be primary, released into the atmosphere due to incomplete combustion, or secondary, formed as a result of physical or chemical transformations. The primary particles can include elemental carbon (soot) or organic carbon particles. The latter can also be secondarily obtained by condensation of hot vapors (resins) [23]. When a fire breaks out, soot particles are released into the air. As is known, they consist of carbon and products of incomplete combustion (wood, tree bark, forest litter, etc.). In turn, these organic substances, including phenolic compounds, are dangerous toxicants [2, 16]. Therefore, smoke can be considered to be one of the factors that have a significant impact on soil and its biota.

Biochemical indicators (soil enzyme activity) can serve as an indicator for assessing quality and fertility of agricultural soils, as well as their status upon different treatment methods [1, 7, 19, 22, 31]. Under the impact of pyrogenic factor, enzymatic activity, being a very sensitive indicator of biogeneity, serves as a reliable indicator for detecting disturbances in soil [21, 33, 34, 36], for chemical pollution identification, in particular [7, 26].

At present, the smoke effect on the enzymatic activity of soils has not been sufficiently studied. Most of the studies in pyrogenic impact deal with the effect of fire (heat) on soil properties and living organisms. A need arises to consider in more detail the effect of smoke produced by combustion sources on the activity of soil enzymes.

This work is aimed at revealing the effect of vegetation (leaves, wood, wheat straw) combustion products on the activity of enzymes (catalase, polyphenol oxidase, peroxidase, and invertase). The research tasks included determination of the smoke penetration depth into chernozem according to the enzyme inhibition degree at different depths (0–1, 1–2, and 4–5 cm); and identification of the enzyme specific reaction in dry and wet soil, as well as their recovery dynamics after fumigation

OBJECTS AND METHODS

Characteristics of the study area. The soil on the arable plot (a layer of 0–20 cm) in the botanical garden of the Southern Federal University, which covers an area of more than 160 ha, was chosen as the object of study. Geographical coordinates of the study site are: 47.23815° N, 39.64194° E. The mean annual precipitation is about 460 mm, the mean annual temperature is 8.2°С. The mean annual temperature in January ranges from –5 to –9°C, and that in July, from 22 to 24°C [14]. The soil is classified as carbonate light clay ordinary chernozem of the South European facies (Haplic Chernozem (Aric, Loamic, Pachic)). Chernozems occur over a vast territory in the south of Russia, being among the most fertile soils in the world [1, 3]. The soil parameters at the experimental plot are the following: the thickness of humus horizons (A + B) is 80 cm, the content of organic carbon in the arable horizon is 2.0%, the content of physical clay is 53% (light clay texture), the content of mobile phosphorus is 3.3 mg Р2О5/100 g, and that of exchangeable potassium К2О and nitrates N–NO3 is 341 and 8.4 mg/kg, respectively.

Model experiments were carried out in laboratory and in the field. Freshly dried (at room temperature in the shade, a day or two after sampling) soil was used for tests. Since high temperature is an important factor of enzyme inhibition [22, 30], the thermal effect from smoke was excluded by the specific design of smoke generators. The gas-measure container of 50 L in volume was identical in all studies. The weight of combustion product ash was controlled with Vibra HTR-220CE analytical balance (Shinco Denshi Co. Ltd., Japan).

Short- and long-term effects of fumigation on the soil. Threshold values of smoke toxicity were determined in the reconnaissance experiment. The short-term experiment included 4 variants studied in triplicate. The air-dry soil weighing 40 g was placed in a layer of 0.6 cm in 200 mL polypropylene containers. The soil surface area that was in contact with smoke was equal to 136 cm2. Next, the containers with soil were placed into a large container that acted as a gas chamber for smoke processing with a smoke generator Kitfort KT-2033 Smoking Gun (Kitfort, China) during 15 minutes at a smoke rate of about 2 L/min. Dry plant materials (winter wheat straw, maple leaves, pine needles, and cigarette tobacco) were used as combustion sources. Up to 10 g of each combustion product affected soil samples. A total mass of all combustibles used was 37.8 g; straw, maple leaves, and pine needles—10 g each; and cigarette tobacco—7.8 g.

A long-term experiment was carried out using a Merkel Standart smoke generator (Helicon, Russia). Smoke was pumped through a gas chamber to soil samples using a Barbus Air 007 (Barbus, China) air compressor, with a capacity of 4.5 L/min. Apple wood chips of 200 g in weight were used as a combustion product. Six soil samples were studied (3 as a control and 3 for smoke treatment). Soil samples weighing 40 g in a 0.6-cm-thick layer and a surface area of 136 cm2 in contact with smoke were placed in a 50 L container (gas chamber) for fumigation. The smoke-processing time of each sample was 60 min during the smoke generator operation. In this experiment, we assessed a longer effect of smoke on the enzymatic activity (catalase, polyphenol oxidase, peroxidase, invertase) of chernozem.

The entire soil sample in a 0.6-cm-thick layer was taken for analysis after a short- and long-term exposure to fumigation.

Depth of smoke penetration into the soil. The experiment was performed with the same equipment. Pine sawdust and shavings weighing 210 g were used as a smoke source. The aim of the experiment was to find the penetration depth of gaseous substances into soil depending on the exposure time (30, 60, and 120 min). Dry soil (310 g in weight) was poured into 350-mL containers in a layer of 5 cm, with the soil surface area of 136 cm2. Nine samples were treated with smoke in three control options. A similar experiment was performed with the soil moistened with water to 40 wt %, with a mass of combustible material having been 225 g.

After the experiments, the studied soil layers (0–1, 1–2, 4–5 cm) were sampled for the analysis layer by layer.

Recovery dynamics of soil enzymes. The experiment was carried out in laboratory with a smoke generator. Soil (40 g) was placed in 200-mL containers in a layer of 0.6 cm, and next placed in a gas chamber (50 L in volume) for fumigation. In contrast to previous experiments, air was supplied by a more powerful Hailea ACO-208 air compressor (China), with a capacity of 17.5 L/min. Chernozem was treated with smoke for 30, 60 and 120 minutes. Apple wood chips were used as a thermal degradation product. In this experiment, the restoration dynamics of soil enzymes under laboratory conditions without the use of biological drugs was revealed; enzymes were investigated immediately after the experiment. The entire soil samples were analyzed after the experiment. The analysis was repeated in a month, with the optimal conditions having been maintained in soil for the development of biological processes during the entire period of sample storage (temperature 25–30°C and moisture 40%). The soil sampling procedure for the analysis is similar to the above-described.

A similar experiment was carried out under natural conditions in the botanical garden of the Southern Federal University. During the study, the soil was covered with a plastic insulator. Equipment as well as the experimental conditions and soil-sample storage are described above. Samples were taken from the surface layer of 0–1 cm. Changes in biochemical parameters were studied immediately after fumigation and a month later.

The air temperature in the gas chamber and in soils was monitored using a Hanna Checktemp electronic thermometer, a DT-810 CEM pyrometer, and Thermochron DS1921 temperature sensors. The effect of combustion products upon fumigation on the pHwater was controlled potentiometrically in a container separate from soil using HANNA HI-98128-pHep-5 instrument (Germany). Soil reaction was found for a soil : water ratio of 1 : 2.5. The content of easily soluble salts was analyzed conductometrically according to the specific electrical conductivity in the solution with a HANNA HI-9034 tool (Germany). Soil exoenzymes were taken as smoke exposure indicators, their content was studied in 3–6 replicates by routine methods of assessing the enzymatic activity of soils [8]. Catalase activity was determined volumetrically by the Galstyan method according to the H2O2 decomposition rate. Peroxidase and polyphenol oxidase activity was measured colorimetrically by the Karyagina and Mikhailova method using the hydroquinone substrate. To determine the invertase, a modified colorimetric method was used, which was based on determining the optical density of the Felling reagent after CuSO4 reduction with glucose formed from inverted sucrose [8].

The reliability of difference between the control and experimental variants was assessed by a single-factor analysis of variance with a significance level p < 0.05, as well as by the Student’s t-test at a significance level of p < 0.01 in Microsoft Excel and Statistica software.

RESULTS

Short- and long-term effects of fumigation on the soil. The results of a reconnaissance study with a Kitfort KT-2033 miniature smoke generator show a significant decrease in enzyme activity (by 7–10%) under the 15-minute-long influence of smoke from a small amount of combustible substances. In cases of treatment with smoke produced by winter wheat straw, maple leaves, and pine needles, inhibition did not depend on the combustion source. Cigarette smoke resulted in a slight increase (by 9%) in enzyme activity. Further studies were carried out with smoke sources from combustion products to find regularities of its effect on the enzymatic activity in soil.

The results of a longer model experiment (60 min) proved that the activity of all enzymes considered (catalase, polyphenol oxidase, peroxidase, and invertase) decreased after the soil exposure to smoke for 60 min. The catalase activity decreased by 25% in respect to control (Fig. 1). The activity of polyphenol oxidase and peroxidase was found to be suppressed by 33 and 15%, respectively, and that of invertase, by 23%.

Fig. 1.
figure 1

Changes in enzyme activity after the 60-minute-long impact of smoke: (1) catalase, (2) polyphenol oxidase, (3) peroxidase, (4) invertase (significant changes at p < 0.01).

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The depth of smoke penetration produced by the combustion of pine sawdust into soil. After 30 min of fumigation, catalase activity decreased by 5–19% in the layers of 0–1, 1–2, and 4–5 cm (Fig. 2). After 60 min, inhibition in the same layers was 10–28%, and 8–37% suppression was recorded after 2 h.

Fig. 2.
figure 2

Penetration depth of smoke from combustion products. Changes in catalase activity in the air-dry soil depending on the treatment time: (1) smoke for 30 min (a layer of 0–1 cm, 1–2 cm, 4–5 cm), (2) smoke for 60 min, (3) smoke for 120 min (significant differences at p < 0.05).

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After 30–120 min of fumigation, the activity of polyphenol oxidase decreased by 6–49% in the 0–1 cm layer. However, no significant differences were recognized in other layers due to a highly varying activity values in the samples, which overlapped the differences between the experimental options.

This experiment also permitted us to reveal variation in pH of distilled water placed in a gas chamber in separate containers. This value decreased reliably by 2.2–2.6 units under the effect of smoke, which testifies to a strong impact of combustion products. Water mineralization also changed significantly during 30-, 60- and 120-minute-long fumigation with sawdust smoke (Table 1).

Table 1.   Changed in mineralization and pH of water and soil after fumigation with smoke of plant substances
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A similar test on the smoke penetration depth was performed for the wet soil pre-moistened with water to the optimal moisture for biological processes (40% of soil mass). Chernozem was wetted in order to compare the smoke effect on wet and air-dry soil subjected to smoke in the previous test. It was assumed that wet soil would absorb more aerosols due to the dissolution of combustion products. It was found out that the activity of soil enzymes (catalase and polyphenol oxidase) decreased the same as in the previous test (Fig. 3). The analysis showed that catalase activity was inhibited in respect to control in the 0–1 cm layer by 52, 41, and 48%, after 30, 60, and 120 min, respectively. These enzyme activity values are somewhat higher than in the previous experiment. Polyphenol oxidase content also decreased in the 0–1 cm layer in respect to control (by 38, 57, and 54% for 30, 60, and 120 min, respectively), which is more than in the air-dry soil (6–46%).

Fig. 3.
figure 3

Changes in the activity of catalase and polyphenol oxidase in wet soil in a layer of 0–1 cm depending on the smoke treatment duration: (1) control, (2) smoke for 30 min, (3) smoke for 60 min, and (4) smoke for 120 min (differences are significant at p < 0.05).

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Rehabilitation dynamics of soil enzymes. As research proved, the values of oxidoreductases (catalase, polyphenol oxidase, peroxidase) has substantially changed in respect to control options immediately after the model experiment on soil fumigation with materials of plant origin (apple wood chips) (Fig. 4). The analyses performed a month later pointed to a gradual restoration of enzymatic activity without adding any biological preparations. Catalase activity restored to control values, except for the 120-min-long exposure to smoke. The same effect was revealed for peroxidase. Polyphenol oxidase turned out to be less susceptible enzyme. As compared to the control variant, no complete recovery was observed for the 60- and 120-minunte-long treatments with smoke (95 and 60%, respectively).

Fig. 4.
figure 4

Effect of fumigation on chernozem in the model experiment (a) and recovery dynamics (b) after 1 month: (1) catalase, (2) peroxidase, (3) polyphenol oxidase (differences are significant at p < 0.05).

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The field test elucidated a significant effect of smoke on biochemical parameters. However, as compared to the model test results, the suppression of enzymes is less pronounced. For example, catalase activity has decreased by 13–42% within 30–120 min, peroxidase activity has decreased by 8–29%, and that of polyphenol oxidase, by 7–30% (Fig. 5). The results were rechecked a month later to find out that none of the indicators had restored their control values, whereas these enzymes restored their activity completely after fumigation for 30–60 minutes in laboratory.

Fig. 5.
figure 5

Effect of fumigation on chernozem under natural conditions (a) and recovery dynamics (b) after 1 month: (1) catalase, (2) peroxidase, and (3) polyphenol oxidase (differences are significant at p < 0.05).

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DISCUSSION

The enzymes were chosen for analysis according to their high sensitivity to anthropogenic impact. Catalase is often analyzed in biomonitoring and biodiagnostics of the soil cover. This enzyme is widespread in soils and appears to be an informative diagnostic indicator in the study of various kinds of anthropogenic impacts [7]. Peroxidase and polyphenol oxidases are very sensitive to impacts of various kind, and their activity may serve as an important diagnostic criterion [7, 8, 32, 35]. Invertase activity is one of the most informative indicators, showing the clearest correlation with fertility and anthropogenic factors [8, 10, 18].

The results of research permitted us to reveal a general pattern of a decrease in the enzymatic activity with time of smoke treatment. Some authors considered the fumigation effect on soil [25, 27, 29, 39, 40]. However, these studies were focused on the change in microbial communities under the smoke impact and its influence on the enzymatic activity in agricultural soils treated with various substances (methyl bromide, dazomet, etc.). Increasing crop yields as well as the impact on soil microbial communities was emphasized. Some authors note the suppression of pathogens by fumigation, as well as an increase in the number of microorganisms promoting vegetation growth and enzymatic activity. This process goes due to soil decontamination with special drugs (dazomet, etc.). Our study examined the change in soil enzymes after pyrogenic impact, where fumigation suppressed the enzyme activity. This difference in results with other researchers points to the toxic composition of fire smoke. It differs significantly from the composition of fumigants when decontaminating the soil with dazomet, methyl bromide and other chemicals.

Formerly, the effect of smoke from the straw combustion products on soil was studied [9]. Transformation of microbiological (microbial biomass, nitrogen-fixing bacteria) and biochemical properties of ordinary chernozem was revealed. In particular, attenuating activity of oxidoreductases and hydrolases was found under different moisture content upon cold (52°C) and hot (139°C) soil treatments with smoke from burning straw. It was the hot smoke that exerted the greatest inhibitory effect on enzymes due to high temperatures. A decrease in the values of biochemical parameters also depends on the soil depth. A change in the microbial biomass and in the abundance of Azotobacter bacteria was registered as a result of exposure to hot smoke [9]. Both the enzymatic activity and the abundance of living organisms depended on the smoke impact duration. It was proved that humus content and soil moisture were the principal parameters affecting soil enzymes and the microbial community [6, 19].

The main effect exerted by the pyrogenic factor was recorded in the layer of 0–1 cm showing the maximal changes in enzymatic activity. The latter decreased due to the absorption of toxic gases and aerosols by soil, since the burning of wood residues releases soot, CO2, CO and various hydrocarbons into the soil. Values of pH fall to those restricting extremely the life of many animals and plants; they change due to CO2, which is readily dissolved in water to form carbonic acid. The soil reaction in suspension decreased to a lesser extent in the tests because of the high buffering capacity of chernozems (Table 1). The effect of pyrogenic impact decreases in deeper soil layers, with the minimal changes being observed in the layer of 4–5 cm. The wildfire impact is known to be limited to the upper soil layer, and the effect of the pyrogenic factor (flame) on soil properties is weakly pronounced deeper [9, 17, 18, 37]. It is established that soil serves as an effective filter capable of absorbing toxic gases and protecting the underlying layers from the hazardous impact. This allows soil to restore its biological activity quickly, spreading it from below to the surface soil layers inhibited by smoke.

The results of field test on the restoration of enzymatic activity in chernozem differ from those in the model experiment. This is due to more favorable temperature and moisture conditions in the laboratory that accelerate biological processes. In the field studies, the soil manifested the same temperature, but lower moisture content (12–22%), which changed the rate of biological processes in it.

In both cases, the results obtained attest to gradual restoration of the soil. To stimulate biological processes and accelerate the recovery of damaged areas, many researchers recommend to use biological drugs [5, 11, 15, 24, 38].

CONCLUSIONS

A high sensitivity of catalase, polyphenol oxidase, peroxidase, invertase to combustion products of plant origin has been stated. A clear dependence has been revealed between the alteration of enzymatic activity in ordinary chernozem and the duration of smoke treatment. Inhibition did not depend on the substance subjected to thermal degradation. Enzymes of oxidoreductase class, i.e., catalase (the most sensitive enzyme) and polyphenol oxidase, turned out to be more sensitive to smoke. Suppression occurred even upon a short (15 min) exposure to gaseous substances. Longer exposure to smoke led to an increase in the suppression of enzymes. Invertase of hydrolase class appeared to be less informative due to worse sensitivity to fumigation and greater variation in soil. The penetration depth of gaseous products into air-dry and wet soil is limited to a few centimeters, the smoke affecting the wet soil stronger. This may testify to a higher sensitivity of enzymes in wet soil, since the soil solution absorbs gaseous combustion products, which affect the biological activity of soils. A tendency to restore the activity of chernozem enzymes after fumigation was revealed. This effect was registered under laboratory conditions, where oxidoreductases (catalase, polyphenol oxidase, peroxidases) almost restored their activity up to their original values a month after a 30- and 60-minute-long exposure to smoke. In nature, the enzyme activity has restored incompletely in the fumigated soil, which testifies to different development of biological processes in laboratory and in nature.

The results obtained and regularities revealed on the effect of one of the pyrogenic factors can be used to assess the state of soils and soil cover in the areas most affected by wildfires.