Vol 52, No 12 (2019)

First data on the specific morphology and physicochemical properties of soils forming in the zone of active discharge of ascending solutions of deep thermal water of Kuchiger hot springs of the fluoride hydrosulfuric bicarbonate–sulfate–sodium type are presented. The soil cover was studied along the transect crossing the mire with active gas hydrothermal griffons, an island within the mire, and the adjacent margin of alluvial plain in the northwestern part of the Barguzin Depression near the village of Kuchiger, Kurumkan district of the Buryat Republic. The features of peat–mucky gley soils (Eutric Rheic Sapric Histosols (Fluvic, Sodic)) predominating within the mire are described. The profiles of weakly developed and alluvial soils (Eutric Sodic Gleyic Fluvisols (Epiloamic, Endoarenic, Humic)) forming under the impact of ascending gas hydrothermal fluids bear the features of turbation and impregnation with condensed organic matter in their mineral and organic horizons. These features are not included in the international and Russian soil classification systems. It is suggested that they should be diagnosed and indicated by symbols imp (various degrees of impregnation), int (intruded material), and IMP (black sandy–silty horizon strongly impregnated with condensed organic matter). The inclusion of these features into the list of diagnostic properties allows us to specify impregnated and hydrothermally turbated soil subtypes in the new Russian soil classification system. Hypotheses of the genesis of soils along the entire transect and the appearance of islands within the mire massif are suggested.

The results of the study of the carbonate profiles of soils in the western (Cis-Baikal) and eastern (the Selenga middle mountains) Baikal region are presented. There is a similarity in their structure: numerous CaCO₃ maxima in the soil profiles are typical, which is associated with the repeated redistribution of carbonates during different stages of pedogenesis. The carbonate profile of soils in the Baikal region is relatively ancient (the youngest accumulations date back to the Middle Holocene) and a rather conservative formation that does not undergo any noticeable rearrangements under modern climatic conditions. The accumulations of secondary carbonates are localized in the carbonate-accumulative horizons of the modern surface soils and buried Kargin (MIS-3) soils and Holocene soils. The exception is hypocoatings that are often found beyond the indicated horizons, which is associated with a more active redistribution of carbonates within the root system of vegetation. The solid-phase effect of carbonatization (carbonate neoformations) is observed in the study area mainly in the soils formed on calcareous rocks and products of their redeposition and does not depend on the type of soil and the nature of the growing vegetation. Based on the analysis of the composition of stable carbon and oxygen isotopes in secondary carbonate accumulations, it was found that carbonate precipitation in soils of the Cis-Baikal region occurs during the degassing of soil solutions in the course of freezing–thawing cycles, dynamic increase and decrease in the soil biological activity, and alternation of the soil moistening with snowmelt and rainwater with its subsequent freezing that can take place in spring and autumn seasons. Under these conditions, atmospheric CO₂ has the main influence on the isotopic composition of carbon in pedogenic carbonates; the isotopic composition of oxygen is controlled by the fractionation of isotopes upon freezing of the soil solutions. Secondary carbonate accumulations in the Selenga middle mountains are formed during the soil drying as a result of active water consumption for plant transpiration, which is most pronounced in the first half of summer.

Tropical agricultural soils have been claimed as a source of carbon. As agricultural systems in the tropics are highly diverse, it is useful to study soil organic C (SOC) of different agricultural systems. We quantified the SOC fractions, available nutrients, and aggregate stability in eight different tropical agricultural systems, including annual crops under different management scenarios, such as organic, inorganic, and combined fertilizer applications. Annual crops treated with organic fertilizer only (A–OF), inorganic fertilizer only (A–IF), both organic and inorganic fertilizers (A–O/IF), perennial crops (PC), home gardens (HG), and abandoned home gardens (AHG) in Eutrustox soils and annual crops with organic fertilizer only (A–OFS) and uncultivated land on Quartzipsamments soil (USR) were studied. The links between SOC fractions, available nutrients, and aggregate stability in these soils were analyzed. Regression models were fitted for SOC fractions and available nutrients. Our results indicated that the different land use types exhibited significant variations in organic carbon fractions, aggregate stability, and available nutrients in soils. The available macro and micronutrients, except for nitrogen, showed a significant positive correlation with either total organic C (TOC) or carbon fractions indicating the synergy between them. The differences in soil C stocks clearly reflected the differences in litter fall and soil disturbance, as indicated by the highest C stocks in AHG. The dry weight of collected litter showed that AHG accumulated the highest litter content (97.38 g/m²) compared to the lowest (37.63 g/m²) in A–I/F. Organic matter addition to soil also increased the C stocks, even in annual crops. Aggregate stability showed a positive correlation with C fractions. The regression models developed in this study can be used to predict available nutrients by measuring TOC or C fractions in similar land use types in the tropics. This study confirmed that tropical agricultural systems that include annual crops have potential for storing and maintaining C in soils, if appropriately managed. The beneficial influence of SOC on available nutrients and aggregate stability could be a driving force to increase carbon stock in tropical agricultural systems.

The reliable prediction of coupled heat and moisture transfer in three-phase soils involves authentic estimation of soil thermal conductivity. In the current work, a steady-state method was developed to measure soil thermal conductivity. Soil samples for two soils were packed in a plastic of 6.6 cm inner diameter and 3 mm thickness. A 130 Ohm resistor as a heat generator being connected to 5 V electricity source was located in the center of the ball. Three thermocouples were installed at the distances of 0.5, 1.5, and 2.5 cm from the center, respectively. After unpacking the ball, thermal conductivity was measured using the single probe method. The thermal conductivity values measured by both methods were almost the same in air dry soils; overall, the observed differences between the two methods were less than 3%. However, in the intermediate moisture content ranges, the values measured by the steady-state method were slightly lower than those obtained by the single probe method. Considering the effect of convective vapor flux due to thermal expansion of soil air in the single probe method, values measured with the new steady-state method may be more reliable.

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.

The objectives of this study were to (i) establish the background variation and determine a threshold values for aqua regia soluble cadmium (Cd) in Terra Rossa soil at the local scale (Dalmatia, Croatia), (ii) check for possible soil Cd enrichment comparing its concentration at two different depths, and (iii) compare the threshold values for Cd calculated applying different statistical techniques with soil guideline values (SGV) as defined in Croatia and other national systems in Europe. The four statistical methods (the mean ± 2 standard deviations [mean ± 2SD], the median ± 2 median absolute deviations [median ± 2MAD], the (upper) Tukey inner fence (TIF), and the percentile-based approach) were used to establish the threshold values for Cd and identify samples with high Cd concentrations deviating from the background variation in a studied dataset. Overall, 74 samples from the A horizon of Terra Rossa soil were analyzed for aqua regia soluble Cd concentration (Cd_A); major soil properties—pH, CaCO₃, soil organic carbon (SOC) content, P₂O₅, K₂O, and particle-size distribution—were also determined in these samples. The underlying cambic B horizon was analyzed only for Cd concentration (Cd_B) The median Cd_A and Cd_B values of 1.84 and 1.70 mg kg^–1, respectively as well as the maximum value of 8.53 mg kg^–1 in the A horizon and 7.56 mg kg^–1 in the B horizon obtained in this study attested to anomalously high Cd concentrations. The median Cd_A/Cd_B ratio was close to unity (1.07) indicating very low Cd enrichment in the A horizon. The [median ± 2MAD] method achieved the lowest threshold Cd value of 4.68 mg kg^–1 and, consequently, a maximum number of outliers; it was followed by the classical [mean ± 2SD] method (5.01 mg kg^–1), the Q95th percentile (5.29 mg kg^–1), the Q98th percentile (5.64 mg kg^–1), and the TIF method with the highest threshold value of 6.93 mg kg^–1. All specified threshold values for Cd concentration repeatedly exceeded the maximum admissible concentrations (MACv) for agricultural land as defined in soil guideline values developed in Croatia and in several countries of the European Union. These findings indicate that more attention should be paid to the unusually high Cd concentrations in Terra Rossa soils originating from the natural sources; a more detailed geochemical survey of these soils should be performed in the future.