Vol 52, No 2 (2019)

This paper provides a review of the current state of soil geography and budding directions for the development of pedogeographic research. We mention some new ideas in the frames of structural approach rooting in the classical concept of soil cover pattern and based on new concepts, such as pedodiversity assessment, graph theory, and geostatistical analysis of soil spatial variation. We note the significance of digital soil mapping in the development of the theory and practice of pedogeography and stress that digital soil mapping is a method that cannot replace soil geography as a scientific discipline. There is a need for deeper integration of mathematical methods in traditional soil geography. We stress that pedogeographical models are required for predicting soil properties and regimes even in digital agriculture. We discuss the necessity for adequate reflection of polygenetic soils in the soil mantle, and recommend using both indirect paleogeographic information and current remote and proximate sensing data. We also note the difficulties in predicting the spatial distribution of anthropogenically transformed soils using state factor theory; we discuss the possibilities of broader use of historical and economical geography data. In conclusion, we suggest developing “new soil geography” not only through integration of mathematical methods but also through closer integration with allied sciences.

Experimental data on 1029 soil profiles examined in the area of the Chashnikovo Experimental and Training Soil-Ecological Center of Moscow State University in Moscow region were used to create an enumerative classification at the level of the groups of soil horizons according to the theoretical concept of a machine generator of soil classifications suggested by V.A. Rozhkov and the concept of logical possibilities suggested by G.A. Zavarzin. Combinatorics of the sets of features of 17 diagnostic horizons separated into eight groups made it possible to characterize 255 possible combinations of the groups of horizons. In turn, the space of logical possibilities consisted of 168 combinations. In the real space, the number of combinations decreased to 46, i.e., by 3.6 and 5.5 times compared to the logical and universal spaces, respectively. The relationship between the separated groups of combinations of soil horizons and the subtypes of soils identified in the Classification and Diagnostics of Soils of the Soviet Union (1977) was as follows: any given combination does not belong to two different subtypes, but a given subtype may include several combinations. This made it possible to link the suggested enumerative classification and the classification system of 1977 to ensure mutual transition between these classification fields.

The content and the composition of soil structural units of different hierarchical levels were studied for Haplic Chernozems (Loamic, Pachic) of Kursk region under contrasting variants of land use (steppe and bare fallow). A detailed scheme of the method used to analyze the content and composition of organic matter in granulodensimetric fractions from the soil aggregates of different sizes and different water stability was developed. The major attention was paid to the study of aggregates isolated by the wet sieving method from the air-dry aggregates of 2–1 mm in size, as the most representative agronomically valuable aggregate fraction, Regardless of the land use, the content of the 2–1 mm air-dry aggretates was approximately the same in the soils under the steppe and the bare fallow. However, their water stability differed considerably between the steppe and the bare fallow. The long-term (52 yr) fallowing of the soil led to considerable losses of organic matter both in the whole soil and in different water-stable aggregates. Minimum losses were revealed for virtually all components of organic matter localized within undestroyed water-stable macroaggregates of 2–1 mm.

Data on the aggregate-size distribution (dry sifting method) in humus horizons of chernozems were processed by principal components analysis (PCA) with the centered logratio transformation of the initial data set. The analysis of the positions of treatments in the space of principal components coupled with the analysis of eigenvector magnitudes made it possible to identify several size fractions of aggregates, whose contents in the soils reflect soil degradation or progradation processes. These groups fitted well to the partial lognormal curves of aggregate sizes. In addition, the distribution of water-stable aggregates in coarse aggregate fractions (>10, 10–7, 7–5, and 5–3 mm) was analyzed. The integral analysis of the obtained data made it possible to propose the following conceptual model of the recovery of the structure of tilled chernozems. In the course of tillage, soil particles <0.25 mm in size are formed due to comminution. These particles are unstable and may stick together to shape large (>10 mm) water-unstable aggregates (clods). With the removal of tillage loads, large aggregates interact with fresh organic matter, and the water stability of aggregates increases. At the same time, the largest aggregates (mainly >10 mm) tend to transform into aggregates of smaller sizes.

Water regime of arable chernozems was simulated for two plots at the Voronezh experimental station. The first plot is under continuous corn monoculture since 1966. The second plot is under permanent bare fallow since the same time. The free HYDRUS-1D software was used. Evaporation from bare soil surface and evapotranspiration from the plot under corn were estimated using the FAO56 method. Simulations were performed for the 2013 and 2014 growing seasons with contrasting precipitation. Precipitation sum in May–September 2013 was 427 mm, and it was only 195 mm in May–September 2014, whereas the long-term average precipitation sum from May 1 to September 30 is equal to 284 mm. The monthly precipitation in July 2014 was only 2 mm. During both growing seasons, the simulated water regimes at the two plots were different. In the 70–140 cm layer, the soil water content was no less than 0.25 cm³/cm³ under bare fallow and it decreased to 0.17 cm³/cm³ under corn. The driest layers were formed under corn at the depths from 70 to 120 cm. At the plot under bare fallow, the cumulative evaporation from the soil surface from May 1 to September 30 in 2014 was higher than the precipitation sum for the same period. At the plot under corn, the cumulative evaporation was less than the precipitation sum, and the lower soil water content at this plot is explained by root water uptake with the cumulative value of 23 cm for the period from May 1 to September 30.

The role of mycorrhizal symbiosis as a control of the biogeochemical cycle of nitrogen in soils and in the nitrogen nutrition of plants is considered. The contribution of ericoid mycorrhiza (ErM) and ectomycorrhiza (EcM) to nitrogen (N) supply of host plants is well known, whereas the role of arbuscular mycorrhiza (ArM) is insufficiently understood. Exoenzymes released into the soil from the ErM and EcM mycelium favor the hydrolysis of high-molecular-weight N-containing organic compounds of plant litter and soils to \({\text{NH}}_{4}^{ + }\) or amino acids that are then transported toward plant roots and are absorbed by them. ArM-producing fungi have a limited capacity to release hydrolytic enzymes capable to decompose high-molecular-weight organic compounds into the soil (or do not have it at all). Therefore, they are specialized on the absorption of inorganic forms of N and amino acids appearing in the soil in the course of decomposition of high-molecular-weight N-containing compounds by saprotrophic microorganisms. The activity of hydrolytic exoenzymes and the role of mycorrhiza in the nitrogen nutrition of plants become more significant under conditions of the low supply with mineral N compounds and decrease upon the rise in availability of mineral N compounds. At the same time, mycorrhizal fungi and host plants may compete for the limited resource. The isotopic composition of N in plants (δ¹⁵N) and the fractionation of ¹⁵N isotope between the mycorrhizal fungi and host plants are considered indicative of the participation of mycorrhiza in the nitrogen nutrition of plants.

Changes in characteristics of the upper soil horizons in dark coniferous forests in the course of their postfire restoration were assessed. The studies were carried out in the northern taiga forests exposed to fire 8 to 400 years ago in the Kola Peninsula and in the middle taiga forests burned 70 and more than 500 years ago in the northern Cis-Ural region (Pechoro-Ilychskii State Biospheric Reserve). Samples from 23 soil pits on 20 test plots were analyzed. The actual acidity, loss on ignition, contents of exchangeable K, Ca, and Mg and mobile Mn, Fe, and Zn were determined. Two boundaries (~80 and 160–200 years after fire) and three main periods in the postfire dynamics of the upper soil horizons properties were established. The ash and Fe content in the litter was found to decreases about 80 years after the fire. In 160–200 years after fire, the thickness of the forest litter is restored and stabilized at the level of about 9.7 (8.3–10.4) cm; the pH value and the Ca, K, and Mn concentrations in the litter become lower. Thus, it can be concluded that the properties of soils under dark coniferous forests of the European North considerably change in the course of postfire successions.