Microtopography influences morphometric characteristics of morphological patterns of Podzols: results of digital image analysis

This study addresses the fundamental need for a quantitative assessment of soil profile heterogeneity and the interpretation of genetic information encoded in macromorphological patterns. Digital image analysis offers a powerful new methodology for this task, particularly for soils with complex internal organisation, such as Podzols. The presented research develops and tests a digital morphometric approach for analysing soil sections. The work was conducted in the middle taiga of Western Siberia on the left-bank terrace of the Bolshoy Yugan River (60°24'29.4840" N, 73°56'56.6808" E). The study objects were Glossic Stagnic/Folic Albic Podzols (WRB) formed on light alluvial deposits within two conjugate microtopographic elements: a ridge and an inter-ridge depression. The methodology utilised high-quality photographic records from large-scale archaeological excavations. In total, 76 horizontal and vertical soil sections with a combined area of 1653 m2 were analysed. The core methodology comprised processing section photographs in Adobe Photoshop CC 2018 for the preliminary delineation of horizons and morphons, followed by quantitative analysis in ImageJ 1.45 software. For each identified morphon of the genetic horizons (O, E, Eh, BF1, BF2, BC, Box, [AY]), key morphometric parameters were calculated: Area (%), Perimeter (cm), Aspect Ratio (AR), Circularity, Solidity, and Roundness. Statistical processing was performed using MS Excel 2019 and STA-TISTICA 12. The key results are as follows. Statistically significant differences in the composition and spatial distribution of morphons between the ridge and the depression were established. In the depression soils, hydromorphic iron-enriched (Box) morphons and relic buried humus ([AY]) morphons were identified, whereas lamellae (pseudofibers) were characteristic of the ridge soils (See Figs. 2 and 3). Radiocarbon dating of charcoal from the [AY] morphons yielded an Early Holocene age of 7801-8546 cal yr BP (See Table 1). Quantitative analysis revealed that the maximum number of eluvial (E) horizon morphons in the ridge soils is twice as high as in the depression at a depth of 25 cm, while their size (perimeter) is larger in the depression (See Fig. 7). The maximum area of the illuvial (BF2) horizon in the depression is located 10 cm higher in the profile compared to the ridge. This shift is associated with the occurrence of Box morphons in the depression, which peak in area at 55-65 cm depth (See Fig. 7). A previously undescribed contact eluvial-illuvial-humus (Eh) morphon, consistently located between the E and BF1 horizons, was identified and characterised (See Fig. 3a and Fig. 7e, f). Principal component analysis of the morphometric data confirmed the paragenesis of most morphons; only the Box morphons formed a distinct cluster due to their different genesis (See Fig. 11 in Supplement 2). In conclusion, the formation of the complex macromorphological pattern in the studied Podzols is conclusively shown to result from three main agents acting in concert: (1) the penetration and die-back of tree root systems, forming deep rounded tongues; (2) windthrow disturbances, creating large elongated or rounded structures; and (3) subsequent eluvial-illuvial differentiation of substances along these biogenic pathways. The developed digital morphometry approach proves highly promising for the objective quantification of intra-profile soil heterogeneity, the reconstruction of pedogenesis, and the refinement of soil organic carbon stock estimates. The article contains 7 Figures, 2 Tables, 66 References, and 2 Supplements. The Authors declare no conflict of interest.

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