Phytoliths: Persistence & Release of Silicon in Soil and Plants – A Review

Phytoliths are formed from silica carried up from groundwater and some plants. The weathering of silicate minerals at the Earth’s surface provides large amounts of soluble silica, some of which is absorbed by growing plants. In solution, silica exists as mono silicic acid Si (OH4) with pH values of 2–9. It is carried upward in the vascular system and becomes concentrated during transpiration around the leaf stomata. The supersaturated solution begins to polymerize or gel then solidifies and forms solid opaline silica (SiO2:nH2O) bodies (phytoliths) within and between some of the plant cells. Phytoliths were extracted from the 7.4 meter loess core and analyzed morphologically and isotopically from the occluded carbon. Rates of isotopic fractionation between plant and phytolith were determined by measurements from many modern tree, fern, and grass species. The use of phytolith biochar as a Si fertilizer offers the undeniable potential to mitigate desilication and to enhance Si ecological services due to soil weathering and biomass removal. Silicon is accumulated at levels equal to or greater than essential nutrients in plant species belonging to the families Poaceae, Equisetaceae, and Cyperaceae. However, the abundance of silicon in soils is not an indication that sufficient supplies of soluble silicon are available for plant uptake.

Soil Related Factors Controlling Erosion And Landslides In Malaysia

The high temperature and rainfall in Malaysia results in intensive tropical weathering and depending on the parent material, the resultant soil can have deep or shallow soil profiles. Thus, a variety of soils can form with different textures (clay content), structure and porosity. With the high rainfall, soils in steep terrain are subjected to surface erosion when exposed or landslides if the rainwater percolates into the soil profile. Key soil-related factors controlling soil erosion and landslides include local climate, parent material of soils and depth of the weathered profile. Slope and geomorphology, vegetation and land use as well as land management practices also influence erosion and landslides. Measures used to control soil erosion and stabilize slopes require improved understanding of soil weathering, erosion, landslide and their linkages in steep terrain.

Hierarchical landform delineation for the habitats of biological communities on the Korean Peninsula

Landforms determine the locations of particular biological communities based on their components and spatial positions. This study hierarchically classified the topographic spaces serving as habitats for biological communities in the Korean Peninsula and established the habitat types that occur on the classified landform types. We classified landform types by applying cell-based modeling, map algebra, and spatial query techniques to spatial data, including digital elevation model (DEM), Sentinel 2 image, land use, and field survey data to model their ecological characteristics. Landforms were classified into four categories (designated Category 1 through 4) according to their spatial scale based on topographical characteristics such as mountains, plains, alluvial landforms, coastal landforms, islands, and special areas (Baekdudaegan, DMZ), which are found throughout the Korean Peninsula. The landforms of the Korean Peninsula were classified into 47 subcategories in Category 1, 16 in Category 2, 36 in Category 3, and 63 in Category 4. There were 62 main types of habitats that were classified based on their topographic spatial units, and there were 437 types of sub-habitats, for which soil weathering, biodiversity, and geodiversity were combined with the main habitat types. When factor analysis was conducted for the environmental factors used to determine the main and sub-habitats, the first primary components were temperature-related factors, followed by biodiversity, geodiversity, aspect, and slope. When the indicator species were analyzed by habitat type, indicator species diversity was high in Jeju Province, Gangwon Province, and Gaema Plateau. Based on these results, landform elements for species habit conservation were assigned conservation values and classified into (I) absolute conservation areas, (II) transition areas, and (III) areas for coexistence with humans. Topographic spaces are being degraded as biological habitats as a result of climate change and human development; our proposed classifications can be applied to the conservation of landforms and biodiversity.

The role of geochemistry in organic carbon stabilization against microbial decomposition in tropical rainforest soils

Stabilization of soil organic carbon (SOC) against microbial decomposition depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical African mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley positions. To do so, we conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil and a sequential extraction of pedogenic oxides. Relationships between our target and predicting variables were investigated using a combination of regression analyses and dimension reduction. Here, we show that reactive secondary mineral phases drive SOC properties and stabilization mechanisms together with, and sometimes more strongly than, other mechanisms such as aggregation or C stabilization by clay content. Key mineral stabilization mechanisms for SOC were strongly related to soil geochemistry, differing across the study regions. These findings were independent of topography in the absence of detectable erosion processes. Instead, fluvial dynamics and changes in soil moisture conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. At several sites, we also detected fossil organic carbon (FOC), which is characterized by high C/N ratios and depletion of N. FOC constitutes up to 52.0 ± 13.2 % of total SOC stock in the C-depleted subsoil. Interestingly, total SOC stocks for these soils did not exceed those of sites without FOC. Additionally, FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under more fertile conditions. Regression models, considering depth intervals of 0–10, 30–40 and 60–70 cm, showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and Δ14C. Furthermore, the same variables explain 44 % of the variability in the relative abundance of C associated with microaggregates vs. free-silt- and-clay-associated C fractions. However, geochemical variables gained or retained importance for explaining SOC target variables when controlling for soil depth. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and mineral-related C stabilization mechanisms. While identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the tropical soils investigated.

Correlative Microscopy: a tool for understanding soil weathering in modern analogues of early terrestrial biospheres

Correlative imaging provides a method of investigating complex systems by combining analytical (chemistry) and imaging (tomography) information across dimensions (2D-3D) and scales (centimetres-nanometres). We studied weathering processes in a modern cryptogamic ground cover from Iceland, containing early colonizing, and evolutionary ancient, communities of mosses, lichens, fungi, and bacteria. Targeted multi-scale X-ray Microscopy of a grain in-situ within a soil core revealed networks of surficial and internal features (tunnels) originating from organic-rich surface holes. Further targeted 2D grain characterisation by optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (SEM–EDS), following an intermediate manual correlative preparation step, revealed Fe-rich nodules within the tunnels. Finally, nanotomographic imaging by focussed ion beam microscopy (FIB-SEM) revealed coccoid and filamentous-like structures within subsurface tunnels, as well as accumulations of Fe and S in grain surface crusts, which may represent a biological rock varnish/glaze. We attribute these features to biological processes. This work highlights the advantages and novelty of the correlative imaging approach, across scales, dimensions, and modes, to investigate biological weathering processes. Further, we demonstrate correlative microscopy as a means of identifying fingerprints of biological communities, which could be used in the geologic rock record and on extra-terrestrial bodies.

Geochemical features of the distribution of major and trace elements in sediments and soils of the Zarafshon River Valley

Purpose The aim of this research was to estimate impact of the geochemical features to distribution of major and trace elements in soil and sediments, assess of anthropogenic and natural factors in terrestrial ecosystems of Zarafshon. As well as to answer the common criticism of the water, soil and sediment pollution in Zarafshon. In this way this research will be reasonable substantial information about distribution of major and trace elements in mountains and piedmont regions. Materials and methods In view of a planned detailed investigation, and to get more data concerning the geochemistry of sediments and soils along the Zarafshon Valley of Western Tajikistan, as well as to assess of the local environmental situation, the mass fraction of eight major, rock-forming elements together with other 38 trace elements were determined by Instrumental Neutron Activation Analysis. To accomplish this task 116 samples of sediment and an equal number of soils were collected around the Tajik sector of the Zarafshon catchment basin and its main tributaries. Results and discussion Both major and trace elements proved a significant similarity between soil and sediments including potentially contaminating elements As, Sb and Hg, whose mass fractions showed in some places to be significantly higher. An analysis of the distribution of major elements oxides as well as of incompatible trace elements Sc, Zr, REE, Th, and U suggested that the analysed soil and sediments have rather a felsic origin. A more careful examination revealed a reduced degree of recycling and for some location, a certain degree of weathering. All these peculiarities could be ascribed to a vast investigated area spread onto a significant diversity of geological formations. Conclusion The process of soil weathering of the studied area takes place relatively quick, but mainly it's has a natural character rather. The high content of accumulation of major and trace element in the sediment also related to rock source but only in rare cases is it of an anthropogenic nature. Based on these results, it can be predicted that the seriously environmental threat was not observed in the Zarafshon river valley. The activities of industrial facilities cover only a small part of this region and their impacts on the ecological state of the Zarafshon river valley is very small.