Genesis and soil environmental implications of intact in-situ rhizoliths in dunes of the Badain Jaran Desert, northwestern China

Abstract

Desert rhizoliths are generally found as weathered, broken and scattered samples on dune field surface, but rarely in-situ in their initial states buried under the soil of desert in the Badain Jaran Desert, northwest China. This study offers an assessment of the morphological, mineralogical, and chemical properties of intact and in-situ rhizoliths found in soils of swales and depressions among dune chains. The characteristics of these rare and precious objects were assessed using optical polarizing microscopy, cathodoluminescence, scanning electronic microscopy, radiocarbon dating, and stable isotopic analyses, providing the opportunity for discussion of the rhizolith formation mechanisms and associated environmental conditions. Field and laboratory investigations showed that the in-situ intact rhizoliths were formed only in the places where Artemisia shrubs are living, and the remaining root relicts within rhizoliths belong to this species. The spatial distribution of rhizoliths also suggested that low topographic positions on a landscape provided soil moisture, and redox environments favored rhizolith formation. A semi-closed redox environment in the subsoil at swales and depressions, where water is always present, along with the sandy soil texture, facilitated fast water percolation to deeper depths and condensation. Such a soil environment not only provides water for Artemisia growth, but also for the weathering of minerals such as felspars and calcite from primary carbonates, and for the decomposition of root relicts. Furthermore, harsh climatic conditions, such as strong winds and solar radiation, led to water evaporation through dead root channels and triggered the calcification along the root relicts. The entrapped lithogenic carbonates and to a lesser extent the decomposition of Artemisia roots provided the carbon sources for the rhizoliths formation, while the weathering of soil minerals, particularly feldspars and carbonates, was the main source of Ca. Rhizoliths in the Badain Jaran desert formed relatively quickly, probably over a few soil drying episodes. This led to the entrapment of a large quantity of lithogenic carbonates (more than 90% of carbon) within rhizolith cement. The re-dissolution of the entrapped lithogenic carbonates in rhizolith tubes should be taken into account in the paleoenvironmental interpretation of 14C ages, the latter suggesting that rhizoliths formed during the Holocene (~ 2053 years cal BP, based on root organic relicts).

Field root ecology and contribution of dominant vegetations in rhizoliths formation in Badain Jaren Desert, China

“…(5) Root diagenesis can happen in any depth of the shallow topsoil (Sun et al, 2022), but this study suggests that their primary diagenesis prefers to happen below the dry sand layer of 10-30 cm depth upmost. This indicates that soil water has an important impact on rhizolith formation and may explain differences between PRs and WERs.…”

“…And so far, it is known that rhizoliths are solely related to Artemisia sp. (Sun et al, 2022(Sun et al, , 2021(Sun et al, , 2020(Sun et al, ,2019a. Therefore, we focus on contribution of three dominant vegetations in the Alashan Deserts including Artemisia, Psammochloa and Phragmites, to study their root ecology and architectures as potentially main drivers and initiators of rhizoliths formation.…”

Where did the water come from? Wetlands and shallow lakes in semi‐arid dunefields from South America during the Pleistocene–Holocene transition

“…Here, the formation of iron plaques, calcium oxalate crystals and cylindrical concretions, within the fluctuating or palustrine area, involves the presence of hydrophytes with welldeveloped root systems (Metcalfe, 1985;Sundby et al, 1998Sundby et al, , 2003Alonso-Zarza, 2003;Khan et al, 2016;Altamirano et al, 2018). Such Fe-rhizoconcretions have been recognized in environments such as Saharan dunes, and related to Pleistocene shallow water bodies (Felix-Henningsen, 2000), whereas Ca-rhizoliths associated with periods of relatively humid conditions in drylands are described from northwest China (Li et al, 2015a(Li et al, , 2015bSun et al, 2019bSun et al, , 2021Sun et al, , 2022.…”

“…In dryland environments, particularly with eolian sedimentation, calcareous and ferruginous rhizoliths, calcareous crusts, tufas, pedogenic carbonate, amorphous Mn-oxide precipitations, hypocoatings and Fe/Mn-oxide depletions are common indicators of past changes in soil saturation, drainage, pedogenesis and watertable depths, which in turn can be related to geomorphological, ecological and/or regional moisture conditions (Alonso-Zarza and Wright, 2010;Sun et al, 2021). Indeed, rhizoliths broadly defined as root traces of massive or concentric mineral precipitations, resulting from total or partial replacement of the root organic matter in dryland eolian sand deposits, were interpreted as notable indicators of changes in hydrogeomorphic conditions (Klappa, 1980;Sun et al, 2021Sun et al, , 2022. These precipitates are derived from plant-soil exchange mechanisms, with bio-induced calcification, and bio-oxidation processes promoted by bacteria, fungi and/or algae associated with live or decayed roots (Klappa, 1980;Brazier et al, 2020).…”