Abstract
Modern cryptogamic ground covers (CGCs), comprising assemblages of bryophytes (hornworts, liverworts, mosses), fungi, bacteria, lichens and algae, are thought to resemble early divergent terrestrial communities. However, limited in situ plant and other fossils in the rock record, and a lack of CGC-like soils reported in the pre-Silurian sedimentological record, have hindered understanding of the structure, composition and interactions within the earliest CGCs. A key question is how the earliest CGC-like organisms drove weathering on primordial terrestrial surfaces (regolith), leading to the early stages of soil development as proto-soils, and subsequently contributing to large-scale biogeochemical shifts in the Earth System. Here, we employed a novel qualitative, quantitative and multi-dimensional imaging approach through X-ray micro-computed tomography, scanning electron, and optical microscopy to investigate whether different combinations of modern CGC organisms from primordial-like settings in Iceland develop organism-specific soil forming features at the macro- and micro-scales. Additionally, we analysed CGCs growing on hard rocky substrates to investigate the initiation of weathering processes non-destructively in 3D. We show that thalloid CGC organisms (liverworts, hornworts) develop thin organic layers at the surface (<1cm) with limited subsurface structural development, whereas leafy mosses and communities of mixed organisms form profiles that are thicker (up to~7cm), structurally more complex, and more organic-rich. We term these thin layers and profiles proto-soils. Component analyses from X-ray micro-computed tomography data show that thickness and structure of these proto-soils are determined by the type of colonising organism(s), suggesting that the evolution of more complex soils through the Palaeozoic may have been driven by a shift in body plan of CGC-like organisms from flattened and appressed to upright and leafy. Our results provide a framework for identifying CGC-like proto-soils in the rock record and a new proxy for understanding organism-soil interactions in ancient terrestrial biospheres and their contribution to the early stages of soil formation.
Highlights
We investigated modern cryptogamic ground covers (CGCs) volcanic substrate soils and CGC-colonised hard substrates from Iceland (Figure 1) and applied a combination of novel imaging techniques (e.g. X-ray microcomputed tomography, optical microscopy (OM), scanning electron microscopy (SEM)) to determine the impact of combinations of CGC organisms on (a) the origination of soils from regolith and weathering residues from hard substrate weathering and (b) on CGC proto-soil structural development
Our overarching goal is to identify variations in processes and structure caused by different CGC organisms that can be used as a framework to recognise potential CGC-like proto-soils in the sedimentological/fossil record and to understand better the impact of various CGC organisms on early soil development as well as how this may have evolved through the Palaeozoic
By studying modern cryptogamic ground covers (CGCs) as analogues of the earliest terrestrial biospheres, it is possible to understand the influence that ancient organisms may have had on the initiation of soils and their structural development
Summary
Cryptogamic ground covers (CGCs) are a type of biological soil crust comprising a complex association of early divergent organisms including non-vascular plants (bryophytes; liverworts, hornworts, mosses), fungi (free-living, saprotrophic and mycorrhizal), bacteria (free-living and symbiotic), algae, and lichens (Edwards et al, 2015; Mitchell et al, 2016; Porada et al, 2014). While improved resolution of these relationships will allow for better understanding of how plants arose on land, it remains that bryophytes represent highly suitable modern analogues to study soil forming processes associated with early plant-based biotas, in particular thalloid liverworts and hornworts, which are known to form fungal associations with members of the early divergent mycorrhizal fungal clades Glomeromycotina and Mucoromycotina (Desiro et al, 2013; Field et al, 2016; Field, Rimington, et al, 2015; Rimington et al, 2020; Rimington et al, 2018, 2019) It is widely assumed, based on fossil, molecular, and physiological evidence that the evolution of mutually beneficial symbioses between plants and fungi was a key factor in terrestrialisation (Field, Rimington, et al, 2015; Rimington et al, 2018; Selosse & Strullu-Derrien, 2015), which augmented mineral weathering (Mitchell et al, 2016) in early proto-soils, reportedly leading to changes in Earth's atmosphere through consumption of CO2 (Porada et al, 2014), and perhaps triggering the Ordovician glaciations (Lenton et al, 2012). Our overarching goal is to identify variations in processes and structure caused by different CGC organisms (thalloid, moss, mixed, lichens) that can be used as a framework to recognise potential CGC-like proto-soils in the sedimentological/fossil record and to understand better the impact of various CGC organisms on early soil development as well as how this may have evolved through the Palaeozoic
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