Peaty soils form in waterlogged conditions with high organic accumulation, whereas laterite soils form in tropical regions via intense leaching
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Retrieved peer-reviewed literature confirms that peat soils form in waterlogged, anoxic conditions promoting organic matter accumulation, while laterite soils are weathered profiles typically formed under tropical climates via intense leaching.
Soils are crucial in regulating ecosystem processes, such as nutrient cycling, and supporting plant growth. To a large extent, these functions are carried out by highly diverse and dynamic soil microbiomes that are in turn governed by numerous environmental factors including weathering profile and vegetation. In this study, we investigate geophysical and vegetation effects on the microbial communities of iron-rich lateritic soils in the highly weathered landscapes of Western Australia (WA). The study site was a lateritic hillslope in southwestern Australia, where gradual erosion of the duricrust has resulted in the exposure of the different weathering zones. High-throughput amplicon sequencing of the 16S rRNA gene was used to investigate soil bacterial community diversity, composition and functioning. We predicted that shifts in the microbial community would reflect variations in certain edaphic properties associated with the different layers of the lateritic profile and vegetation cover. Our results supported this hypothesis, with electrical conductivity, pH and clay content having the strongest correlation with beta diversity, and many of the differentially abundant taxa belonging to the phyla Actinobacteria and Proteobacteria. Soil water repellence, which is associated with <i>Eucalyptus</i> vegetation, also affected beta diversity. This enhanced understanding of the natural system could help to improve future crop management in WA since the physicochemical properties of the agricultural soils in this region are inherited from laterites via the weathering and pedogenesis processes.
Our results supported this hypothesis, with electrical conductivity, pH and clay content having the strongest correlation with beta diversity, and many of the differentially abundant taxa belonging to the phyla Actinobacteria and Proteobacteria. Soil water repellence, which is associated with Eucalyptus vegetation, also affected beta diversity. This enhanced understanding of the natural system could help to improve future crop management in WA since the physicochemical properties of the agricultural soils in this region are inherited from laterites via the weathering and pedogenesis processes.
Keywords: soil microbial community, bacteria, laterite, critical zone, Western Australia status released display-pdf yes is-olf no is-manuscript no is-preprint no is-journal-matter no is-scanned no is-retracted no Received 2019 Mar 8; Accepted 2019 Jun 14; Collection date 2019. Introduction Soils provide a variety of essential ecosystem services which support life above- and below-ground ( Smith et al., 2015 ; Adhikari and Hartemink, 2016 ). They form part of the Earth’s critical zone (CZ), which spans from the canopy to the bedrock, incorporating a complex network of biogeochemical processes and cycles that sustain terrestrial life ( Brantley et al., 2007 ).
Laterites are ancient weathered profiles that are typically formed under tropical climates ( Volkoff, 1998 ) and comprise five horizons underneath the topsoil: ferricrete, mottled zone, pallid zone, saprolite, and (parent) bedrock ( Figure 1A ). The ferricrete can be several meters thick, either occurring as a ferruginous crust (duricrust) with a pisolitic structure ( Anand and Gilkes, 1987 ; Beauvais and Colin, 1993 ), or in a nodular form (ironstone gravel) ( Tille et al., 2001 ).
Nitrogen (N), phosphorus, and potassium are severely depleted, while some nutrients are inaccessible as they bind with lateritic compounds ( Orians and Milewski, 2007 ). This extensive leaching also causes lateritic soils to be acidic, with Eucalyptus litter leachates also stimulating soil acidification and iron mobilization ( Ellis, 1971 ; Bernhard-Reversat, 1999 ; Anand and Paine, 2002 ). Low soil organic matter (SOM) and clay contents in near-surface horizons result in a low cation exchange capacity (CEC), or even confer an anion exchange capacity that promotes further leaching ( Wong and Wittwer, 2009 ).
On T140, this may be attributed to the low clay content which is characteristic of water repellent soils ( Harper et al., 2000 ), with clay additions serving as an effective method to ameliorate SWR ( Hall et al., 2010 ; Shanmugam et al., 2014 ). Another possible contributing factor is the occurrence of forest fires – which are frequent in WA – as these can result in increased soil hydrophobicity ( DeBano, 2000 ). Furthermore, a positive feedback loop can establish as hydrophobic soils commonly form preferential flow paths and soil aggregates which may intensify drought events and inhibit microbial degradation of organic matter ( Goebel et al., 2011 ).
Beijerinckia are well-adapted to lateritic soils, as they are able to grow in environments which lack calcium and have high levels of iron, which are common features of the upper layers of laterites ( Becking, 1961a , b ; Anand and Paine, 2002 ). Also, ferruginous gravels are known to have P-fixing attributes ( Tiessen et al., 1991 ), which may mean that the plateau section has greater P content, thereby alleviating P-limitations on microbial N-fixation ( Crews, 1993 ; Reed et al., 2007 ). The higher number of N-fixing bacteria detected in samples from the bottom section may be related to the potential influence of fertilizers applied to the paddock.
This enhanced understanding of the natural system could help to improve future management of agricultural systems in WA since they use soil with inherited physicochemical properties and bacterial communities. For instance, relative to the rest of T140, the plateau section was enriched in several families known to contain N-fixing bacteria. If the microbial communities of these laterite-derived agricultural soils continue to converge with those of the plateau over time via the weathering process, it could have important implications for agricultural management as it may reduce the need for inorganic N fertilizer applications.
Tropical peatlands are critical for climate mitigation due to their dual role as major carbon sinks and methane sources. In rainforests, high and stable rainfall supports peat accumulation in tropical climates. However, groundwater-fed peatlands in seasonally dry tropical ecosystems remain poorly understood, despite their potential importance in global carbon dynamics. Here, we present an integrated carbon assessment in organic soil ecosystems (locally known as Veredas and Campos úmidos) in the Brazilian savanna. We quantified carbon in soil and biomass, dated carbon using radiocarbon, and evaluated chemical stability using infrared spectrometry. We used machine learning models to map their potential area. Additionally, we measured soil CO<sub>2</sub> and CH<sub>4</sub> efluxes to evaluate the influence of climatic seasonality on emissions. Veredas contained exceptionally high carbon stocks (c. 1200 Mg C ha<sup>-1</sup>) accumulated over c. 20 000 yr and spanning c. 16.7 Mha. However, spectroscopy indicated low carbon stability compared to other tropical peatlands, and c. 70% of annual CO<sub>2</sub> and CH<sub>4</sub> emissions occurred during the dry season. Our findings show that the Brazilian Cerrado harbors one of the largest carbon-storing ecosystems in the tropical Americas, yet one that is highly vulnerable to land-use change and intensified drought. Despite their wide distribution, peat accumulation and the extent of Veredas remain uncertain.
In rainforests, high and stable rainfall supports peat accumulation in tropical climates. However, groundwater‐fed peatlands in seasonally dry tropical ecosystems remain poorly understood, despite their potential importance in global carbon dynamics. Here, we present an integrated carbon assessment in organic soil ecosystems (locally known as Veredas and Campos úmidos) in the Brazilian savanna. We quantified carbon in soil and biomass, dated carbon using radiocarbon, and evaluated chemical stability using infrared spectrometry. We used machine learning models to map their potential area.
Given their importance and vulnerability, the identification, understanding, and protection of carbon‐rich soils, particularly peatlands, are priorities in climate change mitigation actions. Peatlands have a global distribution but are most concentrated in high latitudes of the Northern Hemisphere, where cold climates favor long‐term carbon accumulation in soils (Global Peatlands Initiative, 2022 ). In the wet tropics, high and constant rainfall enables reduced decomposition rates even in the absence of thermal limitations, resulting in extensive peatland areas within rainforest regions (Global Peatlands Initiative, 2022 ). Tropical peatlands began forming c .
(c) Location and distribution of the Cerrado in South America and studied Veredas of the current (red circles) and previous studies (colored diamonds). (d) Detail of the sampled sites of the current study. Photo credits (a) A. Dib; (b) L. Verona. Vereda soils can be classified as peat or organic soils (Boaventura, 1978 ). Although there is no consensus regarding the threshold of soil organic carbon that defines peat soils or peatlands (Lourenco et al ., 2022 ), these ecosystems are generally characterized by waterlogged conditions that promote the accumulation of carbon as undecomposed organic matter.
To test if ‘carbon storage’ measured using samples with SOC > 8% (the threshold for organic soil in Brazilian soil classification; Santos et al ., 2018 ) differed from ‘carbon storage’ including all samples, we ran a t ‐test. To assess if ‘carbon stability’ differed between the two flooding patterns (seasonally vs permanently flooded), we ran linear mixed‐effect models (LMMs) with ‘flooding pattern’ as a fixed effect, and ‘point’ nested in ‘site’ as a random effect to account for variability among sampling points nested within sites. To compare our results with those from other tropical regions reported by Hodgkins et al .
This divergence was further amplified by temperature responses: while CH 4 emissions increased with temperature in permanently flooded areas, a negative relationship was observed in seasonally flooded Veredas (Fig. 8d ; Table S8 ). Discussion Our findings highlight the significant carbon storage potential of open wetlands in Cerrado, driven primarily by the accumulation of peat and organic soils. These carbon stocks have accumulated over the last 20 000 yr, indicating that despite pronounced seasonal fluctuation in hydrological conditions, many Veredas have maintained sufficiently stable waterlogged (anoxic) conditions to support long‐term peat formation and persistence.
30%), these criteria are not universally agreed upon (Lourenco et al ., 2022 ). The Global Peatlands Assessment ( 2022 ) applies a 12% threshold, while the Brazilian Soil Classification uses 8% for organic soils. Veredas are among the most seasonal tropical peatlands, with very few ecosystems under similarly seasonal precipitation regimes capable of accumulating peat. Some sites with comparable climatic seasonality, such as the Barotse Floodplain in Africa, maintain peat formation under fluvial flooding or lacustrine water regimes (Global Peatlands Initiative, 2022 ), whereas Veredas are distinguished by being groundwater‐fed.
( 2022 ) found more stable and fewer unstable compounds in tropical than in non‐tropical peatlands. However, when comparing Vereda soil with other tropical regions, we found c . 60% fewer stable compounds (lignin) than the tropical average (Hodgkins et al ., 2018 ). This is likely due to the different vegetation types forming the organic matter. Amazonian, Indonesian, and Congo peatlands are wet forest environments (Page et al ., 2006 ; Draper et al ., 2014 ; Dargie et al ., 2019 ), while Veredas are an herbaceous ecosystem with a low density of palms, typical of savanna ecosystems (Ribeiro & Walter, 1998 ).
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