Despite their low carbon (C) content, most subsoil horizons contribute to more than half of the total soil C stocks, and therefore need to be considered in the global C cycle. Until recently, the properties and dynamics of C in deep soils was largely ignored. The aim of this review is to synthesize literature concerning the sources, composition, mechanisms of stabilisation and destabilization of soil organic matter (SOM) stored in subsoil horizons. Organic C input into subsoils occurs in dissolved form (DOC) following preferential flow pathways, as aboveground or root litter and exudates along root channels and/or through bioturbation. The relative importance of these inputs for subsoil C distribution and dynamics still needs to be evaluated. Generally, C in deep soil horizons is characterized by high mean residence times of up to several thousand years. With few exceptions, the carbon-to-nitrogen (C/N) ratio is decreasing with soil depth, while the stable C and N isotope ratios of SOM are increasing, indicating that organic matter (OM) in deep soil horizons is highly processed. Several studies suggest that SOM in subsoils is enriched in microbial-derived C compounds and depleted in energy-rich plant material compared to topsoil SOM. However, the chemical composition of SOM in subsoils is soil-type specific and greatly influenced by pedological processes. Interaction with the mineral phase, in particular amorphous iron (Fe) and aluminum (Al) oxides was reported to be the main stabilization mechanism in acid and near neutral soils. In addition, occlusion within soil aggregates has been identified to account for a great proportion of SOM preserved in subsoils. Laboratory studies have shown that the decomposition of subsoil C with high residence times could be stimulated by addition of labile C. Other mechanisms leading to destabilisation of SOM in subsoils include disruption of the physical structure and nutrient supply to soil microorganisms. One of the most important factors leading to protection of SOM in subsoils may be the spatial separation of SOM, microorganisms and extracellular enzyme activity possibly related to the heterogeneity of C input. As a result of the different processes, stabilized SOM in subsoils is horizontally stratified. In order to better understand deep SOM dynamics and to include them into soil C models, quantitative information about C fluxes resulting from C input, stabilization and destabilization processes at the field scale are necessary.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

Peat landscapes will be in focus at the upcoming ‘Global Landscapes Forum: Peatlands Matter’ event, taking place in Jakarta on 18 May 2017. Find out more here.Wetlands are important providers of ecosystem services and key regulators of climate change. They positively contribute to global warming through their greenhouse gas emissions, and negatively through the accumulation of organic material in histosols, particularly in peatlands. Our understanding of wetlands' services is currently constrained by limited knowledge on their distribution, extent, volume, inter-annual flood variability, and disturbance levels. We present an expert system approach to estimate wetland and peatland areas, depths and volumes, which relies on three biophysical indices related to wetland and peat formation: 1. Long-term water supply exceeding atmospheric water demand; 2. Annually or seasonally water-logged soils; 3. A geomorphological position where water is supplied and retained. Tropical and subtropical wetlands estimates reach 4.7 million km2. In line with current understanding, the American continent is the major contributor (45%) and Brazil, with its Amazonian inter-fluvial region, contains the largest tropical wetland area (800,720 km2). Our model suggests, however, unprecedented extents and volumes of peatland in the tropics (1.7 million km2 and 7,268 (6,076-7,368) km3), which more than three-fold current estimates. Unlike current understanding, our estimates suggest that South America and not Asia contributes the most to tropical peatland area and volume (ca. 44% for both) partly related to some yet unaccounted extended deep deposits but mainly to extended but shallow peat in the Amazon Basin. Brazil leads the peatland area and volume contribution. Asia hosts 38% of both tropical peat area and volume with Indonesia as the main regional contributor and still the holder of the deepest and most extended peat areas in the tropics. Africa hosts more peat than previously reported but climatic and topographic contexts leave it as the least peat forming continent. Our results suggest large biases in our current understanding of the distribution, area, and volumes of tropical peat and their continental contributions.

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An expert system model for mapping tropical wetlands and peatlands reveals South America as the largest contributor

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=4164&context=smhpapers

Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes

Andic soils contain a large amount of stabilised soil organic matter (SOM). The present study aims to review and integrate the determining factors and mechanisms of SOM stabilisation in andic compared with other (non-andic) soil types. We have reviewed recent literature regarding the nature of SOM and its stabilisation processes in the top- and sub-soil to address and discuss the interaction between the SOM and the mineral phase. The carbon (C) storage capacity by the metal-humus-complex formation of volcanic soils is also evaluated. The most important stabilisation processes are related to the incorporation and decomposition of microbial-derived C along with the changing C storage capacity with increasing soil development. The priming and destabilisation of adsorbed SOM are crucial mechanisms influencing the soil C sequestration in subsoils. The C storage capacity of andic soils was closely related to the Na-pyrophosphate extractable Al and Fe. The upper boundary for SOM saturation with Al and Fe was a molar metal:C ratio of 0.18. The influence of climate, mineralogy and soil disturbances on the SOM storage capacity of andic soils also require further attention.

Soil carbon storage and stabilisation in andic soils: A review

Biomineralization of iron-phosphates in the water column of Lake Pavin (Massif Central, France)

Tracing source and evolution of suspended particles in the Rio Negro Basin (Brazil) using chemical species of iron

Al speciation in tropical podzols of the upper Amazon Basin: A solid-state 27Al MAS and MQMAS NMR study

In the Rio Negro basin, podzols develop at the expense of clay-depleted laterites through localized and spectacular weathering fronts. This natural process leads to the remobilization of previously accumulated organic matter (OM) which is redistributed within soil profiles and exported towards rivers, hence their typical black coloration. We investigate the fate of OM in the soil–water continuum in the Rio Negro basin through description of the composition of OM potentially mobile at different stages of podzol development and exploration of evidence for contributions from different soil horizons to the exportation of OM into waters. OM was water-extracted from seven key soil samples from a sequence representative of the transition between latosol and podzol on the low elevation plateaux of the Rio Negro basin, thus enabling following both vertical and lateral differentiations. The chemical structure of freeze-dried samples, investigated using 13C nuclear magnetic resonance and thermally assisted hydrolysis and methylation (THM), shows contrasting features depending on the horizon considered. The bulk features of water extracts were first compared with samples collected in the water-tables and rivers draining the soil sequence. A molecular level comparison was then performed with groundwater draining the well-developed podzol. This approach evidenced a contribution from deep horizons of well-developed podzols. It highlights that OM is certainly remobilised after being accumulated in Bh horizons during the development of podzols through accumulation of OM redistributed from surface horizons. The identification of specific compounds opens new perspective to trace a “podzolic origin” of OM in drainage networks.

Podzolisation and exportation of organic matter in black waters of the Rio Negro (upper Amazon basin, Brazil)

https://www.jstor.org/stable/info/41490507

Marion Bardy

Papers

Tracing source and evolution of suspended particles in the Rio Negro Basin (Brazil) using chemical species of iron

Al speciation in tropical podzols of the upper Amazon Basin: A solid-state 27Al MAS and MQMAS NMR study

Podzolisation and exportation of organic matter in black waters of the Rio Negro (upper Amazon basin, Brazil)

Podzolisation and exportation of organic matter in black waters of the Rio Negro (upper Amazon basin, Brazil)