Abstract:Soil organic matter (OM) contains vast stores of carbon, and directly supports microbial, plant, and animal life by retaining essential nutrients and water in the soil. Soil OM plays important roles in biological, chemical, and physical processes within the soil, and arguably plays a major role in maintaining long-term ecological stability in a changing world. Despite its importance, there is a great deal still unknown about soil OM chemical ecology. The development of sophisticated analytical methods have res… Show more
“…Traditionally, the "quality" of SOM (and by inference its intrinsic potential to be further decomposed, transformed and mineralized) has been evaluated by assessing its molecular composition and by applying some type of physical, chemical, or biological fractionation approach to partition the bulk SOM pool into "labile" vs. "recalcitrant" or "stable" forms (Kleber and Johnson, 2010;von Lützow et al, 2007;Simpson and Simpson, 2012). Although the chemical composition of SOM can affect the rate of decomposition, it is now recognized that (1) readily degradable "labile" carbon forms can be stabilized in soil by a variety of mechanisms or conditions that physically or chemically limit microbial access or that impact microbial activity, and (2) even the most "recalcitrant" carbon forms can be mineralized given the "right" conditions (von Lützow et al, 2006;Marschner et al, 2008;Kleber, 2010).…”
Section: Characterization Of the Quality And Decomposability Of Organmentioning
Abstract. Knowledge of soils in the permafrost region has advanced immensely in recent decades, despite the remoteness and inaccessibility of most of the region and the sampling limitations posed by the severe environment. These efforts significantly increased estimates of the amount of organic carbon stored in permafrost-region soils and improved understanding of how pedogenic processes unique to permafrost environments built enormous organic carbon stocks during the Quaternary. This knowledge has also called attention to the importance of permafrost-affected soils to the global carbon cycle and the potential vulnerability of the region's soil organic carbon (SOC) stocks to changing climatic conditions. In this review, we briefly introduce the permafrost characteristics, ice structures, and cryopedogenic processes that shape the development of permafrost-affected soils, and discuss their effects on soil structures and on organic matter distributions within the soil profile. We then examine the quantity of organic carbon stored in permafrost-region soils, as well as the characteristics, intrinsic decomposability, and potential vulnerability of this organic carbon to permafrost thaw under a warming climate. Overall, frozen conditions and cryopedogenic processes, such as cryoturbation, have slowed decomposition and enhanced the sequestration of organic carbon in permafrost-affected soils over millennial timescales. Due to the low temperatures, the organic matter in permafrost soils is often less humified than in more temperate soils, making some portion of this stored organic carbon relatively vulnerable to mineralization upon thawing of permafrost.
“…Traditionally, the "quality" of SOM (and by inference its intrinsic potential to be further decomposed, transformed and mineralized) has been evaluated by assessing its molecular composition and by applying some type of physical, chemical, or biological fractionation approach to partition the bulk SOM pool into "labile" vs. "recalcitrant" or "stable" forms (Kleber and Johnson, 2010;von Lützow et al, 2007;Simpson and Simpson, 2012). Although the chemical composition of SOM can affect the rate of decomposition, it is now recognized that (1) readily degradable "labile" carbon forms can be stabilized in soil by a variety of mechanisms or conditions that physically or chemically limit microbial access or that impact microbial activity, and (2) even the most "recalcitrant" carbon forms can be mineralized given the "right" conditions (von Lützow et al, 2006;Marschner et al, 2008;Kleber, 2010).…”
Section: Characterization Of the Quality And Decomposability Of Organmentioning
Abstract. Knowledge of soils in the permafrost region has advanced immensely in recent decades, despite the remoteness and inaccessibility of most of the region and the sampling limitations posed by the severe environment. These efforts significantly increased estimates of the amount of organic carbon stored in permafrost-region soils and improved understanding of how pedogenic processes unique to permafrost environments built enormous organic carbon stocks during the Quaternary. This knowledge has also called attention to the importance of permafrost-affected soils to the global carbon cycle and the potential vulnerability of the region's soil organic carbon (SOC) stocks to changing climatic conditions. In this review, we briefly introduce the permafrost characteristics, ice structures, and cryopedogenic processes that shape the development of permafrost-affected soils, and discuss their effects on soil structures and on organic matter distributions within the soil profile. We then examine the quantity of organic carbon stored in permafrost-region soils, as well as the characteristics, intrinsic decomposability, and potential vulnerability of this organic carbon to permafrost thaw under a warming climate. Overall, frozen conditions and cryopedogenic processes, such as cryoturbation, have slowed decomposition and enhanced the sequestration of organic carbon in permafrost-affected soils over millennial timescales. Due to the low temperatures, the organic matter in permafrost soils is often less humified than in more temperate soils, making some portion of this stored organic carbon relatively vulnerable to mineralization upon thawing of permafrost.
“…It is recognised to possess a range of turnover times, from less than one year to thousands of years (Amundson 2001), and to comprise complex chemical entities (Stevenson 1986;Kögel-Knabner 2002;Simpson and Simpson 2012). Whereas the chemical complexity was once thought to account for SOM stability, due to molecular recalcitrance, more recent thinking emphasises ecosystem properties notably sorptive protection and hindered microbial access (Schmidt et al 2011;Dungait et al 2012;Lehmann and Kleber 2015).…”
Section: Introductionmentioning
confidence: 99%
“…Whereas the chemical complexity was once thought to account for SOM stability, due to molecular recalcitrance, more recent thinking emphasises ecosystem properties notably sorptive protection and hindered microbial access (Schmidt et al 2011;Dungait et al 2012;Lehmann and Kleber 2015). The chemical structures of SOM have been elucidated principally through NMR spectroscopy (Baldock et al 1992;Hatcher et al 2001;Kögel-Knabner 2002;Simpson and Simpson 2012), while physical techniques have been used to study molecular size and aggregation (Wershaw 1999;Piccolo 2001). Radiocarbon provides information about turnover and age (Torn et al 2009;Trumbore 2009;Mills et al 2014).…”
The formation and turnover of soil organic matter (SOM) includes the biogeochemical processing of the macronutrient elements nitrogen (N), phosphorus (P) and sulphur (S), which alters their stoichiometric relationships to carbon (C) and to each other. We sought patterns among soil organic C, N, P and S in data for c. 2000 globally distributed soil samples, covering all soil horizons. For non-peat soils, strong negative correlations (p \ 0.001) were found between N:C, P:C and S:C ratios and % organic carbon (OC), showing that SOM of soils with low OC concentrations (high in mineral matter) is rich in N, P and S. The results can be described approximately with a simple mixing model in which nutrient-poor SOM (NPSOM) has N:C, P:C and S:C ratios of 0.039, 0.0011 and 0.0054, while nutrient-rich SOM (NRSOM) has corresponding ratios of 0.12, 0.016 and 0.016, so that P is especially enriched in NRSOM compared to NPSOM. The trends hold across a range of ecosystems, for topsoils, including O horizons, and subsoils, and across different soil classes. The major exception is that tropical soils tend to have low P:C ratios especially at low N:C. We suggest that NRSOM comprises compounds selected by their strong adsorption to mineral matter. The stoichiometric patterns established here offer a new quantitative framework for SOM classification and characterisation, and provide important constraints to dynamic soil and ecosystem models of carbon turnover and nutrient dynamics.
“…However, the precise structure of soil OM or specific organic compounds cannot be determined by this method (Kögel-Knabner, 2000). More detailed information is obtained from macromolecular compounds and diagnostic lipids in soil OM (Simpson and Simpson, 2012). The dynamics of organic carbon in physically defined OM fractions are revealed by 14 C analysis.…”
This study investigated soil organic matter (OM) composition of differently stabilized soil OM fractions in the active layer of a polygonal tundra soil in the Lena Delta, Russia, by applying density and particle size fractionation combined with qualitative OM analysis using solid state 13C nuclear magnetic resonance spectroscopy, and lipid analysis combined with 14C analysis. Bulk soil OM was mainly composed of plant-derived, little-decomposed material with surprisingly high and strongly increasing apparent 14C ages with active layer depth suggesting slow microbial OM transformation in cold climate. Most soil organic carbon was stored in clay and fine-silt fractions (< 6.3 μm), which were composed of little-decomposed plant material, indicated by the dominance of long n-alkane and n-fatty acid compounds and low alkyl/O-alkyl C ratios. Organo-mineral associations, which are suggested to be a key mechanism of OM stabilization in temperate soils, seem to be less important in the active layer as the mainly plant-derived clay- and fine-silt-sized OM was surprisingly "young", with 14C contents similar to the bulk soil values. Furthermore, these fractions contained less organic carbon compared to density fractionated OM occluded in soil aggregates – a further important OM stabilization mechanism in temperate soils restricting accessibility of microorganisms. This process seems to be important at greater active layer depth where particulate OM, occluded in soil aggregates, was "older" than free particulate OM
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