Michael W. I. Schmidt

Persistence of soil organic matter as an ecosystem property

Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily—and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.

Black carbon in soils and sediments: analysis, distribution, implications, and current challenges.

This review highlights the ubiquity of black carbon (BC) produced by incomplete combustion of plant material and fossil fuels in peats, soils, and lacustrine and marine sediments. We examine various definitions and analytical approaches and seek to provide a common language. BC represents a continuum from partly charred material to graphite and soot particles, with no general agreement on clear-cut boundaries. Formation of BC can occur in two fundamentally different ways. Volatiles recondense to highly graphitized soot-BC, whereas the solid residues form char-BC. Both forms of BC are relatively inert and are distributed globally by water and wind via fluvial and atmospheric transport. We summarize, chronologically, the ubiquity of BC in soils and sediments since Devonian times, differentiating between BC from vegetation fires and from fossil fuel combustion. BC has important implications for various biological, geochemical and environmental processes. As examples, BC may represent a significant sink in the global carbon cycle, affect the Earths radiative heat balance, be a useful tracer for Earths fire history, build up a significant fraction of carbon buried in soils and sediments, and carry organic pollutants. On land, BC seems to be abundant in dark-colored soils, affected by frequent vegetation burning and fossil fuel combustion, thus probably contributing to the highly stable aromatic components of soil organic matter. We discuss challenges for future research. Despite the great importance of BC, only limited progress has been made in calibrating analytical techniques. Progress in the quantification of BC is likely to come from systematic intercomparison using BCs from different sources and in different natural matrices. BC identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels. The key to estimating BC stocks in soils and sediments is an understanding of the processes involved in BC degradation on a molecular level. A promising approach would be the combination of short-term laboratory experiments and long-term field trials.

Comparison of quantification methods to measure fire‐derived (black/elemental) carbon in soils and sediments using reference materials from soil, water, sediment and the atmosphere

Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified most of the carbon in all three materials as BC, whereas other methods identified only soot carbon as BC. The different methods also gave widely different BC contents for the environmental matrices. However, these variations could be understood in the light of the findings for the other two groups of materials, i.e., that some methods incorrectly identify non-BC carbon as BC, and that the detection efficiency of each technique varies across the BC continuum. We found that atmospheric BC quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of BC as light-absorbing material irrespective of its origin, leading to biases when applied to terrestrial and sedimentary materials. This study shows that any attempt to merge data generated via different methods must consider the different, operationally defined analytical windows of the BC continuum detected by each technique, as well as the limitations and potential biases of each technique. A major goal of this ring trial was to provide a basis on which to choose between the different BC quantification methods in soil and sediment studies. In this paper we summarize the advantages and disadvantages of each method. In future studies, we strongly recommend the evaluation of all methods analyzing for BC in soils and sediments against the set of BC reference materials analyzed here.

Comparative analysis of black carbon in soils

Black carbon (BC), produced by incomplete combustion of fossil fuels and vegetation, occurs ubiquitously in soils and sediments. BC exists as a continuum from partly charred material to highly graphitized soot particles, with no general agreement on clear-cut boundaries of definition or analysis. In a comparative analysis, we measured BC forms in eight soil samples by six established methods. All methods involved removal of the non-BC components from the sample by thermal or chemical means or a combination of both. The remaining carbon, operationally defined as BC, was quantified via mass balance, elemental composition or by exploiting benzenecarboxylic acids as molecular markers or applying 13C MAS NMR (magic angle spinning nuclear magnetic resonance) spectroscopy. BC concentrations measured for individual samples vary over 2 orders of magnitude (up to a factor of 571). One possible explanation for this wide range of results is that the individual BC methods rely on operational definitions with clear-cut but different boundaries and developed for specific scientific questions, whereas BC represents a continuum of materials with widely contrasting physicochemical properties. Thus the methods are inherently designed to analytically determine different parts of the continuum, and it is crucial to know how measurements made by different techniques relate to each other. It is clear from this preliminary comparative analysis that a collection of BC reference materials should be established as soon as possible 1 ) to ensure long-term intralaboratory and interlaboratory data quality and 2) to facilitate comparative analyses between different analytical techniques and scientific approaches

How surface fire in Siberian Scots pine forests affects soil organic carbon in the forest floor: Stocks, molecular structure, and conversion to black carbon (charcoal)

[1] In boreal forests, fire is a frequent disturbance and converts soil organic carbon (OC) to more degradation-resistant aromatic carbon, i.e., black carbon (BC) which might act as a long-term atmospheric-carbon sink. Little is known on the effects of fires on boreal soil OC stocks and molecular composition. We studied how a surface fire affected the composition of the forest floor of Siberian Scots pine forests by comparing the bulk elemental composition, molecular structure (13C-MAS NMR), and the aromatic carbon fraction (BC and potentially interfering constituents like tannins) of unburned and burned forest floor. Fire reduced the mass of the forest floor by 60%, stocks of inorganic elements (Si, Al, Fe, K, Ca, Na, Mg, Mn) by 30–50%, and of OC, nitrogen, and sulfur by 40–50%. In contrast to typical findings from temperate forests, unburned OC consisted mainly of (di-)O-alkyl (polysaccharides) and few aromatic structures, probably due to dominant input of lichen biomass. Fire converted OC into alkyl and aromatic structures, the latter consisting of heterocyclic macromolecules and small clusters of condensed carbon. The small cluster size explained the small BC concentrations determined using a degradative molecular marker method. Fire increased BC stocks (16 g kg−1 OC) by 40% which translates into a net-conversion rate of 0.7% (0.35% of net primary production) unburned OC to BC. Here, however, BC was not a major fraction of soil OC pool in unburned or burned forest floor, either due to rapid in situ degradation or relocation.

Plant compounds and their turnover and stability as soil organic matter

Publisher Summary This chapter reviews current knowledge on the stabilization of organic compounds (SOM). The focus of the chapter is on the chemical stability of molecules, the interactions of organic molecules with clay or metal (Fe or Al) oxides and hydroxides, and the possibility of biological carbon stabilization. The turnover and stability of SOM depends mainly on environmental and biological parameters. Either biomass production or decomposition rates are affected. Additionally, soil matrix and litter quality and fire frequencies stabilize carbon in soils. From the presented results it is obvious that ecosystems have different mechanisms for stabilizing SOM, which lead to different chemistries of the stable compounds. For a better understanding of SOM in the terrestrial carbon cycle and to identify the missing carbon sink one needs: (1) to investigate compound specific mean residence times of stable compounds and biomarkers, (2) to develop new soil carbon models that are able to model the molecular turnover of ∼3C and ∼4C. The combined information provides new insight into soil carbon turnover and helps to understand and to quantify ecosystem specific retention mechanisms for carbon. Additionally, this information may identify the carbon sink capacities of soils.

Impact of brown coal dust on the organic matter in particle-size fractions of a Mollisol

The influence of brown coal emissions from a briquette factory on the organic matter of a Mollisol has been investigated. Non-contaminated, heavily contaminated soils and airborne particles are compared by using chemical and spectroscopic methods ( 13 C CPMAS-NMR, Py-GC-MS, 14 C dating). Bulk soils as well as their particle-size fractions are investigated. Organic carbon content of the bulk contaminated soil (138.6 g C/kg) is higher by a factor of six than the non-contaminated soil. For particle-size fractions, the highest content of organic carbon is found in the fine sand fraction (347.2 g C/kg). Furthermore, organic matter of the contaminated site shows a trend for relatively higher abundance of aliphatic compounds in the fraction ranging from 6 to 2000 μm, whereas for particles <6 μm no trend was observed. The A horizon of the contaminated soil is much older (15,750 yr BP) than the non-contaminated soil, which indicates input of anthropogenic material. Py-GC-MS analyses show for the non-contaminated site a homologous series of mainly alkanes, low amounts of prist-1-ene and a high abundance of lignin- and carbohydrate-derived structures. In contrast, composition of the samples from the contaminated site are similar to a sample of brown coal analyzed and show a homologous series of n-alkane, alkene, a, ω-alkadiene triplets and a high relative abundance of prist-1-ene with few lignin derived compounds, except for the clay fraction. Reflected light microscopy shows large amounts of brown coal dust particles with typical maceral groups, huminite, liptinite, and inertinite, in the fine sand fraction from the contaminated soil. Additionally, minor amounts of thermally altered brown coal-derived particles, i.e. coke particles, are observed.

Organic matter accumulating in Aeh and Bh horizons of a Podzol — chemical characterization in primary organo-mineral associations

The chemical structure of soil organic matter from the eluvial (Aeh) and (illuvial) Bh horizon of a Podzol was studied in primary organo-mineral associations by 13 C CPMAS NMR spectroscopy and acid hydrolysis. In Podzols, organic matter is leached from the forest floor and Aeh horizons into the Bh horizon, where it is intimately associated with the mineral phase. In the Aeh horizon the majority of the residual organic matter was present in methylene structures, contributing 42% to the organic matter associated with the clay fraction. In the Aeh horizon decreasing particle size was typically accompanied by increasing ratios alkyl C-to-O‐alkyl C, suggesting increasing decomposition of polysaccharides for the residual organic matter. The illuvial Bh horizon, containing high proportions of iron- and aluminum-oxides and hydroxides, was higher in aromatic carbon, while polysaccharides were similar and methylene carbon were smaller than in the Aeh horizon. In the particle size separates proportions of polysaccharides were constant, resulting in decreasing ratios alkyl C/O‐alkyl C. Proportions of hydrolyzable amino acids were larger in the Aeh horizon (52% of the total N) than in the Bh horizon (21%), both for bulk soils and size separates. Our results suggested that in Bh horizons of Podzols, aromatic structures and also presumably labile structures like polysaccharides can be stabilized by organo-mineral associations. # 2000 Elsevier Science Ltd. All rights reserved.

Biogeochemistry: Carbon budget in the black

A significant fraction of a common organic component of marine sediments has an unexpected source, providing a fresh context for studies of the global carbon cycle in oceanic and terrestrial settings.

Nature of organic nitrogen in fine particle size separates of sandy soils of highly industrialized areas as revealed by NMR spectroscopy

The structure of nitrogen-containing compounds of the fine particle size fractions (<20 mm) of two Podzols obtained from the highly industrialized Ruhr area in Germany, were examined by means of solid-state 15 N nuclear magnetic resonance (NMR) spectroscopy. In order to improve the signal-to-noise ratio of the spectra, the samples were treated with hydrofluoric acid (HF), prior to NMR analysis. Comparing the solid-state 15 N NMR spectra of plant incubates obtained before and after HF treatment revealed no major alteration of the nitrogen fraction induced by HF. From 60 to 90% of the nitrogen detectable in the solidstate 15 N NMR spectra of the soil particle size fractions were assigned to amides. A smaller signal derives from free amino groups, leading to the conclusion that most of the nitrogen was derived from peptide-like structures. The calculated high contribution of peptides to the total organic carbon and nitrogen of the samples confirms earlier studies demonstrating that peptide-like material plays a more important role in refractory soil organic matter formation than commonly thought. Major contributions of N-containing heterocyclic aromatic compounds, formed by recondensation reactions or deriving from the input of coal and soot particles from coal processing industries, were not identified. Obviously, in these fractions, contamination did not significantly alter the chemical composition of the organic nitrogen. # 2000 Elsevier Science Ltd. All rights reserved.