B11G-01 INVITED
The Physical Chemistry of Recalcitrance – What is Stable Organic Carbon?
The historic concept of recalcitrance was informed by humus theory, i.e. by the notion that a humification process transforms labile organic biomolecules into thermodynamically "stable" = recalcitrant humic macromolecules. A contrasting view defines recalcitrance not in a thermodynamic sense, but as a physical property: the "molecular- level characteristics of organic substances that influence their degradation by microbes and enzymes". In thermodynamics, stability is expressed as energy content and measured in Joule. A stable bond is a short bond: electrons are on low energy levels close to their nuclei. The resulting molecule has less energy than the individual atoms. Unless energy is supplied to the molecule, it will "stably" remain in its current state. In biogeochemistry, a long tradition refers to carbon that persists for a prolonged time within a biogeochemical system as being "stable". Consequently, biogeochemical stability is expressed as residence time and measured in time units (years). The obvious question here is: Does thermodynamic stability (low energy content) lead to biogeochemical stability (long residence time)? This talk attempts to reconcile the "time-concept" with the "energy-concept" by showing how energy rich "labile" organic molecules can persist for very long times, while relatively energy-poor and "stable" organics may be decomposed very rapidly. In the last years experimental data have become available which show that reduced carbon is never "stable" (over time) in any aerobic environment. It appears that resistance against decomposition or "recalcitrance" can not be parameterized as a constant, measurable and specific property of a given organic compound, but results from a delicate interplay between aqueous solubility, oxygen availability, separation from enzymes, sorptive protection and various other controlling factors. It follows that the concepts of "stable humic substances" or "recalcitrant organic matter" need to be revised before they are incorporated in future models of organic matter turnover.
B11G-02
Stability of Soil Carbon Fractions - Aggregation Versus Mineral Association
Models that seek to describe the dynamics of soil organic C typically distinguish between two or more C fractions according to differences of biochemical and microbial degradation. The rates are a consequence of recalcitrance, accessibility and interactions. Soil aggregation is an important mechanism controlling the accessibility of substrates by microbes and enzymes and thus the dynamics of minerals bound C are interacting with soil aggregate dynamics. In this study we focused on C fractions isolated by particle size fractionation. The main objective of our study was to differentiate between C stabilization of soil fractions due to accessibility/aggregation or to association with minerals. For a detailed understanding of these processes and the sources of respired soil CO2 we combined the measurement of heterotrophic respiration, CO2-13C analyses and radiocarbon dating of the respired CO2 in a long-term laboratory mineralization experiment. For the experiment we took soil material from the A horizon of an Albic Luvisol under Norway spruce forest (Picea abies) in southern Germany. The air dried bulk soil (< 2000 µm) was subjected to ultrasonication (1st step 60 J ml-1; 2nd step 440 J ml-1) and separated according to particle size in three fractions: > 63 µm to 2000 µm - sand, > 6.3 µm to 63 µm - silt and silt/clay fraction < 6.3 µm - clay. Solid-state 13C-CPMAS NMR spectroscopy was used to analyze the composition of bulk soil and fractions. The incubation of the three fractions and the bulk soil was done for 250 days in triplicate at 20 degree Celsius and 70% of maximal water holding capacity. A relative enrichment of alkyl C and an increase of the alkyl / O/N-alkyl C ratios in the order of sand < silt < clay were observed by 13C-NMR. On a long term the sand fraction and the bulk soil showed a sustained C bioavailability. For the silt and clay fraction similar respiration rates and a low C bioavailability were detected. The recombined fractions (by calculation) showed 35% higher amounts of respired CO2-C than the bulk soil. This difference accounts for the absence of restricted accessibility due to soil aggregation. Because of the high amounts of mineral bound C the main source of CO2-C (70%) in the recombined fraction is the clay fraction. Nevertheless the recalcitrance of mineral bound C is restricting the positive effects of aggregate disruption on the C turnover. The small fast decomposing C pool of the sand fraction is of minor importance to the total soil respiration balance. CO2-13C signatures showed higher values of the silt and clay fractions in contrast to the sand fraction, indicating a lower bioavailability of 13C-depleted carbon sources in the small fractions. The analyses of CO2-14C showed a shift to the utilization of older C sources with time.
B11G-03 INVITED
Black carbon and organic matter stabilization in soil
Interaction with minerals is key to stabilization of organic matter in soils. Stabilization is commonly perceived to occur due to entrapment in pore spaces, encapsulation within aggregates or interaction with mineral surfaces. Typically only interactions between organic matter and minerals are considered in such a model. Here we demonstrate that black carbon may act very similar to minerals in soil in that it enhances the stabilization of organic matter. Mineralization of added organic matter was slower and incorporation into intra-aggregate fractions more rapid in the presence of black carbon. Added double-labeled organic matter was recovered in fractions with high amounts of black carbon. Synchrotron-based near-edge x-ray fine structure (NEXAFS) spectroscopy coupled to scanning transmission x-ray microscopy (STXM) suggested a possible interaction of microorganisms with black carbon surfaces and metabolization of residues. These findings suggest a conceptual model that includes carbon-carbon interactions and by-passing for more rapid stabilization of litter into what is commonly interpreted as stable carbon pools.
B11G-04 INVITED
Do iron and aluminium oxides stabilise organic matter in soil? A multi-scale statistical analysis, from field to horizon
Although many "in vitro" experiments demonstrated that Fe and Al oxides stabilise organic carbon (OC) in soil, very few field studies validated that result, and none of them took into account the influence of scale and that of depth on the relationships linking OC to metal oxides in soil. In the present study, we characterised the latter as a function of depth and horizon at two different sampling scales: the field scale and the pedological pit scale. Twenty seven pits, 2 m deep, were opened over a 22 ha area. At large scale, samples were collected per horizons, whereas at fine scale, 150 positioned samples were collected over the surface of a selected profile. All samples were analysed for their content in OC, in Fe and Al oxides (amorphous Feo, Alo and crystalline Fed-o, Ald-o), and in both forms of Si associated either with amorphous or crystalline forms of metal oxides (Sio, Sid-o). For both sampling scales, relationships between OC and Feo, Alo, Sio, Fed-o, Ald-o, Sid-o were statistically investigated per whole profiles and per horizons. For whole profile at large and fine scale, results showed that all variables were correlated to OC. However, all variables, except Sio at fine scale, were also correlated with depth, suggesting that most of these correlations might only be due to a simultaneous depth dependency of correlated variables. On the contrary, within single horizons much less correlations were observed suggesting that this mode of investigation limits the confounding effects of depth. At fine scale per horizons, seven correlations independent from depth were observed in deep horizons only. Four of them were positive and implied amorphous form of Fe and Al suggesting that these oxides physically interact with OM in the subsoil, whereas the three last were negative and implied both forms of Si, suggesting that Si compete with OC for the fixation on sorption sites. In the three deepest horizons, slope coefficients of linear correlations between OC and Feo decreased with depth, suggesting that Feo surfaces were less saturated with depth. In the deepest horizon, partial least square regression showed that Feo, Alo, Sio explained 78% of the OC variability while it represents only 0.36 % of the soil mass. At large scale, the quasi absence of correlations proves that the fine scale of investigation is adapted to the minimisation of the lateral variability, which improves the results of correlations. As a conclusion, to lessen the confounding effect of depth and lateral variability, correlations between pedological variables should be performed per single horizons and at fine scale. Results, obtained with sampling procedure, show that even a small amounts of pedogenetic oxides do have an effects on the soil organic carbon content that might lead to its stabilisation.
B11G-05
Physical and Chemical Stabilization of Organic Matter by Iron Oxides: Reconciling Observations at the Nano and Landscape scales
Concentrations of iron oxides are sometimes well correlated with concentration of organic carbon in the soil. But, this is highly dependent on the type of oxides, and their concentrations. On a northern California toposequence we found that high concentrations of crystalline Fe oxides (mainly goethite in subsoils of flat or convergent slopes) is accompanied with low concentrations of organic carbon, that is old (very negative δ14C values). It has been hypothesized that this could be because the concentrations of Fe oxides in some Mediterranean, and temperate soils may be too small to have significant effect on carbon accumulation and stabilization. In this study we employ different field and lab measurements to determine if there exists a threshold oxide concentration for soil organic carbon storage and stabilization; and determine whether the organic matter is primarily stabilized physically (by aggregation) or chemically (organo- mineral complexation). Here we are using (a) selective dissolutions of Fe oxides to determine concentration of crystalline Fe oxides; (b) density fractionations to determine amount of organic carbon chemically bound to the mineral fraction; (c) hydrogen fluoride demineralization to determine the stock of recalcitrant organic carbon; (d) batch sorption/desorption experiments with synthetic goethite and natural organic matter to determine reversibility of the oxide-organic carbon interactions at different concentrations of sorbent and sorbate, along with ultrasonic disruption of aggregates; and (e) concentrations 13C NMR and FTIR to determine aromaticity of the organic constituents. In this study we show that, as the concentration of oxides increases beyond a threshold level (that depends on type of oxide and chemistry of sorbing organic matter) there is no proportional increase in amount of carbon that is stored by the oxides, or in storage and stabilization of aromatic functional groups. But increasing oxide concentrations were well correlated with fraction of stabilized organic carbon.
B11G-06
Characterization of organo-mineral association by thermal analysis of density fractions
The degree of association of soil organic matter with the mineral matrix is considered a major control on organic matter stability. Fractionation of soils by density separation separates organic matter more or less associated with mineral matter, where the lighter fractions have generally been found to be more C-enriched with higher C/N ratios while heavier fractions have less C and narrower C/N ratios. The objective of this study was to characterize density fractions ranging from < 1.65 g cm-3 to > 2.55 g cm-3 isolated from an Oregon andic soil using thermogravimetry and differential scanning calorimetry to assess the quality and stability of the organic matter associated with these fractions. Preliminary results suggest lighter fractions exhibit large exothermic regions at lower temperatures, while heavier fractions exhibit a greater proportion of organic matter oxidized at higher temperatures. The results further support a potential link between thermal stability and long-term soil organic matter stabilization.
B11G-07
Do Variations in Detrital Inputs Influence Stable Soil Organic Matter? – An Experimental Approach
Recognition of the importance of feedbacks from plants in determining soil nutrient dynamics and C storage led to a large number of litter decomposition studies. Despite growing knowledge of short-term litter dynamics, we know relatively little about the fate of plant litter and its role in determining SOM content and nutrient cycling over time scales ranging from decades and centuries. To address this gap, we established long-term studies of controls on soil organic matter formation in an old-growth forest at the H.J. Andrews Experimental Forest, OR. This study complements a network of recently established similar experiments that pan climatic and soil gradients, as well as the original DIRT experiment established in the Wisconsin Arboretum in 1956 in both grassland and forested sites. The central goal of the DIRT project is to assess how rates and sources of plant litter inputs control the accumulation and dynamics of organic matter and nutrients in forest soils over decadal time scales. Treatment plots include doubled litter (needle) inputs , doubled wood, no above ground litter (screened) inputs, no root inputs (trenched), and no inputs (screened and trenched). For the 50th anniversary of the Wisconsin sites and the 10th anniversary of the H.J. Andrews site, we used sequential density fractionation of soils from all treatments to determine if adding or removing either below- or above-ground litter inputs influenced carbon stabilization as soil organic matter. After 50 years, double litter plots in both prairie and forested soils had higher %C in the 0-10 cm horizon. In the forested site, plots showed increased C content of the lightest fraction, which represents relatively young SOM with a short turnover time. However, the first two heavy fractions also showed increases in C with added aboveground litter, suggesting the importance of aboveground litter inputs to SOM in the forest. No such pattern existed for the prairie soil, and we hypothesize that this is because aboveground, labile litter adds very little to stabilized SOM in grasslands, and that root-derived C is the dominant control on SOM stabilization in grasslands. These results were confirmed with analysis of labile C (short –term respiration measurements) and acid hydrolysis resistant C across treatments. The relative contribution of aboveground vs. belowground litter was analyzed through the analysis of cutin and suberin acids, and we found that the detrital source of litter was retained in soils and could be fingerprinted through this analysis. Thermal analysis, including thermogravimetry (TG) and differential scanning calorimetry (DSC) performed simultaneously is currently being applied to explore both SOM quality and stability.
B11G-08 INVITED
A STXM/NEXAFS Investigation of the Role of Particle Composition in Organic Matter Stablization
The detailed role of mineral particles in the fractionation and stabilization of soil organic matter remains unclear. We employed STXM (Scanning Transmission X-ray Microscopy) to obtain images of the C, N, and Fe distribution on individual mineral particles from three different soils with varying chemistries and mineral make-up and obtained NEXAFS spectra from specific locations to investigate organic matter chemistry in relation to chemistry of the mineral surface. Specifically we examined the soil density fraction of 2.3-2.6 g cm-3 from A horizons of an Inceptisol, Oxisol, and Alfisol. Preliminary results indicate an average difference between the NEXAFS spectra taken from the particles from the different soils as well as small scale (nm) intra-particle differences in carbon distribution. These intra-particle differences appear to be driven by the particle composition, but the effects are not uniform within each soil. We will present further details of the organic matter heterogeneity along with its implications for carbon stabilization pathways.