Alain F. Plante

Experimental warming shows that decomposition temperature sensitivity increases with soil organic matter recalcitrance

Soil C decomposition is sensitive to changes in temperature, and even small increases in temperature may prompt large releases of C from soils. But much of what we know about soil C responses to global change is based on short-term incubation data and model output that implicitly assumes soil C pools are composed of organic matter fractions with uniform temperature sensitivities. In contrast, kinetic theory based on chemical reactions suggests that older, more-resistant C fractions may be more temperature sensitive. Recent research on the subject is inconclusive, indicating that the temperature sensitivity of labile soil organic matter (OM) decomposition could either be greater than, less than, or equivalent to that of resistant soil OM. We incubated soils at constant temperature to deplete them of labile soil OM and then successively assessed the CO2-C efflux in response to warming. We found that the decomposition response to experimental warming early during soil incubation (when more labile C remained) was less than that later when labile C was depleted. These results suggest that the temperature sensitivity of resistant soil OM pools is greater than that for labile soil OM and that global change-driven soil C losses may be greater than previously estimated.

Soil aggregate dynamics and the retention of organic matter in laboratory-incubated soil with differing simulated tillage frequencies

It is generally accepted that aggregate dynamics are a significant control on the dynamics of organic C and that aggregate dynamics differ under cultivated and uncultivated (or no-till) conditions. Cultivation may alter soil organic matter (SOM) dynamics by changing its position within the soil matrix, either releasing organic materials from within aggregates during disruption or occluding materials during aggregate formation. The goal of this study was to observe aggregate dynamics under differing regimes of simulated tillage and relate these to organic matter dynamics. The experiment specifically examined the mechanism of physical protection by inducing different rates of soil aggregate turnover without changes in environmental conditions. Soil samples were incubated with dysprosium-labeled tracer spheres and finely ground corn residues for 8 weeks under imposed rates of aggregate turnover: no simulated tillage, three simulated tillage events, or five simulated tillage events. Total soil respiration and evolution were used to determine the relative degree of physical protection afforded by the induced rates of aggregate turnover. Increased frequency of simulated tillage increased the incorporation of tracer spheres into stable macroaggregates, and reduced the total amount of CO2 evolved during the experiment. We propose that organic matter retention in tilled samples was achieved through a reduction of the priming effect afforded by the increased aggregate turnover and the disruption of the microbial biomass decomposing the added POM and native organic matter. While there appears to be a disparity between short-term tillage-enhanced organic matter protection and the long-term decrease in organic matter content observed in cultivated soils, the results suggest that there may be threshold rates of aggregate turnover that will protect rather than release organic C. We propose that the physical protection available under differing rates of soil aggregate turnover will differ for incoming organic materials versus previously protected organic matter, and that soil C sequestration is maximal at an intermediate aggregate turnover rate.

Use of thermal analysis techniques (TG–DSC) for the characterization of diverse organic municipal waste streams to predict biological stability prior to land application

The use of organic municipal wastes as soil amendments is an increasing practice that can divert significant amounts of waste from landfill, and provides a potential source of nutrients and organic matter to ameliorate degraded soils. Due to the high heterogeneity of organic municipal waste streams, it is difficult to rapidly and cost-effectively establish their suitability as soil amendments using a single method. Thermal analysis has been proposed as an evolving technique to assess the stability and composition of the organic matter present in these wastes. In this study, three different organic municipal waste streams (i.e., a municipal waste compost (MC), a composted sewage sludge (CS) and a thermally dried sewage sludge (TS)) were characterized using conventional and thermal methods. The conventional methods used to test organic matter stability included laboratory incubation with measurement of respired C, and spectroscopic methods to characterize chemical composition. Carbon mineralization was measured during a 90-day incubation, and samples before and after incubation were analyzed by chemical (elemental analysis) and spectroscopic (infrared and nuclear magnetic resonance) methods. Results were compared with those obtained by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. Total amounts of CO(2) respired indicated that the organic matter in the TS was the least stable, while that in the CS was the most stable. This was confirmed by changes detected with the spectroscopic methods in the composition of the organic wastes due to C mineralization. Differences were especially pronounced for TS, which showed a remarkable loss of aliphatic and proteinaceous compounds during the incubation process. TG, and especially DSC analysis, clearly reflected these differences between the three organic wastes before and after the incubation. Furthermore, the calculated energy density, which represents the energy available per unit of organic matter, showed a strong correlation with cumulative respiration. Results obtained support the hypothesis of a potential link between the thermal and biological stability of the studied organic materials, and consequently the ability of thermal analysis to characterize the maturity of municipal organic wastes and composts.

A modeling approach to quantifying soil macroaggregate dynamics

While several researchers have suggested that soil aggregate turnover is a significant control on organic matter dynamics, the quantification of soil aggregate dynamics has yet to be achieved. Quantification of soil aggregate turnover is essential to testing any hypothesis concerning the relationship between aggregate turnover and organic matter dynamics. The goal of the current work was to propose a modeling approach to the quantification of soil macroaggregate dynamics. The approach taken was to define model compartments representing water-stable soil aggregate size fractions and describing the flows between compartments using first-order kinetics. Soil aggregate data from a 2-yr field study on two contrasting soils were used to calibrate the model and yielded soil aggregate mean residence times ranging from 4 to 95 d, where aggregate dynamics were generally two to three times more rapid in a Gray Luvisol compared to a Black Chernozem. The model was subsequently used to predict the distribution of appli...

Improved characterization of soil organic matter by thermal analysis using CO2/H2O evolved gas analysis.

Simultaneous thermal analysis [i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)] is frequently used in materials science applications and is increasingly being used to study soil organic matter (SOM) stability. Yet, important questions remain, especially with respect to how the soil mineral matrix affects TG-DSC results, which could confound the interpretation of relationships between thermal and biogeochemical SOM stability. The objective of this study was to explore the viability of using infrared gas analyzer (IRGA) based CO(2)/H(2)O evolved gas analysis (EGA) as a supplement or alternative to TG-DSC to improve the characterization of SOM. Here, we subjected reference samples and a set of 28 diverse soil samples from across the U.S. to TG-DSC coupled with IRGA-based EGA. The results showed the technical validity of coupling TG-DSC and CO(2)-EGA, with more than 80% of the theoretically evolved CO(2)-C recovered during pure cellulose and CaCO(3) analysis. CO(2)-EGA and DSC thermal profiles were highly similar, with correlation coefficients generally >0.90. Additionally, CO(2)/H(2)O-EGA proved useful to improve the accuracy of baseline correction, detect the presence of CaCO(3) in soils, and identify SOM oxidative reactions normally hidden in DSC analysis by simultaneous endothermic reactions of soil minerals. Overall, this study demonstrated that IRGA-based CO(2)/H(2)O-EGA constitutes a valuable complement to conventional TG-DSC analysis for SOM characterization.

SOIL BIOGEOCHEMICAL CYCLING OF INORGANIC NUTRIENTS AND METALS

Publisher Summary This chapter discusses the soil biogeochemical cycling of inorganic nutrients and metals. Soil microorganisms have a profound effect on the transformations involved in a large number of biogeochemical cycles other than carbon (C) and nitrogen (N), such as the macronutrients phosphorus (P) and sulfur (S), and various micronutrients and environmental pollutants. A conceptual model for the cycling of a generic nutrient or metal element illustrates the continuous flow of energy, water, nutrients, and other materials across the ecosystems boundaries. Meteorologic transfers consist of windblown particulate matter, dissolved substances in precipitation, and gases. Geologic fluxes include soluble and particulate matter transported by surface and subsurface water flow and the mass movement of mineral materials during events such as erosion, landslides, or lava flows. This chapter focuses on phosphorus, sulfur, micronutrients and trace metals. Environmental significance of P, S, and metal biogeochemistry is described along with microorganisms as unifiers of elemental cycles in soil.

Characterization of soil organic matter

INTRODUCTION Soil organic matter (SOM) generally refers to the non-living organic material within the soil matrix that was once part of, or produced by, a living organism. It is usually determined on soil that has passed through a 2-mm sieve, and therefore is free of coarse animal residues, surface litter and large roots. Soil organic matter can be of plant, animal or microbial origin, and consists of a continuum of materials in various stages of alteration due to both biotic and abiotic processes (Baldock and Skjemstad, 2000). Methods used in the past to estimate directly SOM content involved the destruction of the organic matter by treatment with hydrogen peroxide (H 2 O 2 ) or by ignition of the soil at high temperature (Nelson and Sommers, 1996). Both of these techniques, however, are subject to significant error: oxidation of SOM by H 2 O 2 is incomplete, and some inorganic soil constituents decompose upon heating. While different elements such as C, N, P, S etc. are bound into organic compounds, we will concentrate on soil organic carbon (SOC) for the purposes of this chapter because it is the dominant element, and because of its role in the global carbon cycle. Organic carbon to SOM conversion factors for surface soils typically range from 1.72 to 2.0 g SOM g −1 C (Nelson and Sommers, 1996). Direct measurement of total soil carbon involves the conversion of all forms of carbon to carbon dioxide (CO 2 ) by wet or dry combustion and subsequent quantification of the evolved CO 2 .

THE DYNAMICS OF SOIL ORGANIC MATTER AND NUTRIENT CYCLING

Publisher Summary This chapter discusses the dynamics of soil organic matter and nutrient cycling. Knowledge of the turnover rates of plant and animal residues, microbial bodies, and soil organic matter (SOM) is a prerequisite for understanding the availability and cycling of nutrients such as carbon, nitrogen, sulfur, and phosphorus. Understanding the dynamics of nutrient, plant residue, or SOM transformations in the field requires meaningful mathematical expressions for the biological, chemical, and physical processes involved. Models can be used to gain an understanding of the processes and controls involved in nutrient cycles, to generate data on the size of various pools and the rates at which nutrients are transformed, and to make predictions when experiments are inappropriate. While conceptual models may be sufficient for the first task, only quantitative models can achieve the latter tasks. Quantitative models of SOM and nutrient dynamics are attempts to describe soil biological processes rather than strictly mathematical expressions and statistical procedures used to find best-fitting curves. This chapter presents the Rothamsted, Van Veen and Paul, and Century models as examples with the focus on how they are constructed. It discusses in detail about reaction kinetics, modeling the dynamics of decomposition and nutrient transformations, and establishing pool sizes and kinetic constants.

Application of a set of complementary techniques to understand how varying the proportion of two wastes affects humic acids produced by vermicomposting

A better understanding of how varying the proportion of different organic wastes affects humic acid (HA) formation during vermicomposting would be useful in producing vermicomposts enriched in HAs. With the aim of improving the knowledge about this issue, a variety of analytical techniques [UV-visible spectroscopic, Fourier transform infrared, fluorescence spectra, solid-state cross-polarization magic-angle spinning (CPMAS) (13)C nuclear magnetic resonance (NMR) spectra, and thermal analysis] was used in the present study to characterize HAs isolated from two mixtures at two different ratios (2:1 and 1:1) of tomato-plant debris (TD) and paper-mill sludge (PS) before and after vermicomposting. The results suggest that vermicomposting increased the HA content in the TD/PS 2:1 and 1:1 mixtures (15.9% and 16.2%, respectively), but the vermicompost produced from the mixture with a higher amount of TD had a greater proportion (24%) of HAs. Both vermicomposting processes caused equal modifications in the humic precursors contained in the different mixtures of TD and PS, and consequently, the HAs in the vermicomposts produced from different waste mixtures exhibited analogous characteristics. Only the set of analytical techniques used in this research was able to detect differences between the HAs isolated from each type of vermicompost. In conclusion, varying the proportion of different wastes may have a stronger influence on the amount of HAs in vermicomposts than on the properties of HAs.

Distribution of Radiocarbon Ages in Soil Organic Matter by Thermal Fractionation

Radiocarbon analysis is an important tool in quantifying soil organic matter (SOM) dynamics within the terrestrial carbon cycle. However, there is increasing appreciation that representing SOM as a single, homogeneous pool with a single, mean 14C concentration is inadequate. We investigate whether the differing patterns in CO2 release during rampedtemperature oxidation reflect organic matter of different ages, and hypothesize that thermally labile SOM (combusting at low temperatures) consists of younger carbon than thermally resistant organic matter. Topsoil samples under contrasting land uses (native vegetation and long-term cultivation) were selected for 14C analysis before and after acid fumigation for the removal of carbonates. Results of bulk 14C analyses showed a significant shift in 14C age from 0.944 Fm under native vegetation to 0.790 Fm under cultivation. Four to 5 “fractions” associated with different CO2-evolution regions were identified by thermal analysis and analyzed for 14C via modifications to NOSAMS’ established “programmed temperature pyrolysis system,” in which discrete CO2 fractions evolved during ramped-temperature oxidation were isotopically characterized by a microwave gas ion source (GIS) continuous-flow AMS (CFAMS) system. Results showed that while acid fumigation removed soil carbonates, the treatment also significantly altered the thermograms and inferred SOM composition. While direct attribution of 14C values to individual peaks is somewhat confounded by overlapping temperature ranges for oxidation of unique populations of carbon, in general, thermally stable fractions of SOM appear to be 14C-depleted compared to thermally reactive (low temperature) fractions regardless of pretreatment.