Total Decomposition Rate Research Articles

Mode and experiments of a straw ditch-burying returning and maturing in cold region of Northeast China

The current agronomic model of returning straw litter to soil (straw returning) involves crushing and mixing the straw with soil, which leads to shallow burial and ineffective decomposition of the straw. To address this issue in the cold region of Northeast China, this study proposed a method of burying and composting rice straw in furrows for returning to the field, and developed a matching machinery. Based on long-term experiments of burying straw in soil in spring and autumn, in this study, the operating parameters, postoperative straw decomposition effects, and influence of soil bulk density were analyzed. The results showed that the straw returning rate was inversely proportional to the forward speed and proportional to the furrow opening rotation speed and exhibited an increasing-then-decreasing trend with increasing collecting rotation speed. Taking into account both straw returning rate and operational efficiency, the recommended optimal operating parameters for the machinery unit were a forward speed of 0.4 m s−1, a raking speed of 180 r min−1, and a ditching speed of 420 r min−1. Under the spring model of burying straw in soil, straw treated with the composting agent was more likely to be fully decomposed in a short period, with a maximum total decomposition rate of up to 97.23%. Burying straw in soil effectively reduced soil bulk density, and straw returning in spring was more conducive to improving soil physical properties than straw returning in autumn. The value of this research lies in the integration of agricultural machinery and agronomy, providing insights for the development and design of related equipment, as well as offering a new approach for the effective treatment and utilization of straw in cold regions.

Mathematical model for predicting fungal growth and decomposition rates based on improved Logistic equations

As the function of the decomposition of fungi has been clearly researched in the global carbon cycle, it is obviously of value to explore the decomposition rate of fungal populations. This study analyzed the relationship between environmental factors and biodiversity step by step. In order to explore the interaction between the fungi and the relationship between the decomposition rate of fungi with time, the model based on the Logistic model was built and the Lotka-Volterra model was employed in the condition of two kinds of fungi existing in an environment with limited resources. The changing trend of population number and decomposition rate of several fungi under different environmental conditions can be predicted through the model. To illustrate the applicability of the model, Laetiporus conifericola and Hyphoderma setigerum were applied as examples. The results showed that the higher the degree of population diversity, the greater the decomposition rate, and the higher the decomposition efficiency of the ecosystem. Its rich species diversity is conducive to accelerating the decomposition of litter, lignocellulose, and the circulation of the entire ecosystem. Based on the above model and using the data from measuring the mycelial elongation rate of each isolate at 10°C, 16°C, and 22°C under standardized laboratory conditions, the growth patterns of the five fungi combinations were simulated. The results revealed a general increase in growth rate with increasing temperature, which verifies the accuracy of the model. Moreover, it also revealed that the total decomposition rate after fungal incorporation was negatively correlated with the decomposition rate of a fungal single action. Based on the above model, predictions can be made for fungal growth in different environments, and suitable environments for fungal growth can be determined. In the future, the model can be further optimized, and lignin and cellulose decomposition factors can be added to fit the decomposition of logs. The application scenarios of the model can be further broadened, which can contribute to the restoration and management of the ecological environment, as well as produce good effects in the fields of fungi assisting the global carbon cycle and soil problem restoration. Keywords: Logistic equations, fungal decomposition, mathematical model DOI: 10.25165/j.ijabe.20231601.7405 Citation: Pan J Y, Le W X, Wang Z A, Chen J. Mathematical model for predicting fungal growth and decomposition rates based on improved Logistic equations. Int J Agric & Biol Eng, 2023; 16(1): 60–65.

Towards a functional assessment of stream integrity: A first large-scale application using leaf litter decomposition

• Litter decomposition exhibits large variability at the regional scale (83 streams). • Litter decomposition does not correlate with the European WFD structural indicators. • Functional and structural indicators are complementary. • A grid is given to translate decomposition rates into functional quality classes. There is increasing recognition that indicators of ecosystem functioning are needed to improve current stream monitoring schemes. However, to date, no attempt has been made to include functional metrics in large-scale routine monitoring programs under the European Water Framework Directive (WFD). One reason is the uncertainty if functional indicators really carry new and independent information about stream status or, to the contrary, remain broadly redundant with existing indicators of biological stream quality (based on stream communities). A second reason is that, despite increasing scientific knowledge on how anthropogenic pressures influence ecosystem process rates, no ‘ready for use’ tool is available to translate them into stream ecosystem status. Litter decomposition probably ranges among the most documented stream ecosystem processes, and its potential for bioindication has been repeatedly demonstrated during the past decade. Here we report an extensive comparison of routine French structural indicators (I 2 M 2 , IBGN, IBD) with alder litter (microbial and total) decomposition rates in 83 streams located in south-western France. Expectedly, microbial decomposition rates were positively correlated with fungal biomass and activity (conidial production rate), while total and invertebrate-driven decomposition rates increased together with detritivore density and diversity in litter bags. By contrast, correlations between litter decomposition rates and routine structural indicators were clearly weak (Spearman’s ρ < 0.20), providing evidence that functional indicators bring new information. We devised a new interpretation grid to translate litter decomposition rates into stream ecological quality in a WFD-compatible design. This system considers the litter decomposition rates measured in both coarse (total decomposition) and fine (microbial decomposition) mesh bags as uncorrelated metrics of ecosystem functioning ( P = 0.898). Streams are considered functionally impaired when total and/or microbial decomposition rates are either higher or lower than in a reference condition. Applying our classification system to the 83 study sites allowed to assign them to 5 functional quality classes: bad (4 streams), poor (26), fair (29), good (21) and high (3). This classification did not match the classes obtained by conventional WFD structural indicators, reflecting that both functional and structural indicators are needed to obtain a comprehensive assessment of stream ecosystem integrity.

Numerical simulation of complex thermal decomposition processes in pyrolysis furnace for recycling solid waste Mg(NO3)2.2H2O

This study aims to reveal the complex thermal decomposition processes in a pilot-scale pyrolysis furnace for recycling solid waste Mg(NO3)2.2H2O produced in nitric acid leaching process of laterite nickel ore, by using CFD method. Specifically, an integrated mathematical model combining the gas-particle flow and the thermal decomposition reaction as well as the heat, mass transfer between gas and particle was developed based on the Euler-Lagrange method. And then, the developed model was used to study the flow characteristics of the gas and particle and the thermal decomposition processes for Mg(NO3)2.2H2O in the pyrolysis furnace. Furthermore, the effect of operating parameters (gas inlet temperature, mass flow rate and particle size of Mg(NO3)2.2H2O) on the performance of pyrolysis furnace (decomposition rate of Mg(NO3)2.2H2O) was studied. The results show that the developed model is reasonable. Increasing gas inlet temperature, decreasing mass flow rate and particle size of Mg(NO3)2.2H2O can effectively promote the decomposition rate of Mg(NO3)2.2H2O. Under the conditions of gas inlet temperature 1123 K, mass flow rate of Mg(NO3)2.2H2O 200 kg/h, and average particle size of Mg(NO3)2.2H2O 75 µm, the dehydration reaction mainly occurs in the furnace height of 11–8 m, while the denitration reaction mainly occurs in the furnace height of 10.5–1 m. The total decomposition rate of Mg(NO3)2.2H2O is 99.75%.

Exploration of the Ability of Fungi for Decomposing Natural Resources Based on Multiple Regression Equation and Cellular Automata

With the development of society, carbon emissions are increasing. The key organisms to maintain the stability of the carbon cycle are fungi that can be easily seen and ignored. In this paper, we selected several fungi to establish the model of decomposition and reproduction so that we can understand the role they played. First of all, we studied several physiological indexes of fungi, and established the degradation model through multiple regression analysis, and multiple linear regression equation for the relationship between decomposition rate, growth rate, unit volume density of mycelium, temperature and humidity tolerance. Next, we established the competitive growth model based on logistic model, simulated the competitive growth process of strains with different growth rates, humidity tolerance, and the total decomposition rate. In order to be closer to the real situation, we set up the competitive growth model among four species. By arranging fungal communities randomly to simulate different biodiversity, we analyzed the effects on the decomposition rate in the case of that the environmental temperature and humidity changed by 10% respectively. After that, we established a growth prediction model based on ARIMA. By querying the climate data of five typical climates, we established the competitive growth model with 4 combinations, and we obtained a short-term model, a medium-term trend and a long-tern forecast to describe growth, reproduction and decomposition rate. In order to refine the strains of the pressure of competition and the influence of the distance between the strains of competition, we have established improved competition evolution model based on the cellular automata theory of population. The model helped us comprehend the competition between species on a micro level. All these analyses showed us the significance of biodiversity and the great role decomposers play in Earth.

Dynamics of water-stable aggregates associated organic carbon assessed from delta C-13 changes following temperate natural forest development in China

In the context of global climate change, the preservation of soil productivity and the estimation of carbon budgets and cycles, the quantification of changes in carbon has important significance. In this study, we investigated the dynamics of soil aggregate associated organic carbon (OC) following temperate natural forest development in China. The objectives of this study were to examine the variation of soil aggregate associated OC decomposition rates, quantify the changes in the proportion of new and old soil aggregate OC, and explore the effects of controlling factors on SOC stocks, rate of total SOC increase and decomposition rate constants. The results showed that soil aggregate associated OC sequestration increased in 0−10 cm soil depth, while decreased in 10−30 cm soil depth. However, rate of aggregate associated OC increase, decomposition rate constants, and proportion of new OC increased at the early stage and then decreased along with the natural vegetation restoration. In addition, land use change had an important effect on soil aggregate associated OC dynamics, and soil particles, BD, MWD, C: N, plant diversity also played an important role. Moreover, SOC stocks had a negative relationship with clay and silt, while had a positive relationship with MWD and sandy soils. decomposition rate constants had a negative relationship with plant diversity, silt, and sand, while had a positive relationship with C: N and MWD. The proportions of new SOC had significant positive relationships with C: N, and it had a negative relationship with clay and silt. Therefore, it is necessary to clarify the formation mechanism of soil particles and aggregates, improve plant biodiversity, regulate the soil C: N ratio, and improve soil particle structure to increase soil carbon sequestration.

An improved method for quantifying total fine root decomposition in plantation forests combining measurements of soil coring and minirhizotrons with a mass balance model.

Accurate measurement of total fine root decomposition (the amount of dead fine roots decomposed per unit soil volume) is essential for constructing a soil carbon budget. However, the ingrowth/soil core-based models are dependent on the assumptions that fine roots in litterbags/intact cores have the same relative decomposition rate as those in intact soils and that fine root growth and death rates remain constant over time, while minirhizotrons cannot quantify the total fine root decomposition. To improve the accuracy of estimates for total fine root decomposition, we propose a new method (balanced hybrid) with two models that integrate measurements of soil coring and minirhizotrons into a mass balance model. Model input parameters were fine root biomass, necromass and turnover rate for Model 1, and fine root biomass, necromass and death rate for Model 2. We tested the balanced hybrid method in a loblolly pine plantation forest in coastal North Carolina, USA. The total decomposition rate of absorptive fine roots (ARs) (a combination of first- and second-order fine roots) using Models 1 and 2 was 107±13gm-2year-1 and 129±12gm-2year-1, respectively. Monthly total AR decomposition was highest from August to November, which corresponded with the highest monthly total ARs mortality. The ARs imaged by minirhizotrons well represent those growing in intact soils, evident by a significant and positive relationship between the standing biomass and the standing length. The total decomposition estimate in both models was sensitive to changes in fine root biomass, turnover rate and death rate but not to change in necromass. Compared with Model 2, Model 1 can avoid the technical difficulty of deciding dead time of individual fine roots but requires greater time and effort to accurately measure fine root biomass dynamics. The balanced hybrid method is an improved technique for measuring total fine root decomposition in plantation forests in which the estimates are based on empirical data from soil coring and minirhizotrons, moving beyond assumptions of traditional approaches.

The effects of climate on decomposition of cattle, sheep and goat manure in Kenyan tropical pastures

AimsDecomposition of manure deposited onto pasture from grazing animals represents an important process for carbon (C) and nitrogen (N) cycles in grassland systems. However, studies investigating manure decomposition are scarce; especially in sub-Saharan Africa (SSA).MethodsIn this study, we measured decomposition of three types of animal manure (cattle, sheep, goat) over >1 year using litter bags at four climatically different sites across Kenya. Results Manure dry matter, total C, total N and ammonium concentrations decreased exponentially, with the most rapid decrease occurring during the first few weeks following application, followed by slower changes during the following 2–3 months. Rates of N mineralization were lower than those for C mineralization, resulting in decreasing C/N ratios over time. Generally, cattle manure decomposed faster than sheep or goat manure despite having a higher initial C/N ratio and lower N concentration, with decomposition rates for dry matter ranging from 0.200 to 0.989 k year−1. Cellulose decomposed first, while lignin concentrations increased among all manure types and at all sites.ConclusionsWe found that total manure decomposition rates were positively correlated with cumulative precipitation and aridity index, but negatively correlated with mean temperature. Our results show much slower decomposition rates of manures in semi-arid tropical environments of East Africa as compared to the few previous studies in temperate climates.

Spatial Variability of Plant Litter Decomposition in Stream Networks: from Litter Bags to Watersheds

The decomposition of plant litter plays a fundamental role in the cycling of carbon and nutrients and is driven by complex interactions of biological and physical controls, yet little is known about its variability and controls across spatial scales. Here we address the indirect effects of riparian canopy cover on litter decomposition and decomposers and their variability within a set of hierarchical scales (watershed, stream segments and reaches) controlling for confounding factors that could co-vary with canopy cover (for example, temperature and nutrients), in high-altitude subtropical streams. Total, microbial and invertebrate-driven decomposition rates were approximately 1.4–6.6 times higher in closed-canopy than in open-canopy watersheds. Riparian canopy cover accounted for 62–69% of total variability of decomposition rates and indirectly (via light availability and litter inputs) promoted fungal facilitation of shredders through leaf litter conditioning. In contrast to what we expected, much of the spatial variability in the decomposition occurred at smaller scale (4–20% of total variability among reaches versus <1% among watersheds) and coincided with the greatest variability in shredder abundance and fungal biomass (70 and 17% among reaches, respectively). We conclude that riparian canopy cover may be an important control of natural variability of litter decomposition at the watershed scale through its effects on fungal decomposers and shredder consumption. We also provide evidence of higher reach and minor watershed variability of litter decomposition in stream networks. Our results point to the importance of identifying the sources of natural variability of decomposition and how they interact within and among spatial scales.

Drought and detritivores determine leaf litter decomposition in calcareous streams of the Ebro catchment (Spain)

Drought, an important environmental factor affecting the functioning of stream ecosystems, is likely to become more prevalent in the Mediterranean region as a consequence of climate change and enhanced water demand. Drought can have profound impacts on leaf litter decomposition, a key ecosystem process in headwater streams, but there is still limited information on its effects at the regional scale. We measured leaf litter decomposition across a gradient of aridity in the Ebro River basin. We deployed coarse- and fine-mesh bags with alder and oak leaves in 11 Mediterranean calcareous streams spanning a range of over 400km, and determined changes in discharge, water quality, leaf-associated macroinvertebrates, leaf quality and decomposition rates. The study streams were subject to different degrees of drought, specific discharge (Ls−1km−2) ranging from 0.62 to 9.99. One of the streams dried out during the experiment, another one reached residual flow, whereas the rest registered uninterrupted flow but with different degrees of flow variability. Decomposition rates differed among sites, being lowest in the 2 most water-stressed sites, but showed no general correlation with specific discharge. Microbial decomposition rates were not correlated with final nutrient content of litter nor to fungal biomass. Total decomposition rate of alder was positively correlated to the density and biomass of shredders; that of oak was not. Shredder density in alder bags showed a positive relationship with specific discharge during the decomposition experiment. Overall, the results point to a complex pattern of litter decomposition at the regional scale, as drought affects decomposition directly by emersion of bags and indirectly by affecting the functional composition and density of detritivores.

In-stream litter decomposition along an altitudinal gradient: does substrate quality matter?

In temperate streams, water temperature and organic matter inputs from surrounding forest vary along the altitude. We tested if the different features of streams of similar size determined by an altitudinal gradient might differentially affect the processing rate of different quality leaves (alder, oak and beech). To distinguish the relative contribution of microbial decomposition from overall decomposition, fine- and coarse-mesh bags were used. We determined decomposition rates, leaf-N and -P concentration, microbial respiration (fine bags), invertebrate colonisation (coarse bags) and density and identity of benthic invertebrates in three second-order streams. Alder decomposed faster than the other species in all three streams and regardless of mesh size due to its lower values of C:N, C:P and N:P. Unexpectedly, microbial decomposition rate did not vary among streams for any of the leaf species. The total decomposition rate of alder and oak showed a negative trend along the altitudinal gradient, the magnitude of the change in decomposition rates being similar for both species. The density and structure of the invertebrate community differed along the altitudinal gradient, related to temperature and surrounding vegetation, determining the decomposition rate. Unexpectedly, sensitivity of decomposition rate of different quality leaves to temperature does not differ along the gradient.

Biotic interactions mediate soil microbial feedbacks to climate change.

Decomposition of organic material by soil microbes generates an annual global release of 50-75 Pg carbon to the atmosphere, ∼7.5-9 times that of anthropogenic emissions worldwide. This process is sensitive to global change factors, which can drive carbon cycle-climate feedbacks with the potential to enhance atmospheric warming. Although the effects of interacting global change factors on soil microbial activity have been a widespread ecological focus, the regulatory effects of interspecific interactions are rarely considered in climate feedback studies. We explore the potential of soil animals to mediate microbial responses to warming and nitrogen enrichment within a long-term, field-based global change study. The combination of global change factors alleviated the bottom-up limitations on fungal growth, stimulating enzyme production and decomposition rates in the absence of soil animals. However, increased fungal biomass also stimulated consumption rates by soil invertebrates, restoring microbial process rates to levels observed under ambient conditions. Our results support the contemporary theory that top-down control in soil food webs is apparent only in the absence of bottom-up limitation. As such, when global change factors alleviate the bottom-up limitations on microbial activity, top-down control becomes an increasingly important regulatory force with the capacity to dampen the strength of positive carbon cycle-climate feedbacks.

The effects of eucalypt plantations on plant litter decomposition and macroinvertebrate communities in Iberian streams

Eucalypt plantations cover over 1.5 million ha in the Iberian Peninsula. The effects of the replacement of native deciduous forests by exotic plantations on stream communities and litter decomposition, a key ecosystem process in forest streams, are poorly understood. We compared microbially driven and total (microbes+invertebrates) decomposition of alder and oak leaf litter (high and low quality resource, respectively) as well as macroinvertebrate communities associated with decomposing litter and in the benthos, in five streams flowing through native deciduous broad-leaved forests and five streams flowing through eucalypt plantations in central Portugal and northern Spain (20 streams total). Total decomposition rate of alder leaf litter was slower in eucalypt than in deciduous streams, which was attributed to lower macroinvertebrate (and also shredder) colonization. No major effects of eucalypt plantations were found on macroinvertebrate colonization and total decomposition of oak litter, likely due to the low contribution of invertebrates to the decomposition of nutrient-poor litter. Microbially driven litter decomposition was generally not affected by forest change, likely due to high functional redundancy among microbes. Eucalypt streams had fewer invertebrates in Portugal than in Spain, which might be attributed to summer droughts in Portugal and the absence of deciduous riparian corridors in eucalypt plantations. In northern Spain, the relatively wet climate allows streams to flow year-round and eucalypt plantations have riparian deciduous trees that mitigate the effects of plantations. This study highlights the need to consider regional differences in climate, native vegetation, and the importance of macroinvertebrates, when assessing the effects of plantations on stream ecosystem processes such as carbon cycling. It also suggests that preservation of native riparian corridors, especially in drier areas, where the native vegetation provides high quality litter to the streams, and where invertebrates play an important role in aquatic processes, may mitigate the effects of plantations on stream communities and processes.

Nutrient Analysis, Decomposition, and Nitrogen Mineralization of Various Organic Amendments Used for Pechay Production

Farmers are increasingly using alternative fertilizer inputs such as manures and composts due to the high cost of inorganic fertilizers. However, different organic amendments have varied characteristics and thus also their effects on soil properties and the growth and yield of crops. The study was conducted to determine the characteristics and nutrient content of organic amendments, evaluate the decomposition rate and nitrogen mineralization rate of the various organic amendments, and examine the effects of different organic amendments on pechay. Four organic amendments used include swine manure, chicken dung, mudpress, and vermicast. These were analyzed for total elemental analysis, microbial decomposition, and nitrogen mineralization rate. A pot experiment was carried out to test the effects of these amendments on the growth and yield of pechay. Results showed that organic amended soil significantly increased the rates of microbial decomposition and nitrogen mineralization, organic material content, and the growth and yield of pechay. In terms of total elemental analysis, chicken dung had the highest content of essential nutrients such as P, K, Ca, and Cl as compared to mudpress, vermicast, swine manure, hence, has good potential as source of organic fertilizer for vegetable production. High amounts of heavy metals such as lead, strontium, nickel, copper, and zinc were detected in the swine manure.

Thermal Decomposition and Oxidation of CH3OH

Thermal decomposition of CH(3)OH diluted in Ar has been studied by monitoring H atoms behind reflected shock waves of 100 ppm CH(3)OH + Ar. The total decomposition rate k(1) for CH(3)OH + M → products obtained in this study is expressed as, ln(k(1)/cm(3) molecule(-1) s(-1)) = -(14.81 ± 1.22) - (38.86 ± 1.82) × 10(3)/T, over 1359-1644 K. The present result on k(1) is indicated to be substantially smaller than the extrapolation of the most of the previous experimental data but consistent with the published theoretical results [Faraday Discuss. 2002, 119, 191-205 and J. Phys. Chem. A 2007, 111, 3932-3950]. Oxidation of CH(3)OH has been studied also by monitoring H atoms behind shock waves of (0.35-100) ppm CH(3)OH + (100-400) ppm O(2) + Ar. For the low concentration CH(3)OH (below 10 ppm) + O(2) mixtures, the initial concentration of CH(3)OH is evaluated by comparing evolutions of H atoms in the same concentration of CH(3)OH with addition of 300 ppm H(2) diluted in Ar. The branching fraction for CH(3)OH + Ar → (1)CH(2) + H(2)O + Ar has been quantitatively evaluated from this comparative measurements with using recent experimental result on the yield of H atoms in the reaction of (1,3)CH(2) + O(2) [J. Phys. Chem. A 2012, 116, 9245-9254]; i.e., the branching fraction for the above reaction is evaluated as, φ(1a) = 0.20 ± 0.04 at T = 1880-2050 K, in the 1.3 and 3.5 ppm CH(3)OH + 100 ppm O(2) samples. An extended reaction mechanism for the pyrolysis and oxidation of CH(3)OH is constructed based on the results of the present study combined with the oxidation mechanism of natural gas [GRI-Mech 3.0]; evolution of H atoms can be predicted very well with this new reaction scheme over a wide concentration range for the pyrolysis (0.36-100 ppm CH(3)OH), and oxidation (0.36-100 ppm CH(3)OH + 100/400 ppm O(2)) of methanol.

Shock Tube Explorations of Roaming Radical Mechanisms: The Decompositions of Isobutane and Neopentane

The thermal decompositions of isobutane and neopentane have been studied using both shock tube experiments and ab initio transition state theory based master equation calculations. Dissociation rate constants for these molecules have been measured at high temperatures (1260-1566 K) behind reflected shock waves using high-sensitivity H-ARAS detection. The two major dissociation channels at high temperature are iso-C(4)H(10) → CH(3) + i-C(3)H(7) (1a) and neo-C(5)H(12) → CH(3) + t-C(4)H(9) (2a). Ultrahigh-sensitivity ARAS detection of H-atoms produced from the rapid decomposition of the product radicals, i-C(3)H(7) in (1a) and t-C(4)H(9) in (2a), through i-C(3)H(7) + M → H + C(3)H(6) + M (3a) and t-C(4)H(9) + M → H + i-C(4)H(8) + M (4a) allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, which were observed to be equivalent in both systems, k(1a)/k(total) and k(2a)/k(total) = 0.79 ± 0.05. Theoretical analyses indicate that in isobutane, the non-H-atom fraction has two contributions, the dominant fraction being due to the roaming radical mechanism leading to molecular products through iso-C(4)H(10) → CH(4) + C(3)H(6) (1b) with k(1b)/k(total) = 0.16, and a minor fraction that involves the isomerization of i-C(3)H(7) to n-C(3)H(7) that then subsequently forms methyl radicals, i-C(3)H(7) + M → n-C(3)H(7) + M → CH(3) + C(2)H(4) + M (3b). In contrast to isobutane, in neopentane, the contribution to the non-H-atom fraction is exclusively through the roaming radical mechanism that leads to neo-C(5)H(12) → CH(4) + i-C(4)H(8) (2b) with k(2b)/k(total) = 0.21. These quantitative measurements of larger contributions from the roaming mechanism for larger molecules are in agreement with the qualitative theoretical arguments that suggest long-range dispersion interactions (which become increasingly important for larger molecules) may enhance roaming.

Shock Tube and Theoretical Studies on the Thermal Decomposition of Propane: Evidence for a Roaming Radical Channel

The thermal decomposition of propane has been studied using both shock tube experiments and ab initio transition state theory-based master equation calculations. Dissociation rate constants for propane have been measured at high temperatures behind reflected shock waves using high-sensitivity H-ARAS detection and CH(3) optical absorption. The two major dissociation channels at high temperature are C(3)H(8) → CH(3) + C(2)H(5) (eq 1a) and C(3)H(8) → CH(4) + C(2)H(4) (eq 1b). Ultra high-sensitivity ARAS detection of H-atoms produced from the decomposition of the product, C(2)H(5), in (1a), allowed measurements of both the total decomposition rate constants, k(total), and the branching to radical products, k(1a)/k(total). Theoretical analyses indicate that the molecular products are formed exclusively through the roaming radical mechanism and that radical products are formed exclusively through channel 1a. The experiments were performed over the temperature range 1417-1819 K and gave a minor contribution of (10 ± 8%) due to roaming. A multipass CH(3) absorption diagnostic using a Zn resonance lamp was also developed and characterized in this work using the thermal decomposition of CH(3)I as a reference reaction. The measured rate constants for CH(3)I decomposition agreed with earlier determinations from this laboratory that were based on I-atom ARAS measurements. This CH(3) diagnostic was then used to detect radicals from channel 1a allowing lower temperature (1202-1543 K) measurements of k(1a) to be determined. Variable reaction coordinate-transition state theory was used to predict the high pressure limits for channel (1a) and other bond fission reactions in C(3)H(8). Conventional transition state theory calculations were also used to estimate rate constants for other tight transition state processes. These calculations predict a negligible contribution (<1%) from all other bond fission and tight transition state processes, indicating that the bond fission channel (1a) and the roaming channel (1b) are indeed the only active channels at the temperature and pressure ranges of the present experiments. The predicted reaction exo- and endothermicities are in excellent agreement with the current version of the Active Thermochemical Tables. Master equation calculations incorporating these transition state theory results yield predictions for the temperature and pressure dependence of the dissociation rate constants for channel 1a. The final theoretical results reliably reproduce the measured dissociation rate constants that are reported here and in the literature. The experimental data are well reproduced over the 500-2500 K and 1 × 10(-4) to 100 bar range (errors of ∼15% or less) by the following Troe parameters for Ar as the bath gas: k(∞) = 1.55 × 10(24)T(-2.034) exp(-45 490/T) s(-1), k(0) = 7.92 × 10(53)T(-16.67) exp(-50 380/T) cm(3) s(-1), and F(c) = 0.190 exp(-T/3091) + 0.810 exp(-T/128) + exp(-8829/T).

Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory

The thermal dissociation of dimethyl ether has been studied with a combination of reflected shock tube experiments and ab initio dynamics simulations coupled with transition state theory based master equation calculations. The experiments use the extraordinary sensitivity provided by H-atom ARAS detection with an unreversed light source to measure both the total decomposition rate and the branching to radical products versus molecular products, with the molecular products arising predominantly through roaming according to the theoretical analysis. The experimental observations also provide a measure of the rate coefficient for H + CH 3OCH 3. An evaluation of the available experimental results for H + CH 3OCH 3 can be expressed by a three parameter Arrhenius expression as, k = 6.54 × 10 - 24 T 4.13 exp ( - 896 / T ) cm 3 molecule - 1 s - 1 ( 273 - 1465 K ) The potential energy surface is explored with high level ab initio electronic structure theory. The dynamics of roaming versus radical formation is studied with a reduced dimensional trajectory approach. The requisite potential energy surface is obtained from an interpolative moving least squares fit to wide-ranging ab initio data for the long-range interactions between methyl and methoxy. The predicted roaming and radical micro-canonical fluxes are incorporated in a master equation treatment of the temperature and pressure dependence of the dissociation process. The tight (i.e., non-roaming) transition states leading to a variety of additional molecular fragments are also included in the master equation analysis, but are predicted to have a negligible contribution to product formation. The final theoretical results reliably reproduce the measured dissociation rate to radical products reported here and are well reproduced over the 500–2000 K temperature range and the 0.01–300 bar pressure range by the following modified Arrhenius parameters for the Troe falloff format: k 1 , ∞ ( T ) = 2.33 × 10 19 T - 0.661 exp ( - 42345 / T ) s - 1 k 1 , 0 ( T ) = 2.86 × 10 35 T - 11.4 exp ( - 46953 / T ) cm 3 molecule - 1 s - 1 F cent ( T ) = exp ( - T / 880 ) The experimentally observed branching ratio of 0.19 ± 0.07 provides a direct measure of the contribution from the roaming radical mechanism. The theoretical analysis predicts a much smaller roaming contribution of 0.02.

ChemInform Abstract: Crystalline Structure and Thermal Stability of Double Strontium and Barium Carbonates.

Abstract The crystalline structures of double salts of composition SrxBa1-xCO3 have been analysed. It has been shown that this series of compounds crystallizes in the orthorhombic system over the entire composition range. Moreover, they conform to Veghard's law. The thermal decomposition of a series of samples of composition SrxBa1-xCO3 (x = 0.2, 0.4, 0.5, 0.6 and 0.8) has been studied by means of the constant rate thermal analysis (CRTA) technique. This method permits monitoring of the vacuum and the sample temperature in such a way that both the pressure in the close vicinity of the sample and the total decomposition rate remain constant over the whole reaction range. The results obtained at a CO2 residual pressure of 4 ×10−5 Torr show that the thermal decomposition of BaSr double salts takes place in a single step, instead of the two step process previously claimed in the literature. The comparative kinetic analysis of both CRTA and TG curves at high vacuum has allowed the reaction mechanism and the activation energy for the process to be established.

Shock Tube Study on the Thermal Decomposition of CH3OH

H atom produced in the thermal decomposition of CH(3)OH highly diluted in Ar (0.48-10 ppm) was monitored behind reflected shock waves by atomic resonance absorption spectrometry (ARAS) at fixed temperatures (and pressures), that is, 1660 (1.73 atm), 1760 (2.34 atm), 1860 (2.04 atm), 1950 (2.18 atm), and 2050 K (1.76 atm) (+/-10 K, respectively). High sensitivity for the H atom has been attained by signal averaging of the ARAS signals down to the concentrations of approximately 1 x 10(11) atoms/cm(3) and enables us to determine the branching fraction for the direct H atom production channel, CH(3)OH --> CH(2)OH + H (channel 1c ) in a mixture of 1 ppm CH(3)OH. Channel 1c is confirmed to be minor, that is, branching fraction for channel 1c is expressed by Log(k(1c)/k(1)) = (- 2.88 +/- 1.88) x 10(3)/T - (0.23 +/- 1.02), which corresponds to k(1c)/k(1) < 0.03 for the present temperature range. By using 0.48 and 1.0 ppm CH(3)OH with (100-1000) ppm H(2), the total decomposition rate k(1) for CH(3)OH --> products is measured from the time dependence of H atom, where the radical products of main channels 1a and 1b , that is, OH, CH(3), and CH(2), were converted rapidly into H atoms. The experimental result is summarized as Log(k(1)/cm(3)molecule(-1)s(-1)) = (-12.82 +/- 0.71) x 10(3)/T - (8.5 +/- 0.38). A theoretical study based on ab initio/TST calculations with high accuracy has been conducted for the reaction: (3)CH(2) + H(2) --> CH(3) + H (reaction 3 ). The rate is given by k(3)/cm(3)molecule(-1) s(-1) = (7.32 x 10(-19))T(2.3) exp (-3699/T). This result is used for numerical simulations to evaluate k(1). Present experimental results on the thermal decomposition rate of CH(3)OH are found to be consistent with previous works. It is also found that time dependence of [H] observed in the 10 ppm CH(3)OH in Ar can be reproduced very well by kinetic simulations by using a reaction mechanism composed of 36 elementary reactions.