Understanding microbial contributions to soil aggregation and organic matter accumulation

The goal of the project investigators was to characterize soil bacterial and fungal communities and the rates at which they break down specific plant-derived carbon (C) molecules within soil aggregates in three farming systems

Soil aggregate distribution and turnover affects soil microbial ecology and ecosystem processes in three bioenergy systems

Soil microorganisms are the drivers of ecosystem biogeochemistry, but the field of soil ecology lags behind other biological disciplines in understanding habitat constraints driving biodiversity and ecosystem functioning of these microorganisms. One approach to characterizing habitat at the micro-scale is to subset soil into naturally occurring physical associations of particles, organic matter, and microbes called aggregates. However, classic methods of separating soil aggregates can drastically change microbial communities and activities. In this dissertation, a methodological approach to isolating soil aggregates for biological analysis is refined and then applied to evaluating microbial activity and fungal community structure in three ecosystems managed for bioenergy production. Subsequently, aggregate-scale processes are scaled out to contrast ecosystem carbon (C) and nitrogen (N) cycling in the three ecosystems. Direct contrast of soil extracellular enzyme activities within soil aggregates isolated by traditional slaking means, dry sieving, and optimal moisture found wet sieved large macroaggregates (\u3e2000 µm) had four times greater activity than macroaggregates isolated by the other methods. Very high activity in wet-sieved macroaggregates led to mass-proportional sums of enzyme activities to exceed 100% of whole soil measurements. This indicates the wet-sieving procedure induces enzyme activity not observed in whole soil or in dry and optimal moisture aggregates. Although there were few differences in enzyme activity between dry-sieved and optimal moisture aggregates, the additional care taken in optimal moisture sieving, including rapid partial-drying under sterile conditions at 4°C, may be more important in other metrics of soil biological analysis including DNA-based assays. Using the optimal moisture approach, I contrasted extracellular enzyme activity and C and N resources within aggregates isolated across two growing seasons from three ecosystems managed for bioenergy feedstock production: row-crop continuous corn agroecosystems and reconstructed tallgrass prairie with and without annual inorganic N fertilizer application. Across aggregate fractions, N-acetyl-glucosaminidase (NAG) activity was greatest in large macroaggregates and cellobiohydrolase activity was greatest in microaggregates (\u3c250 µm). Increased NAG activity in large macroaggregates is likely driven by greater total C and N within that fraction. Increased cellobiohydrolase activity in microaggregates may indicate enzymes are largely stabilized on the outside of microaggregates, interacting with substrates in the surrounding pore networks rather than intra-aggregate organic matter. Aggregate turn-over was detected across the two growing seasons and disintegration of large macroaggregates corresponded with peaks in enzyme activity. Release of organic matter with aggregate turn-over may be a driver of these spikes in enzyme activity. Fungi are major components of soil microbial communities and perform several important decomposition functions. However, there has been less research dedicated to fungal communities and the forces structuring them compared with soil bacteria. I utilized the optimal moisture aggregate isolation approach to evaluate soil fungal communities within soil aggregates from the aforementioned bioenergy cropping systems. Fungal richness was much greater in microaggregates compared with large macroaggregates and proportional sums of aggregate-level richness indicate whole soil sampling approaches are underestimating fungal richness two-fold. However, fungal community structure was affected by ecosystem to greater extent than aggregation. Unfertilized prairies supported greater abundance of members of the Basidiomycota family Strophariaceae, and genus Limonomyces, which were not present in corn or fertilized prairie systems, but were highly abundant in prairie systems. Fertilized prairies contained greater abundance of fungi from the Basidiomycota family Psathyrellaceae and genus Thanatephours as well as Ascomycota family Orbiliaceae and Trichoderma citrinoviride. Corn communities were distinguished with greater abundance of unknown Basidiomycota and arbuscular mycorrhizal fungi from the order Glomerales. Aggregate-scale differences in extracellular enzyme activity and fungal communities contributed to ecosystem-scale differences in C and N cycling between the unfertilized prairie, fertilized prairie, and corn systems. Whole soil analysis showed fertilized prairies accrued more soil C and N than unfertilized prairies and corn systems, driven by increased microbial biomass, enzyme activity, and aggregation in fertilized prairie systems. Fertilized prairies had greater C-inputs in the form of roots than corn systems and greater inorganic N inputs, in the form of fertilizer, than unfertilized prairies. Thus, coupling of C and N inputs in fertilized prairies facilitated microbial growth and activity, which in turn enhanced soil aggregation, protection of microbially-processed organic matter, and total soil C and N pools. In summary, this dissertation provides a replicable method for soil aggregate isolation that can be used to test hypotheses about soil habitat influences on microbial communities and activities. It was determined that soil enzyme activity varied between aggregate fractions similarly in three managed ecosystems, reflecting consistent aggregate-level controls on enzyme activities. Fungal community structure was also influenced by aggregate fraction, but the ecosystem-level response was much greater. Together, differences in aggregate habitat, in concert with plant inputs and management regime, influenced C and N cycling in corn, unfertilized and fertilized prairie ecosystems. Thus, knowledge of microbial responses at the soil aggregate scale can refine and improve scientific understanding of terrestrial ecosystem biogeochemistry

Chirp compensation in active mode-locked semiconductor diode laser using a DFB

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (p. 85-86).by Susan Bach.M.Eng

The Legislative Process on the House Floor: An Introduction

This report discusses the complicated body of rules, precedents, and practices that governs the legislative process on the floor of the House of Representatives

Floor Procedure in the House of Representatives: A Brief Overview

The House considers bills and resolutions on the floor under several different sets of procedures governing the time for debate and the opportunities for amendment. Some procedures allow 40 or 60 minutes for debate; others permit debate to continue until a majority of Members vote to end it. Some procedures prohibit most or all floor amendments; others allow Members to offer any amendments that meet the requirements of the House’s rules and precedents. Notwithstanding these differences, the rules, precedents, and practices of the House generally are designed to permit the majority to work its will in a timely manner. This report provides a brief overview of this procedure

Floor Procedure in the House of Representatives: A Brief Overview

The House considers bills and resolutions on the floor under several different sets of procedures governing the time for debate and the opportunities for amendment. Some procedures allow 40 or 60 minutes for debate; others permit debate to continue until a majority of Members vote to end it. Some procedures prohibit most or all floor amendments; others allow Members to offer any amendments that meet the requirements of the House’s rules and precedents. Notwithstanding these differences, the rules, precedents, and practices of the House generally are designed to permit the majority to work its will in a timely manner. This report provides a brief overview of this procedure

Going to Conference in the Senate

This report discusses the steps that the Senate must take, and one more step that it may take, as it arranges to send a bill to conference committee

Spatial Structuring of Cellulase Gene Abundance and Activity in Soil

Microbial mechanisms controlling cellulose degradation in soil habitats remains a critical knowledge gap in understanding and modeling terrestrial carbon-cycling. We investigated land management and soil micro-habitat influences on soil bacterial communities and distribution of cellulose-degrading enzyme genes in three bioenergy cropping systems (corn, prairie, and fertilized prairie). Within the soil, aggregates have been examined as potential micro- habitats with specific characteristics influencing resource partitioning and regulation, thus we also investigated genes associated with cellulose degradation within soil aggregate fractions from the fertilized prairie system. Soil bacterial communities and carbon-cycling gene presence varied across land management and soil microhabitats. Examination of genes specifically involved in cellulose-degradation pathways showed high levels of redundancy across the bioenergy cropping systems, but medium macroaggregates (1,000–2,000 μm) supported greater cellulose-degrading enzyme gene abundance than other aggregate fractions and whole soil. In medium aggregates, the enriched cellulose-degrading genes were most similar to genes previously observed in Actinobacteria. These findings represent gentic potential only, and our previous work on the same samples found elevated cellulase exo-enzyme activity in microaggregates. These contrasting results emphasize the importance of measuring community, functional genes, and metabolic potentials in a coordinated manner. Together, these data indicate that location within the soil matrix matters. Overall, our results indicate that soil aggregate environments are hot-spots that select for organisms with functional attributes like cellulose degradation, and future work should further explore micro-environmental factors that affect realized C-cycling processes

Precision medicine to improve use of bleeding avoidance strategies and reduce bleeding in patients undergoing percutaneous coronary intervention: Prospective cohort study before and after implementation of personalized bleeding risks

Objective To examine whether prospective bleeding risk estimates for patients undergoing percutaneous coronary intervention could improve the use of bleeding avoidance strategies and reduce bleeding. Design Prospective cohort study comparing the use of bleeding avoidance strategies and bleeding rates before and after implementation of prospective risk stratification for peri-procedural bleeding. Setting Nine hospitals in the United States. Participants All patients undergoing percutaneous coronary intervention for indications other than primary reperfusion for ST elevation myocardial infarction. Main outcome measures Use of bleeding avoidance strategies, including bivalirudin, radial approach, and vascular closure devices, and peri-procedural bleeding rates, stratified by bleeding risk. Observed changes were adjusted for changes observed in a pool of 1135 hospitals without access to pre-procedural risk stratification. Hospital level and physician level variability in use of bleeding avoidance strategies was examined. Results In a comparison of 7408 pre-intervention procedures with 3529 post-intervention procedures, use of bleeding avoidance strategies within intervention sites increased with pre-procedural risk stratification (odds ratio 1.81, 95% confidence interval 1.44 to 2.27), particularly among higher risk patients (2.03, 1.58 to 2.61; 1.41, 1.09 to 1.83 in low risk patients, after adjustment for control sites; P for interaction=0.05). Bleeding rates within intervention sites were significantly lower after implementation of risk stratification (1.0% v 1.7%; odds ratio 0.56, 0.40 to 0.78; 0.62, 0.44 to 0.87, after adjustment); the reduction in bleeding was greatest in high risk patients. Marked variability in use of bleeding avoidance strategies was observed across sites and physicians, both before and after implementation. Conclusions Prospective provision of individualized bleeding risk estimates was associated with increased use of bleeding avoidance strategies and lower bleeding rates. Marked variability between providers highlights an important opportunity to improve the consistency, safety, and quality of care. Study registration Clinicaltrials.gov NCT01383382