Can Carbon Fluxes Explain Differences in Soil Organic Carbon Storage Under Aspen and Conifer Forest Overstories?

Climate- and management-induced changes in tree species distributions are raising questions regarding tree species-specific effects on soil organic carbon (SOC) storage and stability. Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, but fire exclusion often promotes the succession to conifer dominated forests. Aspen in the Western US have been found to store more SOC in the mineral soil than nearby conifers, but we do not yet fully understand the source of this differential SOC accumulation. We measured total SOC storage (0–50 cm), characterized stable and labile SOC pools, and quantified above- and belowground litter inputs and dissolved organic carbon (DOC) fluxes during snowmelt in plots located in N and S Utah, to elucidate the role of foliage vs. root detritus in SOC storage and stabilization in both ecosystems. While leaf litterfall was twice as high under aspen as under conifers, input of litter-derived DOC with snowmelt water was consistently higher under conifers. Fine root (mm) biomass, estimated root detritus input, and root-derived DOC fluxes were also higher under conifers. A strong positive relationship between root and light fraction C content suggests that root detritus mostly fueled the labile fraction of SOC. Overall, neither differences in above- and belowground detritus C inputs nor in detritus-derived DOC fluxes could explain the higher and more stable SOC pools under aspen. We hypothesize that root–microbe–soil interactions in the rhizosphere are more likely to drive these SOC pool differences

3D Shape Optimization with X-FEM and a Level Set Constructive Geometry Approach

This paper extends previous work on structural optimization with the eXtended Finite Element Method (X-FEM) and the Level Set description of the geometry. The proposed method takes advantage of fixed mesh approach by using an X-FEM structural analysis method and from the geometrical shape representation of the Level Set description. In order to allow the optimization of complex geometries represented with a Level Set description, we apply here a Constructive Solid Geometry (CSG) approach with the Level Set geometrical representation. Hence, this extension allows to optimize any boundary of the structure that is defined with a coumpound Level Set. Design variables are the parameters of basic geometric primitives which are described with a Level Set representation and/or the control points of the NURBS curves that act as the definition of an advanced Level Set primitive. The number of design variables of this formulation remains small whereas global (i.e. compliance or eigenfrequency) and local constraints (i.e. stresses) can be considered. Our results illustrate that fixed grid optimization with X-FEM coupled to a Level Set geometrical description is a promising technique to achieve structural shape optimization

Recognition Through Awards: A Source of Gender Inequality in Science?

Drawing from Acker’s gendered organizations perspective, this study analyzes the gender distribution of research and non-research awards in STEM (Science, Technology, Engineering, Mathematics) colleges at a mid-size public doctoral university in the western U.S. This analysis is complemented with a faculty survey (2016) elucidating faculty perceptions of the nomination process and their standing within their department and college. Despite an increase in the number of women among STEM faculty over time, women remain underrepresented among research award recipients, especially at the university level. The ratio of research to nonresearch awards for men is 3 to 6 times that of women faculty. Differences in productivity cannot be invoked as a mechanism for this gendered awards distribution. Women report being overlooked in the nomination process for all awards. This study suggests that the nomination and selection processes put women at an evaluative disadvantage with respect to high-status research awards. Social proximity tends to neutralize some of the evaluation bias at the college level

The Impact of Nitrification on Soil Acidification and Cation Leaching in a Red Alder Ecosystem

The objectives of this study were to investigate the impacts of internal nitrification on soil and soil solution acidity and on the rate of nutrient export through NO3– mediated leaching. This was achieved by comparing soil chemical properties and soil solution composition within a naturally N-rich red alder (Alnus rubra Bong.) ecosystem to those of an adjacent Douglas-fir [Pseudotsuga menziesil (Mirbel) Franco] forest where soil N levels were significantly lower and no measurable HNO3 production could be observed. In the red alder system, where \u3e 100 kg ha–1 yr–1 of N were added through symbiotic N2 fixation, the net annual NO3– leaching past the 40-cm soil depth amounted to 3460 mol charges ha–1, and NO3– concentrations in the solutions collected below 40 cm periodically exceeded drinking water standards of 10 mg L–1. The H+ and NO3– release was most pronounced in the forest floor and top 10 cm of the soil under alder occupancy and caused significant acidification of percolating solutions. Less than 1% of the total H+ input from internal (nitrification) and external (atmospheric) sources leached below the 40-cm depth, which was indicative for the strong buffering capacity of this particular soil. The cation displacement reactions involved in this pH buffering caused a 15% decline in base saturation and a significant acidification of the upper part of the soil profile. The presence of large amounts of mobile NO3– in solution triggered accelerated cation leaching, causing a selective redistribution of primarily exchangeable Ca2+ from the A to the B horizon. These field studies lead us to conclude that the rate and the selectivity of NO3– mediated leaching in a red alder system could significantly lower the exchangeable cation pool in the rooting zone or cause nutrient imbalance, if a site is managed for repeated rotations of red alder

Acidification Sources in Red Alder and Douglas-Fir Soils -- Importance of Nitrification

Precipitation, throughfall, forest floor, and soil leachate samples were monitored continuously in 1981 and 1982 in a N-poor Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] forest and a red alder (Alnus rubra Bong.) forest growing adjacently on a glacial soil in western Washington. The purpose of the study was to quantify the relative importance of atmospheric vs. natural sources of H+ input to forest soil acidification, and to determine the role of N transformation processes in the overall H+ balance of soils with different N status. Rainwater samples had an avg pH of 4.7 and annual H+ deposition via precipitation averaged 320 mol H+ ha–1 yr–1. This was modest compared to internal H+ production associated with HCO-3 and NO-3 formation. In the soil under alder cover, which was naturally enriched in N through symbiotic N2-fixation, nitrification released up to 4500 mol H+ ha–1 annually to the solution percolating through the upper part of the soil profile. In the N-poor soil no nitrification could be observed and N transformation processes had a minor influence on the soil H+ balance. The main internal acidification source in this case was H2CO3 dissociation releasing 420 mol H+ ha–1 yr–1. In both instances, soil solutions appeared well buffered against these external and internal acidification sources, and few H+ leached below the 40-cm soil depth

Analytical sensitivity analysis using the extended finite element method in shape optimization of bimaterial structures

peer reviewedThe present work investigates the shape optimization of bimaterial structures. The problem is formulated using a level set description of the geometry and the extended finite element method (XFEM) to enable an easy treatment of complex geometries. A key issue comes from the sensitivity analysis of the structural responses with respect to the design parameters ruling the boundaries. Even if the approach does not imply any mesh modification, the study shows that shape modifications lead to difficulties when the perturbation of the level sets modifies the set of extended finite elements. To circumvent the problem, an analytical sensitivity analysis of the structural system is developed. Differences between the sensitivity analysis using FEM or XFEM are put in evidence. To conduct the sensitivity analysis, an efficient approach to evaluate the so-called velocity field is developed within the XFEM domain. The proposed approach determines a continuous velocity field in a boundary layer around the zero level set using a local finite element approximation. The analytical sensitivity analysis is validated against the finite differences and a semi- analytical approach. Finally our shape optimization tool for bimaterial structures is illustrated by revisiting the classical problem of the shape of soft and stiff inclusions in plates

Aspen Soils Retain More Dissolved Organic Carbon Than Conifer Soils in a Sorption Experiment

The effect tree species have on soil organic carbon (SOC) has been hotly debated but, so far, few clear patterns have emerged. One example of a differing tree species effect on SOC is aspen forests in North America, which have been found to have more stable SOC than adjacent conifer forest stands. An important source for the formation of stable organo-mineral complexes in the soil is dissolved organic carbon (DOC). DOC concentrations in mineral soil are often higher under the thick O-horizons of conifer forests than under aspen forests, but this does not correspond to more stable mineral SOC. This suggests that, instead of DOC concentration, DOC quality could be driving the observed differences in SOC. Therefore, we quantified the retention of contrasting forest detritus DOC in soils. Using a batch sorption experiment approach, we compared the retention of detritus leachates from four sources – aspen leaves (AL), aspen roots (AR), conifer (subalpine fir) needles (CN), and conifer (subalpine fir) roots (CR) – on soils sampled under aspen and conifer (subalpine fir and Douglas fir) overstories. The calculated sorption isotherms showed higher retention of AL DOC than AR DOC, as indicated by all four sorption parameters – k and n (curve-fitting parameters), null point concentration (NPC; net sorption = net desorption), and endpoint (EP, retention at the highest initial DOC concentration). Leachates from CN and CR showed very similar retention behavior, and between the two species, the retention of root leachates was more similar than the retention of foliage leachates. Soils sampled from aspen forests showed a higher affinity for new DOC than conifer soils [higher sorption rate (n), lower NPC, and higher EP] regardless of the DOC source. The findings suggest that the higher DOC sorption on aspen soils might be a major driver for more stable SOC under aspen stands in North America

Soil Nitrogen Dynamics Following Harvesting and Conversion of Red Alder and Douglas-Fir Stands

Drastic reductions in NO3- leaching have been observed after harvesting of mature red alder (Alnus rubra Bong.) stands. Our objective was to examine whether these reduction were linked to changes in soil N dynamics. Adjacent alder and Douglas fir [Pseudotsuga menziessii (Mirbel.) Franco] stands on young glacial soils (Alderwood; a loamy-skeletal, mixed, mesic, ortstein Aquic Haplorthod) in western Washington were harvested and replanted with either alder or Douglas fir seedlings; reference plots were established in nearby undisturbed stands. Three years after site conversion, when NO3- leaching declined most drastically in the harvested alder plots, net N mineralization and net nitrification in the upper soil were determined seasonally by in situ incubation using the buried-bag technique. There were no significant increases in soil NH4-N during incubation, indicating that nitrification rates closely followed net N mineralization in all plots. In the N-poor soils of the harvested and uncut Douglas fir plots, nitrification was consistently \u3c2 \u3eµg N g–1 per 30 d. In the N-rich alder reference plot, nitrification was highest in summer (52 µg N g–1 per 30 d), lowest in winter (4 µg N g–1), and intermediate in spring (20 µg N g–1). Seasonal fluctuations in nitrification diminished in the harvested alder plots. Nitrification rates were significantly lower than in the uncut alder plot in summer only (5–9 µg N g–1 per 30 d), but this difference was insufficient to account for the lower NO3- leaching rates in the conversion plots. Soils in the harvested plots were drier and experienced greater temperature extremes, but these changes did not correlate well with the variability in N-mineralization rates between uncut and harvested alder plots. Nitrification was positively correlated to soil temperature in the uncut alder plot only. The establishment of red alder seedlings did not appear to have influenced the N dynamics in either N-poor or N-rich soils

Advancement to the Highest Faculty Ranks in Academic STEM: Explaining the Gender Gap at USU

Science and technology (S&T) disciplines at universities are still largely dominated by men, and few women are found in the highest employment ranks. Using the faculty data from Utah State University, this thesis explores the factors that help explain the difference in career trajectory between men and women in the S&T colleges. While there were few women in S&T colleges prior to the 1990s, more women have been hired since then, and the lower ranks of the faculty corps are starting to reflect the gender composition of the doctoral degree holders in the different fields. This is not the case for full professors and leadership positions, where women are still underrepresented. Analysis of the university careers of faculty in S&T colleges revealed that in the absence of formal guidelines, promotions to full professor were inconsistent, but men were generally promoted faster, especially when hired as associate professors. However, conscious measures by the university to make the promotion process more transparent and formalized reduced the variability in time to promotion to full professor for all faculty and minimized differences between men and women. Recognition of faculty achievement through awards still shows gender bias. Women proportionally receive less awards, especially in recognition of research activities. This study illustrates that clear guidelines reduce gender bias in decisions and improve gender equity in the academia

A tree species effect on soil that is consistent across the species\u27 range: the case of aspen and soil carbon in North America.

Trembling aspen covers a large geographic range in North America, and previous studies reported that a better understanding of its singular influence on soil properties and processes is of high relevance for global change questions. Here we investigate the potential impact of a shift in aspen abundance on soil carbon sequestration and soil carbon stability at the continental scale by conducting a systematic literature review using 23 published studies. Our review shows that aspen’s effect on soil carbon is relatively consistent throughout the species range. Aspen stores less C in the forest floor but similar amounts in the mineral soil relative to conifers. However, a robust set of indicators of soil C stability, for example, degree of organo-mineral associations, proportion of readily-available or labile C estimated during long-term soil incubations or using hot-water extraction, pattern of soil C distribution, and temperature sensitivity of soil heterotrophic respiration, reveals that the soil organic carbon (SOC) stock under aspen is more stable, rendering it more protected against environmental changes and soil disturbances. Therefore, our continental-scale analysis highlights that an increase in the abundance of trembling aspen in North American forests may increase the resistance and resilience of soil C stocks against global changes