Riverine Export of Aged Carbon Driven by Flow Path Depth and Residence Time

The flux of terrestrial C to rivers has increased relative to preindustrial levels, a fraction of which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the ocean, or (bio)­chemically processed and released as CO₂. Disturbance changes land cover and hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major ions from 19 rivers draining the coterminous U.S. and Arctic. DOC optics indicated that the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC tended to consist of smaller, microbial degradation products. Aged DOC exports, observed regularly in arid basins and during base flow in arctic rivers, are associated with higher proportion of mineral weathering products, suggesting deeper flows paths. These patterns also indicate potential for production of microbial byproducts as DOC ages in soil and water with longer periods of time between production and transport. Thus, changes in hydrology associated with landscape alteration (e.g., tilling or shifting climates) that can result in deeper flow paths or longer residence times will likely lead to a greater proportion of aged carbon in riverine exports

Biochar-Induced Changes in Soil Hydraulic Conductivity and Dissolved Nutrient Fluxes Constrained by Laboratory Experiments

<div><p>The addition of charcoal (or biochar) to soil has significant carbon sequestration and agronomic potential, making it important to determine how this potentially large anthropogenic carbon influx will alter ecosystem functions. We used column experiments to quantify how hydrologic and nutrient-retention characteristics of three soil materials differed with biochar amendment. We compared three homogeneous soil materials (sand, organic-rich topsoil, and clay-rich Hapludert) to provide a basic understanding of biochar-soil-water interactions. On average, biochar amendment decreased saturated hydraulic conductivity (<i>K</i>) by 92% in sand and 67% in organic soil, but increased <i>K</i> by 328% in clay-rich soil. The change in <i>K</i> for sand was not predicted by the accompanying physical changes to the soil mixture; the sand-biochar mixture was less dense and more porous than sand without biochar. We propose two hydrologic pathways that are potential drivers for this behavior: one through the interstitial biochar-sand space and a second through pores within the biochar grains themselves. This second pathway adds to the porosity of the soil mixture; however, it likely does not add to the effective soil <i>K</i> due to its tortuosity and smaller pore size. Therefore, the addition of biochar can increase or decrease soil drainage, and suggests that any potential improvement of water delivery to plants is dependent on soil type, biochar amendment rate, and biochar properties. Changes in dissolved carbon (C) and nitrogen (N) fluxes also differed; with biochar increasing the C flux from organic-poor sand, decreasing it from organic-rich soils, and retaining small amounts of soil-derived N. The aromaticity of C lost from sand and clay increased, suggesting lost C was biochar-derived; though the loss accounts for only 0.05% of added biochar-C. Thus, the direction and magnitude of hydraulic, C, and N changes associated with biochar amendments are soil type (composition and particle size) dependent.</p></div

The mean (and standard deviation) of physical, hydraulic, and nutrient properties of the three replicates of each soil and soil+biochar treatment.

<p>Two-tailed t-tests were conducted to determine statistical differences between control and +biochar treatments; p-values are shown in italics below mean and standard deviation values for each treatment.</p><p>The mean (and standard deviation) of physical, hydraulic, and nutrient properties of the three replicates of each soil and soil+biochar treatment.</p

A comparison of studies that examined the impact of biochar amendments on soil saturated hydraulic conductivity (<i>K</i>) and bulk density (ρ_d).

a<p> Biochar application rate reported in paper.</p>b<p> Biochar application rate converted using bulk density of soils or column materials provided in the paper and assuming tillage depth of 10 cm.</p><p>NR: not reported.</p><p>NA: not applicable.</p><p>Biochar amendment rates are provided two ways: the units provided by the study and in tons biochar ha⁻¹. The conversion assumed a tillage depth of 10 cm and the bulk density of the soil or column materials provided in the paper. Studies are organized by soil type, top to bottom: organic-rich soils, clay- and silt-rich soils, and sandy soils. Results from this study are in italics.</p><p>A comparison of studies that examined the impact of biochar amendments on soil saturated hydraulic conductivity (<i>K</i>) and bulk density (ρ_d).</p

Impact of biochar amendment on saturated soil hydraulic conductivity.

<p>The saturated hydraulic conductivity (<i>K</i>), as measured using falling head experiments, for six soil treatments over subsequent flushing events: (a) sand and sand+biochar, (b) organic and organic+biochar, and (c) clay and clay+biochar. Note: the different soil treatment flushing events varied in duration with the clay (c) taking up to 10× longer to drain than the sand (a) or organic soil (b). Saturated hydraulic conductivity data and flushing duration for each flushing experiment available in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108340#pone.0108340.s001" target="_blank">Table S1</a>.</p

Promoting professional identity, motivation, and persistence: Benefits of an informal mentoring program for female undergraduate students

<div><p>Women are underrepresented in a number of science, technology, engineering, and mathematics (STEM) disciplines. Limited diversity in the development of the STEM workforce has negative implications for scientific innovation, creativity, and social relevance. The current study reports the first-year results of the PROmoting Geoscience Research, Education, and SuccesS (PROGRESS) program, a novel theory-driven informal mentoring program aimed at supporting first- and second-year female STEM majors. Using a prospective, longitudinal, multi-site (i.e., 7 universities in Colorado/Wyoming Front Range & Carolinas), propensity score matched design, we compare mentoring and persistence outcomes for women in and out of PROGRESS (<i>N</i> = 116). Women in PROGRESS attended an off-site weekend workshop and gained access to a network of volunteer female scientific mentors from on- and off-campus (i.e., university faculty, graduate students, and outside scientific professionals). The results indicate that women in PROGRESS had larger networks of developmental mentoring relationships and were more likely to be mentored by faculty members and peers than matched controls. Mentoring support from a faculty member benefited early-undergraduate women by strengthening their scientific identity and their interest in earth and environmental science career pathways. Further, support from a faculty mentor had a positive indirect impact on women’s scientific persistence intentions, through strengthened scientific identity development. These results imply that first- and second- year undergraduate women’s mentoring support networks can be enhanced through provision of protégé training and access to more senior women in the sciences willing to provide mentoring support.</p></div

Summary of mentor support descriptive statistics as a function of PROGRESS status (<i>N</i> = 116).

<p>Summary of mentor support descriptive statistics as a function of PROGRESS status (<i>N</i> = 116).</p

Conceptual mediation model linking mentoring support to motivation and persistence through professional identity development.

<p>Conceptual mediation model linking mentoring support to motivation and persistence through professional identity development.</p

Summary of the final step of hierarchical regression models predicting outcomes (science identity, deep interest, & persistence intentions) from relevant predictors and controls.

<p>Summary of the final step of hierarchical regression models predicting outcomes (science identity, deep interest, & persistence intentions) from relevant predictors and controls.</p