Slope Sites Research Articles

Early warning system using tilt sensors in Chibo, Kalimpong, Darjeeling Himalayas, India

The Indian Himalayan regions have been significantly affected by the increase in the frequency of landslide occurrence. Thirty percentage of the worldwide landslide incidents occur in the Himalayan region with 42% of India’s landslide region belonging to Darjeeling–Sikkim Himalayas (NDMA Report, 2011). Several studies have been carried out worldwide on early warning systems considering rainfall history. Although rainfall is one of the criteria which can successfully predict the probability of landslides on a regional scale, it is indeed difficult to understand the risk associated with slope failure. The reason is the spatial variation of rainfall intensity, local soil conditions, geology, hydrology and topography. An early warning and monitoring system is one of the most effective techniques to minimise the disasters influenced by slope instability as it is less expensive and easier than slope reinforcement. A reliable and robust system fitted with microelectromechanical systems tilt sensor and volumetric water content sensors were installed. The sensor monitors the tilting angle of the instrument which was installed at shallow depths, and the variation of tilting angle corresponds to lateral displacement at slope surface. The primary objective was to monitor the tilting angles at various slope sites and to formulate a dependable warning system with a low probability of false alarms in Chibo Pashyor region in the Indian state of West Bengal. Such system would assist in the installation of several low-cost sensors over an active slope as it was difficult to determine the particular section of slope which could fail in the next heavy rainfall. The change in tilting rate would help in establishing thresholds determined empirically. Such system would help in developing an early warning system and also help in calibrating thresholds calculated using empirical techniques.

Paleomagnetism of IODP Site U1380: Implications for the Forearc Deformation in the Costa Rican Erosive Convergent Margin

The destructive nature of subduction erosion poses challenges to fully understanding the evolution of erosive convergent margins that are critical to understanding crustal recycling and seismogenesis. Forearc deformation holds important clues to the evolution of erosive convergent margins. Here we present detailed paleomagnetic and structural analyses of IODP Site U1380 cores from the middle slope of the forearc of the Costa Rican erosive convergent margin. The analyses reveal a strong deformation zone from ~490 to ~550 mbsf that is characterized by abundant fissility/foliations shallower than the bedding. Similar relatively strong deformation zones are recognized from the frontal prism and upper slope sites, and are broadly correlative, forming a zone of strong deformation across the forearc. This zone spans ~2.0 to 1.83 Ma and the deformation likely occurred briefly at ~1.80 Ma. The widespread, short-lived, and strong deformation is interpreted as a result of intense subhorizontal shear following the rapid forearc subsidence driven by the dramatic subduction erosion associated with the abrupt onset of the Cocos Ridge subduction. Given the typical occurrence of forearc subsidence by subduction erosion, similar styles of deformation are probably common in other erosive convergent margins as well.

Weed seed dispersal via runoff water and eroded soil

High rainfall intensities on slopes produce runoff and erosion, but seeds are also carried down slope into streams by surface wash, causing weed infestation in lower parts of a river system. The study area was a cultivated valley (Houay Pano, Laos) farmed by smallholders and equipped with eight gauging stations: one draining the main catchment (60 ha) seven draining subcatchments (20–0.6 ha). All stations were used for sediment sampling, then sediments were scrutinized for seeds. The sampling took place simultaneously during, and immediately after, a single high intensity rainstorm. The event produced a total discharge of 1374 m3 measured at the outlet of the main catchment, in this water 4600 Mg suspended soil left the catchment together with approximately 1.34 million floating and suspended seeds. The rainstorm produced 2227 Mg of stream sediments (bedload), containing about 103,300 viable seeds. In total 131 species were identified, most seeds were weeds. The catchments were mostly covered by vegetation (fallow and forest) just 13% was cultivated, unprotected soil. Only unprotected soil produced runoff and erosion, whereas well-protected soil was subject to high seed loss. Seed dispersal without soil erosion occurred because the discharge needed to transport light-weight seeds is less than the force necessary to move soil particles. An enrichment factor of 85 was calculated, comparing number of seeds in running water leaving a field (20 seeds m−3) with seeds in stream water (1706 seeds m−3). Upper slope sites, when cultivated, yielded not only locally eroded seeds, but seed enrichment took place in running water on the way down to the stream, even under dense cover where no soil erosion occurred. As long as the drainage channels on the lower slopes are not fully recovered, any cultivation of uphill sites will re-open the old gullies and the mass of seeds accumulated in the channels will be re-mobilized and delivered directly into the stream. Our experiment demonstrates that the effect of preferential erosion of seeds, i.e. dispersal without erosion, is far greater than dispersal in sediments, making hill slope processes the major contributor to seed dispersal, while the trapping of seeds in wetlands and by the riparian area was only locally important.

Site conditions influence the climate signal of intra-annual density fluctuations in tree rings of Q. ilex L.

The investigation of the relations between climate and intra-annual density fluctuation (IADF) traits (e.g. frequency, width and functional anatomical traits) in Quercus ilex tree rings of stands living on two opposite slopes, highlighted the influence of local site conditions on the climate signal contained in tree rings. Moreover it allowed to identify the main site-specific environmental drivers for IADF formation. Wood functional traits such as IADFs result from the adaptation of tree physiological processes to seasonal climate variability. They could be used to reconstruct tree response to intra-annual variations in environmental conditions. We investigated the relationship between IADF features and climate in Quercus ilex L. populations living on opposite slopes, aiming to evaluate the influence of local site conditions on wood plasticity in response to seasonal climate variability. Dominant trees were sampled and dated. IADF frequency and width, non-lumen fraction, vessel size, and frequency were measured. Time series analyses using precipitation and temperature data were performed. A decrease in IADF frequency over time influenced by summer and autumn temperature was observed at both sites. IADFs were more frequent, wider, with smaller vessels and a higher non-lumen fraction in trees growing at the north slope compared to the south slope site. IADF anatomical parameters of south slope tree rings were mainly influenced by precipitation, while the ones of north-slope tree rings by temperature. Temperature-limited IADF occurrence in Q. ilex tree rings, probably leading to an earlier stop in cambial activity, and IADF anatomical traits were influenced by site conditions, suggesting them as indicators of site-specific responses to climate.

The Dynamics of Coral-Algal Interactions in Space and Time on the Southern Great Barrier Reef

Globally, tropical coral reefs are being degraded by human activities, and as a result, reef-building corals have declined while macroalgae have increased. Recent work has focused on measuring macroalgal abundance in response to anthropogenic stressors. To accurately evaluate the effects of human impacts, however, it is necessary to understand the effects of natural processes on reef condition. To better understand how coral reef communities are influenced by natural processes, we investigated how spatial and seasonal changes in environmental conditions (temperature and PAR) influence benthic community structure, and the composition and frequency of coral-algal interactions across eight distinct zones and over a 23-month period at Heron reef on the southern Great Barrier Reef. Hard coral cover and macroalgal density showed distinct spatio-temporal variations, both within and between zones. Broad hard coral cover was significantly higher at the reef slope sites compared to the lagoon and was not significantly influenced by season. The composition and biomass of macroalgae increased in spring and declined in summer, with maximum macroalgal abundance corresponding with average temperatures of between 22 and 24°C and average 24 h PAR of 300-500 μmol qanta m s. Changes in macroalgal biomass further influenced the composition and frequency of coral-algal interactions, however the incidence of coral-algal contact was best explained by coral cover. The results presented here emphasize that natural levels of macroalgae and coral-algal interactions are context-specific, and vary not only with-in zones, but in somewhat predictable seasonal cycles. Further, these results emphasize that the frequency of coral-algal interactions is dependent on hard coral, not just macroalgal cover, and an increase in coral-algal interactions does not necessarily translate to degradation of coral reefs.

Aggregate stability and associated organic carbon and nitrogen as affected by soil erosion and vegetation rehabilitation on the Loess Plateau

Although soil erosion and land use change have long been focuses in carbon research, the combined influence of soil erosion and vegetation rehabilitation on aggregate stability and the associated soil organic carbon (SOC) and total nitrogen (TN) remains unclear. The current study evaluated the effects of soil erosion on aggregate stability and the associated SOC and TN dynamics in relation to vegetation rehabilitation after the implementation of the “Grain-for-Green” project in the hilly Loess region. A check dam sediment sequence was dated using 137Cs activity and erosive rainfall events. The SOC and TN in the bulk soil and aggregate fractions were measured in soils from rehabilitated grasslands and sloping croplands and in sediments retained by the check dam. The results showed that vegetation rehabilitation led to 78%, 27% and 9% average increases in the macroaggregate amount, mean weight diameter (MWD) and mean geometric diameter (MGD), respectively. In addition, rehabilitation resulted in the highest SOC and TN concentrations and contents in macroaggregates among all the aggregate size fractions. Soil erosion facilitated the modification of the aggregate size distributions along with soil mineralization and induced the incorporation of deeper SOC-poor soils during transport. These processes resulted in the aggregate-associated SOC and TN concentrations and contents in the sediments being significantly lower than those in the eroding sloping cropland soils. The highest reductions were found in microaggregates, which exhibited decreases of 48% and 44% for SOC and TN, respectively. Moreover, reaggregation and gully soils incorporated during soil erosion led to higher values of macroaggregate amount and aggregate stability at depositional sites than those at eroding sloping cropland sites in this study. Our study contributes to the understanding of the effects of soil erosion and vegetation rehabilitation on SOC and TN dynamics, which is crucial for understanding the restoration efficiency in soil erosion control and ecosystem security evaluation.

Effect of site condition below slip surface on prediction of equivalent seismic loading parameters and sliding displacement

The effect of seismic site response below the slip surface is commonly ignored in conventional predictive models of sliding displacement of slopes. In this study, the influence of site condition below the critical slip surface (i.e., the bedrock depth below slip surface) on the dynamic response of sliding mass and the seismic sliding displacement is investigated. The equivalent seismic loading time history which represents the average acceleration within the sliding mass is derived for sliding masses with different site conditions (i.e., various depths to the overlying bedrock). The frequency-dependent effects of bedrock depth are emphasized for accurate assessment of the stability of slopes during earthquakes. For the same sliding mass, the equivalent seismic loading parameters and sliding displacements are strongly dependent on the depth to the underlying bedrock. Empirical models are developed for predicting the amplitude and frequency parameters of equivalent seismic loading by introducing the ratio of the depth of sliding mass and bedrock to capture the seismic responses of the sliding mass as well as the full slope site. The performance of existing empirical models for rigid block sliding displacement is examined and simplified modifications are performed to appropriately predict the sliding displacement by including the dynamic response of sites. Two example applications are presented, and the results indicate that the proposed empirical models including the site condition below the slip surface provide a more accurate assessment of dynamic response and sliding displacement of slopes.

Evaluating 2D numerical simulations of granular columns in level and gently sloping liquefiable sites using centrifuge experiments

The response of a layered liquefiable soil profile, with granular columns as a mitigation strategy, was evaluated via numerical and centrifuge modeling. Comparisons were made for a level site containing a single granular column and for a pair of gentle slopes, one of which was mitigated with a network of dense granular columns. The results reveal the abilities and limitations of two state-of-the-art soil constitutive models. All simulations were performed in 2-dimensions using: 1) the pressure-dependent, multi-yield-surface, plasticity-based soil constitutive model (PDMY02); and 2) the bounding surface, plasticity-based, Manzari-Dafalias (M-D) soil constitutive model, both implemented in OpenSees. Numerical model parameters were previously calibrated via element testing. Both constitutive models under-predicted PGA near the surface at different distances from the granular column, but they better predicted spectral accelerations at periods exceeding 0.5 s (particularly M-D). The M-D model generally predicted seismic settlements well, while PDMY02 notably underestimated soil's volumetric compressibility and strains. Both models accurately predicted the peak value and generation of excess pore pressures during shaking for the unmitigated slope, leading to a successful prediction of lateral deformations. However, lateral movement of the treated slope was poorly predicted by both models due to inaccuracies in predicting the dissipation rate in the presence of drains. Both models came close to predicting the performance of gently sloping, liquefiable sites when untreated. But further advances are required to better predict the rate of excess pore pressure dissipation and seismic performance when the slope is treated with granular columns.

Variation in soil saturated hydraulic conductivity along the hillslope of collapsing granite gullies

ABSTRACTSaturated hydraulic conductivity (Ks) affects the soil hydrological process and is influenced by many factors that exhibit strong spatial variations. To accurately measure Ks and its scale, spatial variability and relationship with collapsing gullies, we analysed four double-ring infiltrometer diameters in three soil layers during in situ experiments designed to measure Ks in two typical collapsing gullies (three slope sites) in Tongcheng County of China. The results showed that Ks increased with increasing inner ring diameter, but no significant difference existed between inner diameters of 30 and 40 cm. The Ks in red soil layers was higher than that in sandy soil layers, the transition layers had the lowest values. Ks also varied with slope position, gradually decreasing with distance from the gully head. The suggestion is that the spatial variation in Ks is affected not only by the intrinsic soil properties but also by the interaction with the collapsing gully.

Tree community variation in a tropical continental island according to slope aspect and human interference.

Associating description of unrecorded tropical tree community structure to sampling approaches that can help determine mechanisms behind floristic variation is important to further the comprehension of how plant species coexist at tropical forests. Thus, this study had the goals of (i) evaluating tree community structure on the continental island of Marambaia (23°4'37.09"S; 43°59'2.15"W) and (ii) testing the prediction that there are local scale changes in a tropical tree community structure between slopes facing different geographic orientation and with distinct human interference history. We established 60 (0.6 ha) sampling units in three different slope sites with distinct predominant geographic orientation and human interference. We sampled all woody trees with diameter at breast height (dbh) ≥ 5 cm. We found a total of 1.170 individuals representing 220 species, 120 genera and 50 families. The overall tree community structure and structural descriptors (abundance of individuals, basal area, species richness and diversity) varied extensively between the sites. The evidence presented here supports that local scale topography variations and human interference history can be important factors contributing to the known floristic heterogeneity of the Atlantic Rainforest. Future work on the study area should focus on disentangling effects from distinct causal factors over tree community variation and species occurrence.

Suprabenthic crustacean assemblages subjected to high-energy hydrodynamic events in the Blanes Canyon and adjacent slope (NW Mediterranean Sea)

Submarine canyons and adjacent slopes from the NW Mediterranean Sea are subjected to high seasonal and inter-annual variability in hydrodynamic processes. Such an environmental setting has a profound influence in the spatial and temporal variability of deep-sea benthic assemblages. We analyzed the suprabenthic crustacean assemblages from the Blanes Canyon and the adjacent slope along a bathymetric gradient. Macer-Giroq sledge hauls were taken from 900 to 2250 m depth in the canyon axis and southwestern adjacent slope in the autumn of 2008, 2009 and 2012. From the twenty four hauls sampled, one hundred and seventy one species were identified. Major changes in densities, number of species and diversity were only found at particular depths or years. In general the samples yielded variable densities (from approx. 10 to 539 ind.100 m−2), mostly owing to temporal fluctuations in motile fauna such as amphipods and mysids, in relation to the occurrence of energy-driven events. The environmental conditions associated to these events coupled with particular topographic features at the shallowest sites (namely at 900 m depth) lead to contrasting biodiversity patterns in the canyon and adjacent slope: high dominance and subsequent depressed diversity in the canyon and the highest diversity on the slope at similar depths. At the deepest sites, suprabenthic diversity reached similar values in the canyon and adjacent slope, probably owing to the decreased influence of disturbance processes at greater depths. Trophic diversity decreased with depth only in the slope, while in the canyon it remained constant likely owing to the relatively enhanced trophic conditions found in the canyon at all depths. However, the community composition differed significantly between the canyon and adjacent slope and at contrasting depths (<1200 m vs. >1200 m). At shallowest depths, the canyon sites were characterized by a high contribution of motile omnivores while the slope sites were characterized by a high contribution of detritivores to the total abundance. Highly motile predators on zooplankton were dominant at deepest sites in both environments. Our results suggest that taxonomic and trophic diversity in the northwestern Mediterranean may be influenced by the abrupt topography and its interaction with hydrodynamic conditions (intensification of currents; variable intensity of disturbance/hydrodynamic events; food pulses linked to food availability). These are major drivers which influence the spatial and temporal availability of food sources to the various faunal compartments eventually propagating up the food web.

Influence of predation risk on the sheltering behaviour of the coral-dwelling damselfish, Pomacentrus moluccensis

Predation is a key ecosystem function, especially in high diversity systems such as coral reefs. Not only is predation one of the strongest top-down controls of prey population density, but it also is a strong driver of prey behaviour and function through non-lethal effects. We ask whether predation risk influences sheltering behaviour of damselfish living in mutualism with branching corals. Host corals gain multiple advantages from the mutualistic relationship which are determined by the strength of damselfish sheltering. Distance travelled by the Lemon Damselfish Pomacentrus moluccensis away from their host colony was measured here as a proxy for sheltering strength and was expected to be shortest under highest predation risk. Predation risk, defined as a function of predator abundance and activity, turbidity and habitat complexity, was quantified at four reef slope sites in Kepulauan Seribu, Indonesia. Damselfish sheltering strength was measured using stationary unmanned video cameras. Small damselfish (< 2 cm) increased their sheltering strength under high turbidity. Predator feeding activity, but not abundance, influenced damselfish sheltering strength. Contrary to our expectations, sheltering behaviour of adult damselfish decreased under high predator activity. While these observations are in line with risk-averse behaviour by juvenile P. moluccensis, they may indicate the presence of sentinel behaviour in the adults of this species. Habitat complexity seemed to be less important as a driver of damselfish behaviour. These counterintuitive results may indicate complex social behaviour and age-specific strategies for predator avoidance.

Landslide Movement at Lokanthali during the 2015 Earthquake in Gorkha, Nepal

AbstractThe Gorkha earthquake (Mw=7.8) of April 25, 2015, caused widespread damage in the Kathmandu Valley. At a gently sloping site, known as Lokanthali, large ground fissures and tension cracks w...

Differential Responses of Taxonomic, Structural, and Functional Diversity to Local-Scale Environmental Variation in Afromontane Forests in South Africa

Exploring taxonomic, functional, and structural diversity can provide additional insights into our understanding of diversity responses to environment. Using altitude, slope, and relative radiation index as well as floristic and functional data from a South Africa Afromontane forest, we examined how taxonomic, structural, and functional diversity varied with local environmental variation. Taxonomic and structural diversity were quantified through species richness- and diameter class-based Shannon index and evenness, respectively. Skewness and coefficient of variation of diameter distribution were additionally computed for structural diversity. As for functional diversity, we used functional richness, evenness, divergence, and dispersion based on functional traits. Data were analyzed using multimodel inference and subset regression. We found little evidence of environmental effects on local-scale taxonomic diversity patterns. In contrast, structural and functional diversity metrics varied significantly along environmental gradients. Accordingly, diameter class-based Shannon evenness declined with increasing slope while skewness and coefficient of variation of diameter distribution increased with increasing slope. Functional evenness and divergence decreased with increasing altitude and radiation, respectively, while functional richness and dispersion increased with increasing slope. The results showed that taxonomic diversity patterns were less responsive to local-scale topographical variation than structural and functional diversity. Lower functional diversity on lower slope sites suggests weak environmental filtering effect promoting competitive exclusion and dominance of species with acquisitive traits. On higher slope sites, environmental filtering associated with slope gradient seems to favor coexistence of species with conservative traits and adapted to harsh conditions.

Caractérisation de la végétation des friches situées au sein d’un réseau d’infrastructures de transport : le cas de la vallée de l’Huveaune – Marseille / Characterisation of the vegetation of urban wastelands situated within transport infrastructure sites : the case of the Huveaune valley – Marseille

Often considered as abandoned sites of no value, colonised by vegetation that is depreciated, disordered and poorly maintained, urban wastelands are today in France the focus of major territorial issues, in particular with regard to the preservation of the biodiversity and the development of Green and Blue Infrastructure. Featuring among the last ‘wild’ natural spaces, but also the last areas in the city that are still available for urban development, they are universally coveted. From the ecological point of view, the biological diversity of these areas in the Mediterranean region remains largely unknown. A study was therefore undertaken on 24 urban wastelands situated within the transport infrastructure sites of the lower Huveaune valley between Marseille and Aubagne, in south-east France, in order to characterise the vegetation of these areas with regard to the taxonomic, morphological and biological aspects. The analysis of the floristic data resulting from vegetation transects shows that these infrastucture wastelands, rich and diversified, are dominated by five herbaceous species of the Poaceae family, in which a few shrub and tree species cohabit. The dominant plant species are characteristic of ruderal, open urban environments, and are accompanied by roadside and field edge plant species that are evidence of the long history of changes of land occupation and use that the valley has been witness to and of recent disturbances to which it has been subjected. The micro-topography related to the transport infrastructure system does not appear to have any influence on the structure and composition of the floristic composition of wasteland sites, except on the frequency of certain biological and morphological traits. The maintenance carried out on the transport infrastructure embankment wastelands may be a factor that explains the differences observed. These visually perceptible differences may also contribute from the aesthetic and affective points of view to the variation of the degree of appreciation by the general public of the plant diversity between the sloping and non-sloping wasteland sites.

Litter dynamics of Olea europaea subsp. Europaea residues related to soil properties and microbial N-biomass in a Mediterranean agroecosystem

Residues decomposition is an important process for plant nutritional demands. The present work was conducted in order to investigate the olive tree residues decomposition (leaves and shoots: buried in a horizontal and a sloping site of an organic olive grove) and its impact on soil properties and microbial N-biomass as affected by soil slope. Mean mass loss and C, N, K, and Mg release rates were higher in leaf than in shoots residues in both soil slopes. The incorporation of leaves and shoots in the soil of the olive grove contributed to its enrichment in certain nutrients such as N, K, Ca, and Mg. Soil organic carbon losses were compensated by the residue released soluble C compounds. The macroaggregates formed by fungal hyphae were not stable in water as Water Aggregate Stability tended to decline in the horizontal site and showed no change in the sloping site. The added amounts of N, P, and K by olive tree residues soil incorporation were 6.1, 0.4, and 7.2 kg ha−1 y−1 (sloping site) and 6.8, 0.3, and 8.8 kg ha−1 y−1 (horizontal site), respectively. These amounts are lower than the needed ones for olive trees (15–35 N, 8 P, ≤50 K kg ha−1 y−1), so olive tree growers should use additional fertilization in order to have strong and productive olive trees.

Deconstructing human-shaped treelines: Microsite topography and distance to seed source control Pinus nigra colonization of treeless areas in the Italian Apennines

Human-shaped treelines are a common feature in mountain landscapes across Europe, and particularly in secularly managed Mediterranean high-elevation areas. The abandonment of traditional land use, and especially the reduction in grazing pressure at high elevations, triggered secondary succession in treeless grassland areas, and favored the upward shift of anthropogenic treelines in some cases. We investigated this process in four anthropogenic treeline ecotone sites in the Central Apennines, Italy, populated by European black pine (Pinus nigra Arn.). The upward treeline shift was controlled by microsite topography and the proximity to plantations acting as seed source. We found a 50% probability of producing cones in trees with basal diameters of 15–25 cm, heights of 2–5 m and ages of 20–25 years. The role played by climate on growth and the recruitment processes seems to be secondary, or could be masked by human-shaped processes. The presence of reproductive age trees at the treeline, mainly growing on debris-rich and steep slope sites, could indicate that the recruitment process will increase in future, leading to patchy tree patterns at different elevations. The high growth and encroachment rates observed at these human-shaped treelines would indicate that general growth dynamics are speeding up, including the tree colonization of treeless areas. These succession processes could cause a significant long-term decline in plant diversity in species-rich grasslands. Nonetheless, tree encroachment could increase forest protection against landslides and avalanches in the context of global change.

Anna Meares Velodrome

The Anna Meares Velodrome (AMV) is Queensland’s first indoor velodrome, located on a sloping site alongside the Brisbane Super X BMX Track at Sleeman Sports Complex in Brisbane. The AMV was designed with the planning flexibility to not only host the track cycling competition for the Gold Coast 2018 Commonwealth GamesTM, but to provide a legacy venue for track cycling and a range of other indoor sports and community uses. To provide the required planning flexibility with world-class spectator and user amenity, the AMV has a large column-free arena with an elliptical footprint. The arena roof clear spans up to 118 m between inclined perimeter supports, making it one of the largest clear-span roofs in Australia. The roof is gently curved in two directions, and the resulting saddle shape combined with the inclined perimeter walls creates a striking and memorable building, befitting the important role that the venue will play in 2018 and beyond. The building was successfully delivered quickly and to a tight budget, in large part due to the efficiency of the design of the steel roof and wall structure, and to the adoption of a ‘construction-led’ design approach. This approach ensured that the steel roof structure could be economically, safely and quickly fabricated and erected in parallel with the construction of the concrete substructure and bowl. The paper describes key aspects of the project, including some of the design innovations that played a crucial role in the successful delivery of the project and the realisation of the competition-winning architectural vision. These innovations included parametric design, geometric rationalisation, a ‘construction-led’ approach and structural design optimisation.

Ca2+ Effects on a Pt/C Catalyst for the Oxygen Reduction Reaction

The proton exchange membrane fuel cell (PEMFC) is one of the most promising renewable energy sources due to benign exhaust emissions and high energy efficiency. Pt/C is currently the primary catalyst for the slow oxygen reduction reaction (ORR) kinetics at the cathode of a PEMFC. The ORR, O2 + 4H+ + 4e- → 2H2O, indicates that this process preferentially occurs at a triple phase boundary where reactant, electrolyte, and catalyst are in contact [1, 2]. The depletion of O2, H+ or e- would significantly affect reaction kinetics. The Nafion® perfluorosulfonic acid (PFSA) ionomer, composed of a carbon-fluorine backbone and side chains ending with sulfonic acid groups, is conventionally used to prepare catalyst layers by exploiting its binding, proton-conducting and O2 transport properties. Foreign cations replace ionomer protons and occupy sulfonic acid sites by ion exchange [3-6]. The replacement of H+ by foreign cations in the ionomer decreases not only the proton conductivity but also O2 permeability. Uddin et al. [6] model results showed that the presence of Na+ in the ionomer of the cathode catalyst layer, which decreases the O2 permeability, causes a decrease in dissolved O2 concentration. Durst et al. [7] found that the diffusion coefficient and the concentration of O2 in the H2SO4 electrolyte, and the kinematic viscosity of the electrolyte decrease in the presence of Co2+. Cationic impurities decrease the membrane water content in turn shrinking the ionomer and lowering the active catalyst surface area. Also, foreign cations promote the generation of H2O2, a side product of the ORR, which exerts an undetermined influence on the performance and durability of PEMFCs. Literature and our group studies indicated that foreign cations have severe effects on the performance and durability of the PEMFC. Among all cations, Ca2+ deserves special attention because it is one of the most abundant elements within the earth’s crust. Prior efforts have focused on the effects of Ca2+ on PEMFC operation [8-10] and membrane transport properties [11]. However, the effects of Ca2+ on the ORR kinetics have hardly been studied. Okada et al. [12] studied the effects of Ca2+ on the O2 reduction kinetics at a Nafion® film covered Pt rotating disk electrode. Their results showed that impurity ions, even in small amounts, significantly hinder the rate of the O2 reduction charge transfer step. However, the Nafion® film covered Pt electrode is different from the supported catalyst layer for a PEMFC. The O2 transport properties in a Ca2+ form ionomer are poorly understood. In addition, the H2O2 yield has not been measured. In this presentation, detailed analyses of the changes in the Pt/C electrochemical surface area, ORR diffusion-limited and kinetic currents, Tafel slope, H2O2 yield, O2 permeability and ionomer sulfonate site occupation will be presented and discussed. Acknowledgments Authors are grateful to the United States Department of Energy (award DE-EE0000467), the Office of Naval Research (award N00014-12-1-0496), and the Hawaiian Electric Company.

Spatial and temporal variability of soil nitric oxide emissions in N-saturated subtropical forest

Acidic subtropical forest soils, receiving high atmospheric nitrogen (N) deposition, are characterized by fast N turnover, making them potential “hot spots” for nitrous oxide (N2O) and nitric oxide (NO) emissions. Production of NO in soils is mediated by microbial nitrification and denitrification and/or chemical decomposition of microbially produced nitrite (NO2 −). In monsoonal forests, soil microbial processes are exposed to strong soil moisture and temperature fluctuations, known to cause pronounced variability in NO emission. We quantified in situ fluxes of NO throughout two summers in the N-saturated TieShanPing forest, Chongqing, SW China. Additional N2O measurements were carried out in the second summer to infer likely source processes. To address spatial variability, fluxes were measured along two transects, one covering well-drained Acrisols on a forested hill slope, and the other hydrologically connected terraces with poorly drained Cambisols situated in a groundwater discharge zone. Hill slope sites emitted on average 48.4 ± 60.6 µg NO–N m−2 h−1 in the relatively wet summer of 2015 and 88.4 ± 94.6 µg NO–N m−2 h−1 in the dry summer of 2016, illustrating the strong effect of soil moisture regime. The NO emission in the groundwater discharge zone, measured in the dry summer of 2016 only, while it was flooded in the summer of 2015, was significantly smaller (26.4 ± 52.7 µg N m−2 h−1). The mean N2O-N emission on the hillslope (18.1 ± 13.9 µg N m−2 h−1) was smaller than NO-N flux, while it was larger (38.3 ± 39.3 µg N m−2 h−1) in the groundwater discharge zone. The temporal variability of the NO flux was largely explained by soil moisture fluctuations, whereas soil organic matter pool sizes contributed most to spatial variability. Hourly measurements showed that the NO flux doubled with 8 °C increase of soil temperature, indicating that soil temperature was another important driver for NO emissions. Our study indicates that acid N-saturated subtropical forest soils are a large source for NO, particularly during dry summers.