3,214 research outputs found
Land use influences the spatiotemporal controls on nitrification and denitrification in headwater streams
N and C cycles in headwater streams are coupled, and land use can modify these cycles by increasing N availability and removing riparian vegetation. To increase our understanding of how land use modifies the controls on N cycling, we quantified rates of 2 microbial N transformations in a total of 18 agricultural and urban streams (with and without riparian buffers) for 3 y to examine how riparian vegetation and land use influence sediment nitrification and denitrification. Nitrification rates were highest in agricultural streams in late spring. Nitrification was not related to streamwater NH4+ concentrations but was positively related to sediment C content (linear regression, r2 = 0.72, p \u3c 0.001). This result suggests that benthic decomposition provided NH4+ (via mineralization) to increase sediment nitrification. Denitrification rates did not differ among landuse types but were positively related to sediment C content and streamwater NO3– concentration (multiple linear regression, R2 = 0.78, p \u3c 0.001). Sediment C content, the primary predictor of denitrification rates, did not differ among land uses, but streamwater NO3– concentration, the secondary predictor of denitrification rates, was highest in winter and in agricultural streams, indicating that land use and season were more important determinants of denitrification than coupled nitrification. Substrate availability (N and C) for N transformations generally did not differ between buffered and unbuffered streams within a similar landuse type, probably because of the confounding influence of tile drainage systems, which effectively decouple stream channels from their riparian zones. Land use influenced the delivery of the necessary substrates for N transformations but decreased the role of riparian zones in stream N cycling by simplifying the drainage network of headwater streams
Nitrous oxide emission from denitrification in stream and river networks
Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that \u3c1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change
From sensorimotor dependencies to perceptual practices: making enactivism social
Proponents of enactivism should be interested in exploring what notion of action best captures the type of action-perception link that the view proposes, such that it covers all the aspects in which our doings constitute and are constituted by our perceiving. This article proposes and defends the thesis that the notion of sensorimotor dependencies is insufficient to account for the reality of human perception, and that the central enactive notion should be that of perceptual practices. Sensorimotor enactivism is insufficient because it has no traction on socially dependent perceptions, which are essential to the role and significance of perception in our lives. Since the social dimension is a central desideratum in a theory of human perception, enactivism needs a notion that accounts for such an aspect. This article sketches the main features of the Wittgenstein-inspired notion of perceptual practices as the central notion to understand perception. Perception, I claim, is properly understood as woven into a type of social practices that includes food, dance, dress, music, etc. More specifically, perceptual practices are the enactment of culturally structured, normatively rich techniques of commerce of meaningful multi- and inter-modal perceptible material. I argue that perceptual practices explain three central features of socially dependent perception: attentional focus, aspects’ saliency, and modal-specific harmony-like relations
Abundance and size of birds determine the position of the species in plant-frugivore interaction networks in fragmented forests
Abstract
The relationships between frugivorous animals and plants are of vital importance particularly in tropical forests. The way species interact and how they are organized within interaction networks could be determined by their ecological and morphological characteristics. This study evaluates the hypothesis that the topological position of species within an interaction network is determined by their degree of frugivory, body size, and abundance. Thus, we constructed the frugivory network between birds and plants in a rainforest fragment in northwestern Colombia. The position of the species within the network was calculated based on three centrality measures (degree, betweenness, and closeness), and its association with relative abundance, degree of frugivory, and body size of each bird species was evaluated by means of a generalized linear model. We found that the species that were most abundant and had the smallest body size had central positions in the interaction network. This pattern is contrary to what has been observed in pristine forests, where species with large body size are more important for network stability. Our results suggest that forest fragmentation modifies the roles of species within the network structure, in part, due to changes in the makeup of the original frugivore community. The information presented may be useful to evaluate the effects of the loss of species as a result of anthropic actions, with the aim of generating ecosystem restoration strategies
Improvement and application of a methodology to perform the Global Energy Balance in internal combustion engines. Part 1: Global Energy Balance tool development and calibration
[EN] The increasingly stringent internal combustion engines emissions regulations, has led to the extended
use of after-treatment systems, giving progressively more importance to the engine efficiency optimization.
In this framework, the combined modelling and experimental methodologies to perform and
analyse the energy balance are key to evaluate the potential of different engine strategies aimed at the
consumption optimization and the identification of the improvement paths. This work has been divided
into two parts, dealing separately with the development and application of a Global Energy Balance tool.
This article corresponds to the first part, which comprises the description of the models required to
perform a detailed energy balance and the calibration methodologies followed to achieve accurate energy
terms estimation. The models are calibrated based on experimental information, thus, a thermodynamic
analysis aimed at defining comparable quantities between experimental and modelled terms is
performed. The uncertainty analysis of the tool shows a deviation in the determination of the heat
transfer to the coolant and the oil of about ±2%, and in terms of fuel energy about ±1%.This work was partially funded by the Government of Spain through Project TRA2013-41348-R. In addition, the authors acknowledge that some equipment used in this work has been partially supported by FEDER project funds (FEDER-ICTS-2012-06), framed in the operational program of unique scientific and technical infrastructure of the Ministry of Science and Innovation of Spain.López, JJ.; Payri, F.; Martín, J.; Carreño-Arango, R. (2018). Improvement and application of a methodology to perform the Global Energy Balance in internal combustion engines. Part 1: Global Energy Balance tool development and calibration. Energy. 152:666-681. https://doi.org/10.1016/j.energy.2018.03.118S66668115
The Influence of the Strength of Financial Institutions and the Investment-Production Delay on Commodity Price Cycles: A Framed Field Experiment with Coffee Farmers in Colombia
Impacto de la regulación en la eficiencia asignativa del mercado spot eléctrico colombiano
Este paper utilizando un modelo de regresión con estructura ARCH, analiza el
efecto de la implementación de algunas medidas regulatorias y de variables
fundamentales (relación entre la demanda comercial y la disponibilidad real, El
Niño y los aportes hidrológicos) en el Mercado de Energía Mayorista en
Colombia sobre el precio spot. Los resultados obtenidos evidencian que las
regulaciones establecidas por la Comisión de Regulación de Energía y Gas,
han impactado estadística y significativamente el precio spot. Además, El Niño
y los aportes hidrológicos presentan un impacto positivo y negativo,
respectivamente sobre el precio spot, debido a la gran participación hidráulica
de este mercado.This paper uses an ARCH regression model to analyze the effect of several
regulatory measures and fundamental factors (the relationship between
commercial demand and real availability, El Niño, and water supplies) on the
spot price in the Colombian wholesale power market. The results indicate that
the regulations established by the Electricity and Gas Regulatory Commission
have had a substantial and statistically-significant effect on spot prices. In
addition, El Niño and hydro supplies have a positive and negative respectively
effect on the spot price, due to the large share of hydropower in this market
Técnicas de lógica difusa en la predicción de índices de mercados de valores: una revisión de literatura.
El pronóstico de índices de mercados de valores es una tarea importante en ingeniería financiera, porque es una información necesaria para la toma de decisiones. Este estudio tiene como objetivo evaluar el estado del arte en el progreso del pronóstico del mercado de valores, usando metodologías basadas en sistemas de inferencia borrosa y redes neuronales neuro-difusas, enfatizando el caso del Índice General de la Bolsa de Colombia (IGBC). Se empleó la revisión sistemática de literatura para responder cuatro preguntas de investigación. Existe una tendencia importante sobre el uso de las metodologías basadas en inferencia difusa para predecir los índices de los mercados de valores, explicada por la precisión del pronóstico en comparación con otras metodologías tradicionales. La mayoría de las investigaciones se enfocan en metodologías de “series de tiempo difusas” y ANFIS, pero, hay otras aproximaciones prometedoras que no han sido evaluadas aún. Existe un vacío de investigación en el caso del mercado accionario colombiano
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