Transferencia de nitrógeno de ecosistemas agrícolas recientemente intensificados a acuíferos: efecto de la variación en las precipitaciones

A partir del monitoreo a campo de cultivos, suelo y agua subterránea, modelos de simulación y un experimento en lisímetros se estudió el intercambio de nitrógeno (N) inorgánico entre cultivos y el agua subterránea en sistemas agrícolas recientemente intensificados con suelos ricos en materia orgánica (MO). El monitoreo (1998-2010) en tres posiciones del paisaje de la Pampa Ondulada permitió captar períodos plurianuales húmedos y secos (1506-444 mm año-1) disparadores de cambios intensos en las profundidades freáticas (mayor 1 a menor a 6 m) que afectaron la función del agua subterránea y de los cultivos como moduladores de los flujos de agua y N. Con niveles freáticos superficiales, se redujo la capacidad de almacenaje del agua drenada hacia la zona no saturada, produciéndose un flujo lateral subsuperficial que redistribuyó el N lixiviado desde posiciones altas hacia los bajos y contribuyó a la recarga del acuífero aguas abajo. Este flujo, favorecido por fuertes gradientes hidráulicos, y la concentración local de solutos disparada por el consumo de agua subterránea por los cultivos en la posición de pie de loma, produjeron picos de concentración de cloruro (menor a 500 mg Cl. l-1) y nitrato (menor a 45 mg N-NO3. l-1) en el agua freática superficial. Durante períodos normales a secos los drenajes de diferentes eventos de lluvia se alojan en la zona no saturada, disipando y retrasando la respuesta del nivel freático a eventos individuales. El agua de lluvia fluye verticalmente arrastrando N del suelo hacia una superficie freática profunda (con mayor desfasaje temporal entre la concentración de nitrato del drenaje y la del agua freática a medida que el nivel se profundiza) y luego descarga en el arroyo. La aplicación de un fertilizante enriquecido con 15N a un cultivo de maíz en lisímetros permitió demostrar que la fertilización representa un aporte insignificante (1 por ciento) al flujo de N lixiviado y que el N derivado de la mineralización de la MO del suelo constituye una fuente importante de N lixiviable durante períodos con balance hídrico positivo y/o baja demanda de los cultivos. A partir de los resultados obtenidos se propone un modelo conceptual de los flujos hidrológicos y de N en paisajes ondulados de uso agrícola para situaciones climáticas contrastantes

La lixiviación de nitratos en la Región Pampeana : análisis de procesos y factores determinantes

La fertilización nitrogenada es esencial para satisfacer la demanda global de alimentos, pero es uno de los principales causantes de contaminación de acuíferos subterráneos con nitratos a nivel mundial y nacional. El objetivo de esta tesis fue determinar los principales factores condicionantes de la distribución y lixiviación de nitratos en suelos de la Pampa Ondulada y analizar los destinos del nitrógeno (N) de los fertilizantes. Se determinó a campo el contenido de nitratos en suelos en diferentes situaciones productivas y se realizaron experimentos a campo e invernáculo con fertilizantes marcados isotópicamente (15N) para evaluar los destinos del mismo. Se utilizó el modelo NLEAP para simular la acumulación y lixiviación de nitratos en diferentes suelos. La precipitación pluvial fue el principal factor que determinó la lixiviación de nitratos en maíz, regulando la absorción de N por el cultivo y su desplazamiento vertical. La dinámica de los nitratos presentó pulsos de acumulación en el perfil o salida del mismo, cuando las precipitaciones fueron escasas o elevadas, respectivamente. La materia orgánica del suelo fue el principal destino del fertilizante ante un estrés hídrico en maíz, seguido por su acumulación en las plantas. Lo inverso ocurrió cuando no existió estrés hídrico. El N lixiviado, derivado del fertilizante aplicado en una determinada campaña agrícola fue mínimo, en relación a las pérdidas del N de fuentes preexistente en el suelo. Los Argiudoles (texturas más finas) fueron más susceptibles a sufrir pérdidas por lixiviación que los Hapludoles (texturas más gruesas). De acuerdo con el modelo utilizado, el período crítico para la ocurrencia de lixiviación de nitratos fue entre dos cultivos de verano. La inclusión de un cultivo de cobertura en dicho período redujo las pérdidas por lixiviación, incrementando la disponibilidad de N para un cultivo posterior. La retención del N del fertilizante en los residuos del cultivo de cobertura y en la materia orgánica del suelo podría atenuar la lixiviación de nitratos sólo en un corto plazo. El modelo de simulación utilizado fue apto para estimar situaciones de pérdidas de N, cuando las dosis de fertilización fueron las normales en la región.

Effects of formalin preservation on stable carbon and nitrogen isotope signatures in Calanoid copepods: implications for the use of Continuous Plankton Recorder Survey samples in stable isotope analyses

Preserved and archived organic material offers huge potential for the conduct of retrospective and long-term historical ecosystem reconstructions using stable isotope analyses, but because of isotopic exchange with preservatives the obtained values require validation. The Continuous Plankton Recorder (CPR) Survey is the most extensive long-term monitoring program for plankton communities worldwide and has utilised ships of opportunity to collect samples since 1931. To keep the samples intact for subsequent analysis, they are collected and preserved in formalin; however, previous studies have found that this may alter stable carbon and nitrogen isotope ratios in zooplankton. A maximum ~0.9‰ increase of δ15N and a time dependent maximum ~1.0‰ decrease of δ13C were observed when the copepod, Calanus helgolandicus, was experimentally exposed to two formalin preservatives for 12 months. Applying specific correction factors to δ15N and δ13C values for similarly preserved Calanoid species collected by the CPR Survey within 12 months of analysis may be appropriate to enable their use in stable isotope studies. The isotope values of samples stored frozen did not differ significantly from those of controls. Although the impact of formalin preservation was relatively small in this and other studies of marine zooplankton, changes in isotope signatures are not consistent across taxa, especially for δ15N, indicating that species-specific studies may be required. Copyright © 2011 John Wiley & Sons, Ltd.

Comparison of new and primary production models using SeaWiFS data in contrasting hydrographic zones of the northern North Atlantic.

The accuracy of two satellite models of marine primary (PP) and new production (NP) were assessed against 14C and 15N uptake measurements taken during six research cruises in the northern North Atlantic. The wavelength resolving model (WRM) was more accurate than the Vertical General Production Model (VGPM) for computation of both PP and NP. Mean monthly satellite maps of PP and NP for both models were generated from 1997 to 2010 using SeaWiFS data for the Irminger basin and North Atlantic. Intra- and inter-annual variability of the two models was compared in six hydrographic zones. Both models exhibited similar spatio-temporal patterns: PP and NP increased from April to June and decreased by August. Higher values were associated with the East Greenland Current (EGC), Iceland Basin (ICB) and the Reykjanes Ridge (RKR) and lower values occurred in the Central Irminger Current (CIC), North Irminger Current (NIC) and Southern Irminger Current (SIC). The annual PP and NP over the SeaWiFS record was 258 and 82 gC m-2 yr-1 respectively for the VGPM and 190 and 41 gC m-2 yr-1 for the WRM. Average annual cumulative sum in the anomalies of NP for the VGPM were positively correlated with the North Atlantic Oscillation (NAO) in the EGC, CIC and SIC and negatively correlated with the multivariate ENSO index (MEI) in the ICB. By contrast, cumulative sum of the anomalies of NP for the WRM were significantly correlated with NAO only in the EGC and CIC. NP from both VGPM and WRM exhibited significant negative correlations with Arctic Oscillation (AO) in all hydrographic zones. The differences in estimates of PP and NP in these hydrographic zones arise principally from the parameterisation of the euphotic depth and the SST dependence of photo-physiological term in the VGPM, which has a greater sensitivity to variations in temperature than the WRM. In waters of 0 to 5C PP using the VGPM was 43% higher than WRM, from 5 to 10C the VGPM was 29% higher and from 10 to 15C the VGPM was 27% higher.

Gross and net nitrogen transformation rates and availability in late summer in tall shrub and birch hummock ecosystems of the Canadian low Arctic

Climate change is occurring most rapidly in the Arctic where warming has been twice as fast as the rest of the globe over the last few decades. Arctic soils contain a vast store of carbon and warmer arctic soils may mediate current atmospheric CO2 concentrations and global warming trends. Warmer soils could increase nutrient availability to plants, leading to increased primary production and sequestration of CO2. Presumably because of these effects of warming on shrub ecosystems, shrubs have been expanding across the arctic over the last 50 years, Arctic shrub expansion may track or cause changes in nutrient cycling and availability that favour growth of larger, denser shrubs. This study aimed at measuring gross and net nitrogen cycling rates, major soil nitrogen and carbon pool sizes, and elucidating controls on nutrient cycling and availability between a mesic birch (Betula nana) hummock tundra ecosystem and an ecosystem of dense, tall, birch (B. nana) shrubs. Nitrogen cycling and availability was enhanced at the tall shrub ecosystem compared to the birch hummock ecosystem. Net nitrogen immobilization by microbes was approximately threefold greater at the tall shrub ecosystem. This was in part because of larger microbial biomass nitrogen and carbon (interpreted as a larger microbial community) at the tall shrub ecosystem. Nitrogen inputs via litter were significantly larger at the tall shrub ecosystem and were hypothesized to be the major contributor to the higher dissolved organic and inorganic nitrogen pools in the soil at the tall shrub ecosystem. The results from this study suggest a positive feedback mechanism between litter nitrogen inputs and the enhancement of nitrogen cycling and availability as a driver of shrub expansion across the Arctic.

Isotopic variation complicates analysis of trophic relations within the fish community of Plußsee: a small, deep, stratifying lake

Analysis of carbon and nitrogen stable isotopes has allowed freshwater ecologists to examine lake food webs in increasing detail. Many such studies have highlighted the existence of separate within-lake pelagic and benthic-littoral food webs but are typically conducted on large (> 10 km2) lakes, whereas the majority of lakes are actually relatively small. We used stable isotope analysis (δ13C & δ15N) to examine trophic interactions between fish and their prey in Plu�see, as an example of a small, stratifying lake, and to determine whether separate pelagic/benthic-littoral food webs could be distinguished in such systems. Our results indicate that the Plu�see food web was complicated, and due to extensive intra-annual isotopic variation in zooplankton (e.g. cladoceran δ13C annual range = 25.6�), it may be impossible to definitively assign consumers from small, eutrophic stratified lakes to pelagic or benthic-littoral food webs. We present evidence that some components of the Plu�see food web (large bream) may be subsidised by carbon of methanogenic origin.

Isotopic composition of tropospheric and soil N2O from successive depths of agricultural plots with contrasting crops and nitrogen amendments

Agricultural soils are the dominant contributor to increases in atmospheric nitrous oxide (N2O). Few studies have investigated the natural N and O isotopic composition of soil N2O. We collected soil gas samples using horizontal sampling tubes installed at successive depths under five contrasting agricultural crops (e.g., unamended alfalfa, fertilized cereal), and tropospheric air samples. Mean d 15N and d 18O values of soil N2O ranged from -28.0 to +8.9‰, and from +29.0 to +53.6‰. The mean d 15N and d 18O values of tropospheric N2O were +4.6 ± 0.7‰ and +48.3 ± 0.2‰, respectively. In general, d values were lowest at depth, they were negatively correlated to soil [N2O], and d 15N was positively correlated to d 18O for every treatment on all sampling dates. N2O from the different agricultural treatments had distinct d 15N and d 18O values that varied among sampling dates. Fertilized treatments had soil N2O with low d values, but the unamended alfalfa yielded N2O with the lowest d values. Diffusion was not the predominant process controlling N2O concentration profiles. Based on isotopic and concentration data, it appears that soil N2O was consumed, as it moved from deeper to shallower soil layers. To better assess the main process(es) controlling N2O within a soil profile, we propose a conceptual model that integrates data on net N2O production or consumption and isotopic data. The direct local impact of agricultural N2O on the isotopic composition of tropospheric N2O was recorded by a shift toward lower d values of locally measured tropospheric N2O on a day with very high soil N2O emissions.

Nitrogen-15 and Oxygen-18 Natural Abundance of Potassium Chloride Extractable Soil Nitrate Using the Denitrifier Method

In agroecosystems, most isotopic investigations of NO3- involve the use of tracers that are artificially enriched in 15N. Although the dual isotope composition of NO3-— d15N and d18O is especially beneficial for understanding the origin and fate of NO3-, its use for KCl-extractable soil NO3- has been hampered by the lack of a suitable analytical technique. Our objective was to test whether the denitrifier method, whereby NO3- is reduced to N2O before mass spectrometric analysis, can be used to determine the N and O isotopic composition of NO3- from 2 M KCl soil extracts. Several internationally accepted NO3- standards were dissolved in 2 M KCl, the conventional extractant for soil inorganic N, and inoculated with the bacterial strain Pseudomonas aureofaciens (ATCC no. 13985). The standard deviation of the NO3- standards analyzed did not exceed 0.2‰ for d15N and 0.3‰ for d18O values. After appropriate corrections, differences between our measured and consensus d15N and d18O values of standard NO3- generally were within the standard deviations given for the consensus values. Both d15N and d18O values were reproducible among separate analytical runs. The method was also tested on genuine 2 M KCl extracts from unfertilized and fertilized soils. Depending on N fertilization, the soils had distinct d15N and d18O values, which were attributed to amendment with NH4NO3 fertilizer. Hence, our data indicate that the denitrifier method provides a fast, reliable, precise, and accurate way of simultaneously analyzing the natural abundances of 15N and 18O in KCl-extractable soil NO3-.

The effect of simultaneous contractions of ipsilateral muscles on changes in corticospinal excitability induced by paired associative stimulation (PAS)

Consideration was given to means of increasing the reliability and muscle specificity of paired associative stimulation (PAS) by utilising the phenomenon of crossed-facilitation. Eight participants completed three separate sessions: isometric flexor contractions of the left wrist at 20% of maximum voluntary contraction (MVC) simultaneously with PAS (20s intervals; 14 min duration) delivered at the right median nerve and left primary motor cortex (MI); isometric contractions at 20% of MVC: and PAS only ( 14 min). Eight further participants completed two sessions of longer duration PAS (28 min): either alone or in conjunction with flexion contractions of the left wrist. Thirty motor potentials (MEPs) were evoked in the right flexor (rFCR) and extensor (rECR) carpi radialis muscles by magnetic stimulation of left M1 Prior to the interventions, immediately post-intervention, and 10 min post-intervention. Both 14 and 28 min of combined PAS and (left wrist flexion) contractions resulted in reliable increases in rFCR MEP amplitude, which were not present in rECR. In the PAS only conditions, 14 min of stimulation gave rise to unreliable increases in MEP amplitudes in rFCR and rECR, whereas 28 min of PAS induced small (unreliable) changes only for rFCR. These results support the conclusion that changes in the excitability of the corticospinal pathway induced by PAS interact with those associated with contraction of the muscles ipsilateral to the site of cortical stimulation. Furthermore, focal contractions applied by the opposite limb increase the extent and muscle specificity of the induced changes in excitability associated with PAS. (C) 2008 Elsevier Ireland Ltd. All rights reserved.

Women artists of the international avant-garde: the case of "Abstraction-Création" (1931-1936)

Women’s contribution to abstract art in the interwar period is a subject that, to date, has received very little attention. In this article we deal with the untold story of the participation of women artists in Abstraction-Création, the foremost international group dedicated to abstract art in the 1930s. Founded in Paris in 1931, the group took on the work of two previous collectives to become a platform for the dissemination and promotion of abstract art and consisted of around a hundred members. Twelve of these were women, whose writings and works were published in the group’s annual magazine, abstraction creátion art non figuratif (1932-1936), and who participated in a number of the group’s exhibitions. Compared to what had occurred in previous groups, the participation of women, although reduced in number, was comparable to that of the male artists and being members of the group had a generally positive impact on the women’s careers. However, all this came at the expense of relinquishing any gender specificity in their work and the public presentation of it, and demonstrates that the normalization of women’s contributions to the avant-garde could only be brought about alongside a questioning of the more dogmatic views of modernity.

Primary motor cortex involvement in initial learning during visuomotor adaptation

Human motor behaviour is continually modified on the basis of errors between desired and actual movement outcomes. It is emerging that the role played by the primary motor cortex (M1) in this process is contingent upon a variety of factors, including the nature of the task being performed, and the stage of learning. Here we used repetitive TMS to test the hypothesis that M1 is intimately involved in the initial phase of sensorimotor adaptation. Inhibitory theta burst stimulation was applied to M1 prior to a task requiring modification of torques generated about the elbow/forearm complex in response to rotations of a visual feedback display. Participants were first exposed to a 30° clockwise (CW) rotation (Block A), then a 60° counterclockwise rotation (Block B), followed immediately by a second block of 30° CW rotation (A2). In the STIM condition, participants received 20s of continuous theta burst stimulation (cTBS) prior to the initial A Block. In the conventional (CON) condition, no stimulation was applied. The overt characteristics of performance in the two conditions were essentially equivalent with respect to the errors exhibited upon exposure to a new variant of the task. There were however, profound differences between the conditions in the latency of response preparation, and the excitability of corticospinal projections from M1, which accompanied phases of de-adaptation and re-adaptation (during Blocks B and A2). Upon subsequent exposure to the A rotation 24h later, the rate of re-adaptation was lower in the stimulation condition than that present in the conventional condition. These results support the assertion that primary motor cortex assumes a key role in a network that mediates adaptation to visuomotor perturbation, and emphasise that it is engaged functionally during the early phase of learning.

Has habitat heterogeneity promoted phenotypic and ecological sub-structuring among a Coregonus lavaretus population in a large Scottish lake?

Differences in stable-isotope values, morphology and ecology in whitefish Coregonus lavaretus were investigated between the three basins of Loch Lomond. The results are discussed with reference to a genetic investigation to elucidate any substructuring or spawning site fidelity. Foraging fidelity between basins of Loch Lomond was indicated by delta 13C and delta 15N values of C. lavaretus muscle tissue. There was, however, no evidence of the existence of sympatric morphs in the C. lavaretus population. A previous report of two C. lavaretus 'species' in Loch Lomond probably reflects natural variation between individuals within a single mixed population.

Accounting for the effects of lipids in stable isotope (δ13C and δ15N values) analysis of skin and blubber of balaenopterid whales

RATIONALE Stable isotope values (d¹³C and d¹⁵N) of darted skin and blubber biopsies can shed light on habitat use and diet of cetaceans, which are otherwise difficult to study. Non-dietary factors affect isotopic variability, chiefly the depletion of C due to the presence of C-rich lipids. The efficacy of post hoc lipid-correction models (normalization) must be tested. METHODS For tissues with high natural lipid content (e.g., whale skin and blubber), chemical lipid extraction or normalization is necessary. C:N ratios, d¹³C values and d¹⁵N values were determined for duplicate control and lipid-extracted skin and blubber of fin (Balaenoptera physalus), humpback (Megaptera novaeangliae) and minke whales (B. acutorostrata) by continuous-flow elemental analysis isotope ratio mass spectrometry (CF-EA-IRMS). Six different normalization models were tested to correct d¹³C values for the presence of lipids. RESULTS Following lipid extraction, significant increases in d¹³C values were observed for both tissues in the three species. Significant increases were also found for d¹⁵N values in minke whale skin and fin whale blubber. In fin whale skin, the d¹⁵N values decreased, with no change observed in humpback whale skin. Non-linear models generally out-performed linear models and the suitability of models varied by species and tissue, indicating the need for high model specificity, even among these closely related taxa. CONCLUSIONS Given the poor predictive power of the models to estimate lipid-free d13C values, and the unpredictable changes in d N values due to lipid-extraction, we recommend against arithmetical normalization in accounting for lipid effects on d13C values for balaenopterid skin or blubber samples. Rather, we recommend that duplicate analysis of lipid-extracted (d13C values) and non-treated tissues (d15N values) be used. Copyright © 2012 John Wiley & Sons, Ltd.

Interstitial Nitrogen in Diamond – Optical and EPR Studies

Nitrogen is one of the most common impurities in diamond. On a substitutional site it acts as a deep donor, approximately 1.7 eV below the conduction band. Irradiation of nitrogen containing diamond and subsequent annealing creates the nitrogen vacancy centre, which has recently attracted much attention for quantum information processing application. Another possible product of irradiation and annealing of nitrogen containing diamond is interstitial nitrogen. Presumably, a mobile carbon interstitial migrates to a substitutional nitrogen to produce an interstitial nitrogen complex which may or may not be mobile. The configuration(s) of interstitial nitrogen related defects (e.g. bond centred, [001]-split) are not known. An infra-red (IR) absorption peak at 1450 cm-1 labelled H1a has been associated with an nitrogen interstitial complex. [1] Theoretical modelling suggested that this IR local mode is due to a bond centred nitrogen interstitial [2]. However, more recent modelling [3] suggests that this defect is mobile at temperatures were H1a is stable and instead assign H1a to two nitrogen atoms occupying a single lattice site in a [001]-split configuration. To date no electron paramagnetic resonance (EPR) spectra have been conclusively associated with an interstitial nitrogen defect.

In this study we present data from new EPR and optical absorption studies in combination with uniaxial stress of nitrogen interstitial related defects in electron irradiated and annealed nitrogen doped diamond. These measurements yield symmetry information about the defects allowing us to determine which of the proposed models are possible. EPR spectra of nitrogen interstitial related defects in samples isotopically enriched with 15N are reported and we show that these explain the lack of previous EPR data for these defects. Correlations between the IR absorbance and the integrated intensity of the new EPR defects are studied for varying irradiation doses and annealing temperatures.

Scyphozoan jellyfish provide short-term reproductive habitat for hyperiid amphipods in a temperate near-shore environment

Hyperiid amphipods (Order Amphipoda, Suborder Hyperiidea) are known to infest gelatinous zooplankton. However, the temporal backdrop to these associations is less clear, given that data are often gathered during discrete sampling events rather than over time. In general, hyperiids are considered to be pelagic: however, for individuals associated with metagenic jellyfishes in temperate shallow shelf seas, this may not always be the case, as the majority of their gelatinous hosts are present in the water column from spring to the onset of autumn. Here, we explored the temporal patterns of colonisation and overall duration of the association between Hyperia galba and 3 scyphozoan jellyfish species (Aurelia aurita, Cyanea capillata and C. lamarckii) in a temperate coastal system (Strangford Lough, Northern Ireland) during 2010 and 2012. Concomitantly, we used carbon and nitrogen stable isotope ratios to examine whether hyperiid infestation represented a permanent association with their host or was part of a more complex life history. We found that jellyfish were colonised by H. galba ca. 2 mo after they are first observed in the lough and that H. galba reached 100% prevalence in the different jellyfish species shortly before the medusae of each species disappeared from the water column. It is possible that some jellyfish overwintered in deeper water, prolonging the association between H. galba and their hosts. However, all the medusae sampled during the spring and early summer (whether they were newly emerged or had overwintered from the previous season) were not infected with hyperiids, suggesting that such behaviour was uncommon or that individuals had become dissociated from their host during the winter. Further evidence of temporary association came from stable isotope data, where δ13C and δ15N isotope ratios were indicative of feeding outside of their host prior to jellyfish colonisation. In combination, these findings suggest alternating habitat associations for H. galba, with the amphipods spending the majority of the year outside of the 3 scyphozoan species considered here.