Nitrogen circulation in a Mediterranean holm oak forest, La Castanya, Montseny, northeastern Spain

The importance of storm frequency as well as the groundwater and hyporheic inputs on nitrate (NO₃-N) and ammonium (NH₄-N) levels in stream water were studied in a small perennial Mediterranean catchment, Riera Major, in northeast Spain. NO₃-N concentrations ranged from 0.15 to 1.9 mg l^-1. Discharge explained 47% of the annual NO₃-N concentration variance, but this percentage increased to 97% when single floods were analysed. The rate of change in nitrate concentration with respect to flow, ΔNO₃-N/ΔQ, ranged widely from 0 to 20 μg NO₃-N s l^-2. The ΔNO₃-N/ΔQ values fitted to a non linear model with respect to the storm flow magnitude (ΔQ) (r²=0.48, d.f.=22, P<0.01). High values of ΔNO₃-N/ΔQ occurred at intermediate ΔQ values, whereas low ΔNO₃-N/ΔQ values occurred during severe storms (ΔQ > 400 l s^-1). N₃-N concentrations exhibit anticlockwise hysteresis patterns with changing flow and the patterns observed for autumnal and winter storms indicated that groundwater was the main N₃-N source for stream and hyporheic water. At baseflow, NO₃-N concentration in groundwater was higher (t=4.75, d.f.=29, P>0.001) and co-varied with concentrations in the stream (r=0.91, d.f.=28, P<0.001). In contrast, NO₃-N concentration in hyporheic water was identical to that in stream water. The role of the hyporheic zone as source or sink for ammonium was studied hyporheic was studied comparing its concentrations in stream and hyporheic zone before and after a major storm occurred in October 1994 that removed particulate organic matter stored in sediments. Results showed high ammonium concentrations (75±28 s.d. μg NH₄-N l^-1) before the storm flow in the hyporheic zone. After the storm, the ammonium concentration in the hyporheic dropped by 80% (13.6±8 μg N₄-N l^-1) and approached to the level found in stream water (11±8 μg NH₄-N l^-1) indicating that indisturbed hyporheic sediments act as a source for ammonium. After the storm, the ammonium concentrations in the stream, hyporheic and groundwater zones were very similar suggesting that stream ammonium concentrations are sustained mainly by input from groundwater. The present study provides evidence that storm flow magnitude is an important source of variability of nitrate concentration and fluxes in Mediterranean streams subjected to an irregular precipitation regime with prolonged dry periods.</p> <p style='line-height: 20px;'><b>Keywords: </b>nitrate, discharge regime, hyporheic zone, groundwater, Mediterranean, stream, Riera Major</p

Anisotropic superconducting properties of aligned MgB2 crystallites

Samples of aligned MgB2 crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy Hc2^{ab}/Hc2^{c}= xi_{ab}/xi_{c} ~ 1.73; with xi_{o,ab} ~ 70 A, xi_{o,c} ~ 40 A, and a mass anisotropy ratio m_{ab}/m_{c} ~ 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.Comment: 4 pages, 4 figures; Revised version to appear in Phys. Rev. Let

Nitrogen circulation in a Mediterranean holm oak forest, La Castanya, Montseny, northeastern Spain

International audienceThe importance of storm frequency as well as the groundwater and hyporheic inputs on nitrate (NO3-N) and ammonium (NH4-N) levels in stream water were studied in a small perennial Mediterranean catchment, Riera Major, in northeast Spain. NO3-N concentrations ranged from 0.15 to 1.9 mg l-1. Discharge explained 47% of the annual NO3-N concentration variance, but this percentage increased to 97% when single floods were analysed. The rate of change in nitrate concentration with respect to flow, ?NO3-N/?Q, ranged widely from 0 to 20 ?g NO3-N s l-2. The ?NO3-N/?Q values fitted to a non linear model with respect to the storm flow magnitude (?Q) (r2=0.48, d.f.=22, P3-N/?Q occurred at intermediate ?Q values, whereas low ?NO3-N/?Q values occurred during severe storms (?Q > 400 l s-1). N3-N concentrations exhibit anticlockwise hysteresis patterns with changing flow and the patterns observed for autumnal and winter storms indicated that groundwater was the main N3-N source for stream and hyporheic water. At baseflow, NO3-N concentration in groundwater was higher (t=4.75, d.f.=29, P>0.001) and co-varied with concentrations in the stream (r=0.91, d.f.=28, P3-N concentration in hyporheic water was identical to that in stream water. The role of the hyporheic zone as source or sink for ammonium was studied hyporheic was studied comparing its concentrations in stream and hyporheic zone before and after a major storm occurred in October 1994 that removed particulate organic matter stored in sediments. Results showed high ammonium concentrations (75±28 s.d. ?g NH4-N l-1) before the storm flow in the hyporheic zone. After the storm, the ammonium concentration in the hyporheic dropped by 80% (13.6±8 ?g N4-N l-1) and approached to the level found in stream water (11±8 ?g NH4-N l-1) indicating that indisturbed hyporheic sediments act as a source for ammonium. After the storm, the ammonium concentrations in the stream, hyporheic and groundwater zones were very similar suggesting that stream ammonium concentrations are sustained mainly by input from groundwater. The present study provides evidence that storm flow magnitude is an important source of variability of nitrate concentration and fluxes in Mediterranean streams subjected to an irregular precipitation regime with prolonged dry periods

Collapse of the Gd3+Gd^{3+} ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor CeFe4P12CeFe_{4}P_{12}

Experiments on the $Gd^{3+}$ Electron Spin Resonance (ESR) in the filled skutterudite $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ ($x \approx 0.001$), at temperatures where the host resistivity manifests a smooth insulator-metal crossover, provides evidence of the underlying Kondo physics associated with this system. At low temperatures (below $T \approx K$), $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$ behaves as a Kondo-insulator with a relatively large hybridization gap, and the $Gd^{3+}$ ESR spectra displays a fine structure with lorentzian line shape, typical of insulating media. The electronic gap is attributed to the large hybridization present in the coherent regime of a Kondo lattice, when Ce 4f-electrons cooperate with band properties at half-filling. Mean-field calculations suggest that the electron-phonon interaction is fundamental at explaining the strong 4f-electron hybridization in this filled skutterudite. The resulting electronic structure is strongly temperature dependent, and at about $T^{*} \approx 160 K$ the system undergoes an insulator-to-metal transition induced by the withdrawal of 4f-electrons from the Fermi volume, the system becoming metallic and non-magnetic. The $Gd^{3+}$ ESR fine structure coalesces into a single dysonian resonance, as in metals. Still, our simulations suggest that exchange-narrowing via the usual Korringa mechanism, alone, is not capable of describing the thermal behavior of the ESR spectra in the entire temperature region ($4.2$ - $300$ K). We propose that temperature activated fluctuating-valence of the Ce ions is the missing ingredient that, added to the usual exchange-narrowing mechanism, fully describes this unique temperature dependence of the $Gd^{3+}$ ESR fine structure observed in $Ce_{1-x}Gd_{x}Fe_{4}P_{12}$.Comment: 19 pages, 6 figure

Irreversible magnetization under rotating fields and lock-in effect on ErBa_2Cu_3O_7 single crystal with columnar defects

We have measured the irreversible magnetization M_i of an ErBa_2Cu_3O_7 single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This method is valid for samples whose magnetization vector remains perpendicular to the sample surface over a wide angle range - which is the case for platelets and thin films - and presents several advantages over measurements of M_L(H) loops at fixed angles. The resulting M_i(\Theta) curves for several temperatures show a peak in the CD direction at high fields. At lower fields, a very well defined plateau indicative of the vortex lock-in to the CD develops. The H dependence of the lock-in angle \phi_L follows the H^{-1} theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to short range vortex-defects interactions.Comment: 7 pages, 6 figures, to be published in Phys. Rev.

Exponential dynamical localization for the almost Mathieu operator

We prove that the exponential moments of the position operator stay bounded for the supercritical almost Mathieu operator with Diophantine frequency