The Florida Everglades wetland ecosystem is subject to changes in hydroperiod and nutrient loading, resulting in soil P enrichment and changes in vegetation communities. The objectives of this study were to: (i) quantify the forms of inorganic and organic P in soils from four hydrologic units of the Everglades, and (ii) develop empirical relationships among various soil P forms. Soil samples from selected hydrologic units, including the Water Conservation Areas (WCAs) and the Holey Land Wildlife Management Area (HWMA), were obtained at various locations along transects perpendicular to each nutrient input source, while selected field sites were sampled in the Everglades Agricultural Area (EAA). Spatial distribution of total P in the surface 0-to 10-cm soil depth showed distinct gradients in the WCAs and HWMA soils, with high total P in soils closer to sources (canals and inflow structures) than in interior, unimpacted areas. Soil ash content and bulk density were also altered as a result of soil subsidence (for EAA soils), hydrology, and nutrient loading (for the WCAs and the HWMA soils). Influence of P loading was primarily confined to the top 30-cin soil depth, with about one-third of the P stored in the inorganic pool (primarily as Ca-and Mg-bound P), and the remainder present as organic P. Inorganic P content was higher in surface soils and decreased with depth. Soil P enrichment indicated that for approximately 5 km from the inflow structures or canals, soils have been impacted by nutrient loading. Empirical relationships developed in this study should be useful for estimating soil P forms at the landscape level, using total P data available for a large number of sites throughout the Everglades region. M OBILITY AND REACTIVITY OF P in wetlands under variable hydrologic conditions are controlled by the chemical composition of P in soil and water, relative sizes of various P pools in the soil, interactions of soluble fractions with solid phases, and decomposition of soil organic matter. Phosphorus is present in both organic and inorganic forms, with organic forms present as the dominant pool in many wetlands. Forms of inorganic P (Pi) in soils are usually determined by sequential extractions with acid and alkaline reagents, as proposed by Chang and Jackson (1957) and later modified by others for soils and sediments (Psenner et al., 1988; Ruttenburg, 1992; Olila et al., 1994). A modification of this scheme has been adopted for wetland soils (Quails and Richardson, 1995; Reddy et al., 1995). These schemes typically identify P in the following groups: (i) labile P; loosely adsorbed; (ii) P; associated with Fe and Al; (iii) Pi associated with Ca and Mg; (iv) alkali-extractable organic P (fulvic-and humic-bound P); and (v) residual organic P. Forms of organic P (P 0) have also been distin
In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, such as in d-wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of ironpnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe 2 (As 1 À x P x ) 2 crystals via electron irradiation, we find from the low-temperature penetration depth measurements that the nodal state changes to a nodeless state with fully gapped excitations. Moreover, under further irradiation the gapped state evolves into another gapless state, providing bulk evidence of unconventional sign-changing s-wave superconductivity. This demonstrates that the topology of the superconducting gap can be controlled by disorder, which is a strikingly unique feature of iron pnictides.
We reconsider the effect of disorder on the properties of a superconductor characterized by a signchanging order parameter appropriate for Fe-based materials. Within a simple two band model, we calculate simultaneously Tc, the change in residual resistivity ∆ρ0, and the zero-energy density of states, and show how these results change for various types of gap structure and assumptions regarding the impurity scattering. The rate of Tc suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the s± type, and propose that observation of a particular evolution of the penetration depth, nuclear magnetic resonance relaxation rate, or thermal conductivity temperature dependence with disorder would suffice.
The lack of nesting of the electron and hole Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a critical temperature of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Both angle-resolved photoemission and quasiparticle interference experiments have reported fully gapped superconducting order parameters with significant anisotropy. The system is also of interest because relatively strong correlations seem to be responsible for significant renormalization of the hole bands. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from measured photoemission spectra. The qualitative features of the gaps obtained in these calculations are shown to be different from previous two-dimensional theoretical works and in good agreement with experiment on the main Fermi surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets that are believed to dominate the pairing in other Fe-based superconductors continue to do so in LiFeAs despite the lack of nesting, leading to gaps with anisotropic s± structure. Some interesting differences relating to the enhanced dxy orbital content of the LiFeAs Fermi surface are noted.
The sequential weight loss-on-ignition (WLOI) method for determination of organic and carbonate or inorganic carbon (C) content was evaluated on sediments from diverse sources with a great range of C contents. The sediments were collected from canal, wetland, river, estuary, lake, and marine sites. The organic and inorganic C contents of these samples ranged from 1 to 430 g kg( -1) and from 4 to 97 g kg( -1), respectively. Combinations of the combustion time and temperature and optimal weight ranges of representative samples were tested, and comparisons of the WLOI method with other methods, including dry combustion and wet combustion, were made. These methods were (1) use of the carbon-nitrogen-sulfur (CNS) autoanalyzer with normal and reduced temperatures for total and organic C, (2) thermogravimetry for both organic and inorganic C, (3) use of the CNS autoanalyzer after removal of inorganic (carbonate) C by fumigating samples with concentrated HCl for organic C, (4) Walkley-Black wet combustion method for organic C, and (5) pressure-calcimeter associated with subtraction method (total C minus inorganic C) for organic C determinations. The results of analyzing samples of sediments of diverse origins showed that the optimal combination of temperature and time of WLOI depended mostly on the sources of the analyzed sediment. The WLOI analysis of sediment samples for organic C from wetlands, canal, estuary, or river sites needed a relatively low temperature but that of sediment samples from lake and marine sites required a relatively high temperature. Overall, to obtain reliable analysis results of samples from widely varied sediment sources except marine sediments, 500°C for 12 h was optimal for organic C content determination, and 800°C for yet another 12 h was optimal for inorganic C content determination. The temperature could even be reduced to 475°C if only wetland and stream sediments were included, but for marine sediments, 550°C for 12 h was necessary. Precise C content determinations for most sediment sources could be obtained by WLOI when sample quantities ranged from 2.0 to 4.0 g. The WLOI method, when conducted properly, resulted in precise measurements of C contents in "standard samples" used for calibration, and these values were closely comparable to results obtained with other dry combustion methods (R (2) ≥ 0.96). We conclude that WLOI, which has advantages of simplicity, cost-effectiveness, and no waste disposal over other methods, can provide precise measurements of organic and inorganic C contents in sediments from a wide range of sources, but the selection of heating temperature and exposure time should be carefully considered based on sediment sources.
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