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Abstract: The inclusion of legumes in semi‐arid native grasslands may promote the productivity and nutritive value of forage. This study was designed to assess the effect of legumes (the introduced legume Medicago sativa or the native legume Dalea purpurea) and soil P fertility (addition of 0, 50, or 200 P2O5 kg/ha at seeding) on the dry matter and nutrient content of native grasses mixtures, compared with the commonly used introduced forage grass Bromus biebersteinii grown with M. sativa. Plant harvests were performed in September 2008, July 2009 and September 2009. Plants nutrient content, δ15N value and dry matter were analysed. Results show that the M. sativa enhanced the N and P concentrations of native grass mixtures early in the summer, as well as the N concentration in Bouteloua gracilis in late summer of the driest year, 2009. The higher AM fungal diversity promoted by M. sativa was positively correlated with the dry matter and nitrogen uptake of M. sativa and with the P concentration of native grasses, in early summer. Overall, this study shows that M. sativa promoted beneficial AM fungal taxa and improved forage production in the semi‐arid prairies. [ABSTRACT FROM AUTHOR]

Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia (NH x(sw)), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here we compare [NH x(sw)] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [NH x(sw)] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NH x within observed ranges. The resulting global ocean emissions is 2.5 TgN a−1, much lower than current literature values (7-23 TgN a−1), including the widely used Global Emissions InitiAtive (GEIA) inventory (8 TgN a−1). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NH x over most of the ocean in the Northern Hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2-5 TgN a−1, comparable in magnitude to other natural sources from open fires and soils. [ABSTRACT FROM AUTHOR]

As a part of the Herschel key programme PRISMAS, we have used the Herschel/HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards eight high-mass star-forming regions in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. Here, we report observations towards W49N of the NH N = 1-0, J = 2-1, and J = 1-0, ortho-NH2 NKa,KcJ = 11,13/2-00,01/2, ortho-NH3 JK = 10-00 and 20-10, para-NH3 JK = 21-11 transitions, and unsuccessful searches for NH+. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH, which is found to correlate with H2 in the solar neighbourhood diffuse gas. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2 × 10-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities ≤2-14% of N(NH), which is in contrast to the behaviour of the abundances of CH+ and OH+ relative to the values determined for the corresponding neutrals CH and OH. Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ≈ 0.5-0.7 ± 0.1, in the strongest absorption components. This result cannot be explained by current models as we had expected to find a value of unity or higher. [ABSTRACT FROM AUTHOR]

Plasma austenitic nitrocarburising as well as plasma ferritic nitrocarburising treatments of pure iron have been carried out in a modified dc plasma unit at 700°C with a gas mixture of nitrogen, hydrogen and organic vapour as the carbon–nitrogen media supplier. The composition, phase structure, microstructure and hardness of the plasma nitrocarburised surface were characterised by a number of materials analytical techniques. The load bearing capacities of ferritic nitrocarburised and austenitic carburised samples were evaluated using a Falex tribometer, and the effect of the sublayer in determining the load bearing capacity of plasma nitrocarburised material was investigated. The experimental results show that the load bearing capacity of plasma nitrocarburised pure iron is mainly determined by the hardness of the sublayer. The load bearing capacity of plasma nitrocarburised pure iron increases in the order of (i) plasma ferritic nitrocarburised, (ii) plasma austenitic nitrocarburised and slow cooled, and (iii) plasma austenitic nitrocarburised, quenched and subzero treated. Based on the experimental results, the role of the sublayer in determining the load bearing capacity of plasma nitrocarburised material is discussed. [ABSTRACT FROM AUTHOR]

AISI 316 austenitic stainless steel has been plasma nitrided using the active screen plasma nitriding (ASPN) technique. Corrosion properties of the untreated and AS plasma nitrided 316 steel have been evaluated using various techniques, including qualitative evaluation after etching in 50%HCl + 25%HNO3 + 25%H2O, weight loss measurement after immersion in 10% HCl, and anodic polarisation tests in 3.5% NaCl solution. The results showed that the untreated 316 stainless steel suffered severe localised pitting and crevice corrosion under the testing conditions. AS plasma nitriding at low temperature (420 °C) produced a single phase nitrided layer of nitrogen expanded austenite (S-phase), which considerably improved the corrosion properties of the 316 austenitic stainless steel. In contrast, AS plasma nitriding at a high temperature (500 °C) resulted in chromium nitride precipitation so that the bulk of the nitrided case had very poor corrosion resistance. However, a thin deposition layer on top of the nitrided case, which seems to be unique to AS plasma nitriding, could have alleviated the corrosion attack of the higher temperature nitrided 316 steel. [Copyright &y& Elsevier]

Surface modification of ultra high molecular weight polyethylene (UHMWPE) was explored using ion implantation with different dosages (1 × 10[sup 15] - 1 × 10[sup 17] ions cm [sup -2]) of nitrogen. The modified surfaces were characterised by SEM, nanoindentation and tribological tests. The experimental results showed that ion implantation can significantly increase the surface hardness of UHMWPE. However, the improvement in wear resistance of the modified UHMWPE is highly dose dependent. Compared with the untreated material, the low dose implanted UHMWPE exhibited a three fold improvement in wear resistance, while the high dose ion implanted material showed hardly any improvement although it possessed the highest hardness. [ABSTRACT FROM AUTHOR]

Abstract: This research was directed at improving the wear resistance of Stellite 21 alloy by developing novel surface engineering technologies. To this end, a series of plasma surface alloying (PSA) treatments with both carbon and nitrogen were conducted using a DC plasma unit with C–N bearing gas mixture at temperatures ranging from 400 to 550°C. The surface alloyed layers were analyzed by XRD, GDS, SEM/EDX, TEM and a microhardness tester. Detailed tribological characterization was carried out by using a reciprocating tribometer in air and in 3.5% NaCl solution. Experimental results have demonstrated that C and N supersaturated metastable phase, i.e. S-phase, can be formed by the PSA treatments with both carbon and nitrogen but at a high temperature of 550°C, chromium nitrides precipitated in the surface treated layer. The surface hardness of Stellite 21 alloy can be significantly increased by 2–3 times following the PSA treatments. The wear factor of Stellite 21 alloy in air and in 3.5% NaCl solution can be reduced by 99% and 96%, respectively. [Copyright &y& Elsevier]

Abstract: Austenitic plasma nitrocarburising treatments of a medium carbon steel (0.47 wt.% C) have been carried out using a modified DC plasma furnace in a gas mixture of nitrogen, hydrogen and organic vapour of C6H6. Various gas compositions and treatment procedures have been used in order to produce a thicker monophase ε carbonitride compound layer on the carbon steel surface. The nitrocarburised layers have been examined with several analytical techniques including X-ray diffraction, glow discharge spectrometry, metallographic and microhardness tests. It has been found that austenitic plasma nitrocarburising with organic vapour as the carbon source can readily produce a desirable monophase ε compound layer on the investigated plain carbon steel surface. The ε compound layer is supported by an austenite layer followed by a diffusion layer which is expected to give higher load bearing capacity. It has also been found that the “two-step” plasma nitrocarburising process is advantageous over the “one-step” process since the monophase ε compound layer produced by the two-step process can be thicker, and the treatment temperature or the total treatment time can be reduced. [Copyright &y& Elsevier]

Abstract: The active screen plasma and DC plasma nitriding of the low alloy steel 722M24 are investigated. Experimental results showed that the metallurgical characteristics and hardening effect on 722M24 steel nitrided by AS plasma nitriding at both floating potential and grounded potential were similar to those nitrided by DC plasma nitriding. Particles sputtered from the active screen and deposited on the specimen surface play the role of the nitrogen carrier in AS plasma nitriding. XRD and high-resolution SEM analysis indicated that the particles with sizes in sub-micron scale were Fe x N (x >2). Based on metallurgical analysis and Optical Emission Spectrometer (OES) experimental results, an AS plasma nitriding model has been proposed considering that AS plasma nitriding is a multi-stage process, involving sputtering, physical adsorption, desorption, diffusion and deposition. [Copyright &y& Elsevier]

Plasma nitrocarburising was carried out at a low temperature to improve the surface hardness of AISI 316 austenitic stainless steel without degradation of its corrosion resistance. It was found that nitrogen and carbon atoms can simultaneously be dissolved into the austenite lattice during the nitrocarburising process, forming a nitrogen and carbon supersaturated solid solution, which has a face centre tetragonal (fct) structure (i.e. the S phase). GDS analysis showed that the maximum nitrogen and carbon concentrations in the nitrocarburised S-phase layer occur at different depths from the surface. The nitrocarburised layers have not only high hardness like the nitrided layer, but also high thickness and gradually reduced hardness profile like the carburised layer. [Copyright &y& Elsevier]