Supramolecular synthon pattern in solid clioquinol and cloxiquine (APIs of antibacterial,antifungal, antiaging and antituberculosis drugs) studied by 35Cl NQR, 1H-17O and 1H-14N NQDR and DFT/QTAIM

The quinolinol derivatives clioquinol (5-chloro-7-iodo-8-quinolinol, Quinoform) and cloxiquine (5-chloro-8-quinolinol) were studied experimentally in the solid state via ³⁵Cl NQR, ¹H-¹⁷O and ¹H-¹⁴N NQDR spectroscopies, and theoretically by density functional theory (DFT). The supramolecular synthon pattern of O–H···N hydrogen bonds linking dimers and π–π stacking interactions were described within the QTAIM (quantum theory of atoms in molecules) /DFT (density functional theory) formalism. Both proton donor and acceptor sites in O–H···N bonds were characterized using ¹H-¹⁷O and ¹H-¹⁴N NQDR spectroscopies and QTAIM. The possibility of the existence of O–H···H–O dihydrogen bonds was excluded. The weak intermolecular interactions in the crystals of clioquinol and cloxiquine were detected and examined. The results obtained in this work suggest that considerable differences in the NQR parameters for the planar and twisted supramolecular synthons permit differentiation between specific polymorphic forms, and indicate that the more planar supramolecular synthons are accompanied by a greater number of weaker hydrogen bonds linking them and stronger π···π stacking interactions.     

Microsolvation effect and hydrogen-bonding pattern of taurine-water TA-(H2O)n (n = 1-3) complexes

The microsolvation of taurine (TA) with one, two or three water molecules was investigated by a density functional theory (DFT) approach. Quantum theory of atoms in molecules (QTAIM) analyses were employed to elucidate the hydrogen bond (H-bond) interaction characteristics in TA-(H₂O)_n (n = 1–3) complexes. The results showed that the intramolecular H-bond formed between the hydroxyl and the N atom of TA are retained in most TA-(H₂O)_n (n = 1–3) complexes, and are strengthened via cooperative effects among multiple H-bonds from n = 1–3. A trend of proton transformation exists from the hydroxyl to the N atom, which finally results in the cleavage of the origin intramolecular H-bond and the formation of a new intramolecular H-bond between the amino and the O atom of TA. Therefore, the most stable TA-(H₂O)₃ complex becomes a zwitterionic complex rather than a neutral type. A many-body interaction analysis showed that the major contributors to the binding energies for complexes are the two-body energies, while three-body energies and relaxation energies make significant contributions to the binding energies for some complexes, whereas the four-body energies are too small to be significant.     

Microsolvation of aminoethanol: a study using DFT combined with QTAIM

The microsolvation of aminoethanol (AE) with one, two, three or four water molecules was investigated using a density functional theory (DFT) approach. Quantum theory of atoms in molecules (QTAIM) analyses were employed to elucidate the hydrogen-bonding characteristics of AE–(H₂O) _n (n = 1–4) complexes. The results showed that AE tends to break its intramolecular OH^AE···N^AE hydrogen bond (H-bond) upon microsolvation and form intermolecular H-bonds with water molecules, while complexes that retain the intramolecular OH^AE···N^AE H-bond show reduced stabilities. The intermolecular H-bond that forms between the nitrogen atom of AE and the hydroxyl of a water molecule is the strongest one for the most stable AE–(H₂O) _n (n = 1–4) complexes, and as n increases from 1 to 4 they grow stronger. The partial covalent character of this H-bond was confirmed by QTAIM analyses. Many-body interaction analysis showed that the relaxation energies and two- and three-body energies make significant contributions to the binding energies of the complexes.     

ENDOR and ESEEM investigation of the Ni-containing superoxide dismutase

Superoxide dismutases (SODs) protect cells against oxidative stress by disproportionating O₂ ⁻ to H₂O₂ and O₂. The recent finding of a nickel-containing SOD (Ni-SOD) has widened the diversity of SODs in terms of metal contents and SOD catalytic mechanisms. The coordination and geometrical structure of the metal site and the related electronic structure are the keys to understanding the dismutase mechanism of the enzyme. We performed Q-band ¹⁴N,^1/2H continuous wave (CW) and pulsed electron–nuclear double resonance (ENDOR) and X-band ¹⁴N electron spin echo envelope modulation (ESEEM) on the resting-state Ni-SOD extracted from Streptomyces seoulensis. In-depth analysis of the data obtained from the multifrequency advanced electron paramagnetic resonance techniques detailed the electronic structure of the active site of Ni-SOD. The analysis of the field-dependent Q-band ¹⁴N CW ENDOR yielded the nuclear hyperfine and quadrupole coupling tensors of the axial N_δ of the His-1 imidazole ligand. The tensors are coaxial with the g-tensor frame, implying the g-tensor direction is modulated by the imidazole plane. X-band ¹⁴N ESEEM characterized the hyperfine coupling of N_ε of His-1 imidazole. The nuclear quadrupole coupling constant of the nitrogen suggests that the hydrogen-bonding between N_ε–H and O_Glu-17 present for the reduced-state Ni-SOD is weakened or broken upon oxidizing the enzyme. Q-band ¹H CW ENDOR and pulsed ²H Mims ENDOR showed a strong hyperfine coupling to the protons(s) of the equatorially coordinated His-1 amine and a weak hyperfine coupling to either the proton(s) of a water in the pocket at the side opposite the axial N_δ or the proton of a water hydrogen-bonded to the equatorial thiolate ligand.     

Azide adducts of stearoyl-ACP desaturase: a model for μ-1,2 bridging by dioxygen in the binuclear iron active site

 The stearoyl-acyl carrier protein Δ⁹ desaturase (Δ9D) uses an oxo-bridged diiron center to catalyze the NAD(P)H– and O₂–dependent desaturation of stearoyl-ACP. Δ9D, ribonucleotide reductase, and methane monooxygenase have substantial similarities in their amino acid primary sequences and the physical properties of their diiron centers. These three enzymes also appear to share common features of their reaction cycles, including the binding of O₂ to the diferrous state and the subsequent generation of transient diferric-peroxo and diferryl species. In order to investigate the coordination environment of the proposed diferric-peroxo intermediate, we have studied the binding of azide to the diiron center of Δ9D using optical, resonance Raman (RR), and transient kinetic spectroscopic methods. The addition of azide results in the appearance of new absorption bands at 325 nm and 440 nm (k _app≈3.5 s^–1 in 0.7 M NaN₃, pH 7.8). RR experiments demonstrate the existence of two different adducts: an η¹–terminal structure at pH 7.8 (¹⁴N₃ ^– asymmetric stretch at 2073 cm^–1, resolved into two bands with ¹⁵N¹⁴N₂ ^–) and a μ-1,3 bridging structure at pH<7 (¹⁴N₃ ^– asymmetric stretch at 2100 cm^–1, shifted as a single band with ¹⁵N¹⁴N₂ ^–). Both adducts also exhibit an Fe–N₃ stretching mode at ≈380 cm^–1, but no accompanying Fe–O–Fe stretching mode, presumably due to either protonation or loss of the oxo bridge. The ability to form a μ-1,3 bridging azide supports the likelihood of a μ-1,2 bridging peroxide as a catalytic intermediate in the Δ9D reaction cycle and underscores the adaptability of binuclear sites to different bridging geometries. Received: 23 August 1996 / Accepted: 4 October 1996     

Probing the ground state of the purple mixed valence CuA center in nitrous oxide reductase: a CW ENDOR (X-band) study of the 65Cu, 15N-histidine labeled enzyme and interpretation of hyperfine couplings by molecular orbital calculations

 CW ENDOR (X-band) spectra for the purple mixed-valence [Cu(1.5+)...Cu(1.5+)], S = 1/2, Cu_A site in nitrous oxide reductase were obtained after insertion of ⁶⁵Cu or both ⁶⁵Cu and ¹⁵N-histidine. The ¹⁴N/¹⁵N isotopic substitution allowed for an unambiguous deconvolution of proton and nitrogen hyperfine couplings in the spectra. A single nitrogen coupling with a value of 12.9 ± 0.4 MHz for ¹⁴N was detected. Its anisotropy was characteristic for imidazole bound to copper. A spin density of 3–5% was estimated for the nitrogen donors to Cu_A, indicating that the ground state is ²B_3u. Proton hyperfine structure was detected from four C_β protons of coordinating cysteine residues. Their isotropic and anisotropic parts were deconvoluted by spectral simulation. From the anisotropic couplings a spin density of 16–24% was estimated for each of the cysteine thiolate donors of Cu_A. The [N_HisCu(RS)₂CuN_His]⁺ core structure of Cu_A in nitrous oxide reductase from Pseudomonas stutzeri is predicted to be similar to the crystallographically determined Cu_A* structure (Wilmanns M, Lappalainen P, Kelly M, Sauer-Eriksson E, Saraste M (1995) Proc Natl Acad Sci USA 92 : 11955–11959), but distinct from the Cu_A structure of Paracoccus denitrificans cytochrome c oxidase (Iwata S, Ostermeier C, Ludwig B, Michel H (1995) Nature 376 : 660–669). The angular dependence of the isotropic couplings as a function of the electronic ground state was calculated by the INDO/S method. The Mulliken atomic-spin populations calculated by a gradient-corrected density functional method and the semiempirical INDO/S method were compared with experimentally derived spin populations, and good agreement between theory and experiment was found for both calculations. The ground state of Cu_A is best represented by the resonance structures of the form [Cu^IS^–S^–Cu^II↔ Cu^IS^•S^–Cu^I↔ Cu^IS^–S^•Cu^I↔ Cu^IIS^–S^–Cu^I]. It is proposed that the Cu 4s,p as well as sulfur 3d orbitals play a role in the stabilization of this novel type of cluster. Received: 17 September 1997 / Accepted: 28 October 1997     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

Evaluation of denitrification in an urban stream receiving wastewater effluent

Urban streams often contain elevated concentrations of nitrogen (N) which can be amplified in systems receiving effluent from wastewater treatment plants (WWTP). In this study, we evaluated the importance of denitrification in a stream draining urban Greensboro, NC, USA, using two approaches: (1) natural abundance of ¹⁵N–NO₃⁻ in conjunction with background NO₃⁻–N concentrations along a 7 km transect downstream of a WWTP; and (2) C₂H₂ block experiments at three sites and at three habitat types within each site. Overall lack of a longitudinal pattern of δ¹⁵N–NO₃⁻ and NO₃⁻–N, combined with high concentrations of NO₃⁻–N suggested that other factors were controlling NO₃⁻–N flux in the study transect. However, denitrification did appear to be significant along one portion of the transect. C₂H₂ block experiments showed that denitrification rates were much higher downstream of the WWTP compared to upstream, and showed that denitrification rates were highest in erosional and depositional areas downstream of the WWTP and in erosional areas upstream of the plant. Thus, the combination of the two methods for evaluating denitrification provided more insight into the spatial dynamics of denitrification activity than either approach alone. Denitrification appeared to be a significant sink for NO₃⁻–N upstream of the WWTP, but not downstream. Approximately 46% of the total NO₃⁻–N load was removed via denitrification in the upstream, urban section of the stream, while only 2.3% of NO₃⁻–N was lost downstream of the plant. This result suggests that controlling NO₃⁻–N loading from the plant could result in considerable improvement of downstream water quality.     

δ<Superscript>15</Superscript>N constraints on long-term nitrogen balances in temperate forests

Biogeochemical theory emphasizes nitrogen (N) limitation and the many factors that can restrict N accumulation in temperate forests, yet lacks a working model of conditions that can promote naturally high N accumulation. We used a dynamic simulation model of ecosystem N and δ¹⁵N to evaluate which combination of N input and loss pathways could produce a range of high ecosystem N contents characteristic of forests in the Oregon Coast Range. Total ecosystem N at nine study sites ranged from 8,788 to 22,667 kg ha⁻¹ and carbon (C) ranged from 188 to 460 Mg ha⁻¹, with highest values near the coast. Ecosystem δ¹⁵N displayed a curvilinear relationship with ecosystem N content, and largely reflected mineral soil, which accounted for 96–98% of total ecosystem N. Model simulations of ecosystem N balances parameterized with field rates of N leaching required long-term average N inputs that exceed atmospheric deposition and asymbiotic and epiphytic N₂-fixation, and that were consistent with cycles of post-fire N₂-fixation by early-successional red alder. Soil water δ¹⁵NO₃ ⁻ patterns suggested a shift in relative N losses from denitrification to nitrate leaching as N accumulated, and simulations identified nitrate leaching as the primary N loss pathway that constrains maximum N accumulation. Whereas current theory emphasizes constraints on biological N₂-fixation and disturbance-mediated N losses as factors that limit N accumulation in temperate forests, our results suggest that wildfire can foster substantial long-term N accumulation in ecosystems that are colonized by symbiotic N₂-fixing vegetation.     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

Soil-Atmosphere Exchange of N<Subscript>2</Subscript>O and NO in Near-Natural Savanna and Agricultural Land in Burkina Faso (W. Africa)

In a combined field and laboratory study in the southwest of Burkina Faso, we quantified soil-atmosphere N₂O and NO exchange. N₂O emissions were measured during two field campaigns throughout the growing seasons 2005 and 2006 at five different experimental sites, that is, a natural savanna site and four agricultural sites planted with sorghum (n = 2), cotton and peanut. The agricultural fields were not irrigated and not fertilized. Although N₂O exchange mostly fluctuated between −2 and 8 μg N₂O–N m⁻² h⁻¹, peak N₂O emissions of 10–35 μg N₂O–N m⁻² h⁻¹ during the second half of June 2005, and up to 150 μg N₂O–N m⁻² h⁻¹ at the onset of the rainy season 2006, were observed at the native savanna site, whereas the effect of the first rain event on N₂O emissions at the crop sites was low or even not detectable. Additionally, a fertilizer experiment was conducted at a sorghum field that was divided into three plots receiving different amounts of N fertilizer (plot A: 140 kg N ha⁻¹; plot B: 52.5 kg N ha⁻¹; plot C: control). During the first 3 weeks after fertilization, only a minor increase in N₂O emissions at the two fertilized plots was detected. After 24 days, however, N₂O emission rates increased exponentially at plot A up to a mean of 80 μg N₂O–N m⁻² h⁻¹, whereas daily mean values at plot B reached only 19 μg N₂O–N m⁻² h⁻¹, whereas N₂O flux rates at plot C remained unchanged. The calculated annual N₂O emission of the nature reserve site amounted to 0.52 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and to 0.67 kg N₂O–N ha⁻¹ a⁻¹ in 2006, whereas the calculated average annual N₂O release of the crop sites was only 0.19 kg N₂O–N ha⁻¹ a⁻¹ and 0.20 kg N₂O–N ha⁻¹ a⁻¹ in 2005 and 2006, respectively. In a laboratory study, potential N₂O and NO formation under different soil moisture regimes were determined. Single wetting of dry soil to medium soil water content with subsequent drying caused the highest increase in N₂O and NO emissions with maximum fluxes occurring 1 day after wetting. The stimulating effect lasted for 3–4 days. A weaker stimulation of N₂O and NO fluxes was detected during daily wetting of soil to medium water content, whereas no significant stimulating effect of single or daily wetting to high soil water content (>67% WHC_max) was observed. This study demonstrates that the impact of land-use change in West African savanna on N trace gas emissions is smaller—with the caveat that there could have been potentially higher N₂O and NO emissions during the initial conversion—than the effect of timing and distribution of rainfall and of the likely increase in nitrogen fertilization in the future.     

Contribution of N2 fixing cyanobacteria to rice production: availability of nitrogen from 15N-labelled cyanobacteria and ammonium sulphate to rice

This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N₂-fixing cyanobacteria abundance and N₂ fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha^-1. No further increase was observed with 140 kg N ha^-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from ¹⁵N fertilizer. Recovery of applied ¹⁵N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of ¹⁵N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application. This revised version was published online in June 2006 with corrections to the Cover Date.     

Atmospheric Deposition to the Turkey Lakes Watershed: Temporal Variations and Characteristics

We investigated the atmospheric concentrations and deposition fluxes of major ions to the Turkey Lakes Watershed (TLW) between 1980 and 1996. During that time, daily SO₄ ²⁻ concentrations in precipitation decreased markedly, while NO₃ ⁻, NH₄ ⁺, and H⁺ concentrations remained roughly constant. It appears that precipitation acidity did not decrease in spite of declining SO₄ ²⁻ concentrations due to a concurrent and counterbalancing decrease in the concentrations of Ca²⁺, Mg²⁺, and K⁺ in precipitation. The reasons for the decline in base cations are unknown, but this decline is probably related to decreasing emissions of soil-derived particles from agricultural, industrial, and road sources. A similar situation was seen during the same period in other parts of Canada, the eastern United States, and Europe. Wet, dry, and total (wet + dry) deposition fluxes of sulphur (S) and nitrogen (N) were estimated annually for the years 1980–96. The 17-year mean annual total (wet + dry) deposition of S to the watershed was estimated at 38.5 mmol m⁻² y⁻¹ (range 24.3–50.3). Total S deposition decreased by 35% from the early 1980s (1982–84) to the mid-1990s (1994–96), a decline consistent with the 23% decline in annual SO₂ emissions in eastern North America during the same period. In contrast, the annual total (wet + dry) deposition of oxidized N ranged from 39.8 to 60.4 mmol m⁻² y⁻¹, with a 15-year mean of 50.1 mmol m⁻² y⁻¹ and a net increase of 10% between the early 1980s (1983–85) and the mid-1990s (1994–96). This is in keeping with a 10% increase in NO_x emissions in eastern North America during the same period. For both S and N (oxidized), wet deposition dominated over dry deposition as the major mechanism for atmospheric input to the watershed. Annually, wet deposition accounted for approximately two-thirds of the total atmospheric deposition of both S and N. Dry S deposition was due more to gaseous SO₂ deposition (two-thirds of dry S deposition) than to particulate SO₄ ²⁻ deposition (one-third of dry S deposition). Dry deposition of oxidized N, however, was dominated (95%) by gaseous HNO₃ deposition, with minimal input from particulate NO₃ ⁻ deposition. Compared to several selected watershed/forest sites in Canada, the United States, and Europe, the estimated total deposition of S and N at the TLW was relatively high during the measurement period. Received 5 October 1999; accepted 1 March 2001.     

ADP and ATP binding to noncatalytic sites of thiol-modulated chloroplast ATP synthase

A modified ‘cold chase’ technique was used to study tight [¹⁴C]ADP and [¹⁴C]ATP binding to noncatalytic sites of chloroplast ATP synthase (CF₀F₁). The binding was very low in the dark and sharply increased with light intensity. Dissociation of labeled nucleotides incorporated into noncatalytic sites of CF₀F₁ or CF₁ reconstituted with EDTA-treated thylakoid membranes was also found to be light-dependent. Time dependence of nucleotide dissociation is described by the first order equation with a k _d of about 5 min⁻¹. The exposure of thylakoid membranes to 0.7–24.8 μM nucleotides leads to filling of up to two noncatalytic sites of CF₀F₁. The sites differ in their specificity: one preferentially binds ADP, whereas the other – ATP. A much higher ATP/ADP ratio of nucleotides bound at noncatalytic sites of isolated CF₁ dramatically decreases upon its reconstitution with EDTA-treated thylakoid membranes. It is suggested that the decrease is caused by conformational changes in one of the α subunits induced by its interaction with the δ subunit and/or subunit I–II when CF₁ becomes bound to a thylakoid membrane.     

Soluble Organic Nitrogen Pools in Forest soils of Subtropical Australia

Soil soluble organic N (SON) plays an important role in N biogeochemical cycling. In this study, 22 surface forest soils (0–10 cm) were collected from southeast Queensland, Australia, to investigate the size of SON pools extracted by water and salt solutions. Approximately 5–45 mg SON kg⁻¹, 2–42 mg SON kg⁻¹ and 1–24 SON mg kg⁻¹ were extracted by 2 M KCl, 0.5 M K₂SO₄ and water, on average, corresponding to about 21.1, 13.5 and 7.0 kg SON ha⁻¹ at the 0–10 cm forest soils, respectively. These SON pools, on average, accounted for 39% (KCl extracts), 42% (K₂SO₄ extracts) and 43% (water extracts) of total soluble N (TSN), and 2.3% (KCl extracts), 1.3% (K₂SO₄ extracts) and 0.7% (water extracts) of soil total N, respectively. Large variation in SON pools observed across the sites in the present study may be attributed to a combination of factors including soil types, tree species, management practices and environmental conditions. Significant relationships were observed among the SON pools extracted by water, KCl and K₂SO₄ and microbial biomass N (MBN). In general, KCl and K₂SO₄ extracted more SON than water from the forest soils, while KCl extracted more SON than K₂SO₄. The SON and soluble organic C (SOC) in KCl, K₂SO₄ and water extracts were all positively related to soil organic C, total N and clay contents. This indicates that clay and soil organic matter play a key role in the retention of SON in soil.     

Impact of long-term nitrogen addition on carbon stocks in trees and soils in northern Europe

The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0–10 cm mineral soil) by analysing data from 15 long-term (14–30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30–50 kg N ha⁻¹ year⁻¹) were always more efficient per unit of N than high application rates (50–200 kg N ha⁻¹ year⁻¹). Addition of a cumulative amount of N of 600–1800 kg N ha⁻¹ resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg⁻¹ (N added) (“N-use efficiency”), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg⁻¹ (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg⁻¹ (N) at C/N 35 and decreased again to about 20 kg (C) kg⁻¹ (N) at C/N 50 when N only was added. In contrast, addition of NPK resulted in high (40–50 kg (C) kg⁻¹ (N)) N-use efficiency also at N-rich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at N-rich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3–4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg⁻¹ (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha⁻¹ year⁻¹ higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m⁻² more tree C and 1.3 ± 0.5 kg m⁻² more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70–80% of the difference in SOC can be explained by different N deposition.     

Conformational stabilities, infrared, and vibrational dichroism spectroscopy studies of tris(ethylenediamine) zinc(II) chloride

The conformational stabilities of the transition metal complex of Zn (en)₃Cl₂ were studied using density functional theory (DFT). Deformational potential energy profiles (PEPs), and pathways between the different isomeric conformational energies were calculated using DFT/B3LYP/6–31G. The relative conformational energies of Δ(λλλ), Δ(λλδ), Δ(λδδ) and Δ(δδδ) are 10.48, 7.08, 3.56, and 0.0 kcal/mol, respectively, which are small compared to the barrier heights for reversible phase transitions (49.56, 49.55, 49.52 kcal/mol, respectively). Frequency assignment was carried out by decomposing Fourier transform infrared (FTIR) spectra using Gaussian and Gaussview. The theoretical IR and vibrational dichroism spectroscopy (VCD) absorption spectra are presented for all conformations within the range of 400–3,500 cm^-1.     

Nitrogen dynamics and mineralization in degraded agricultural soil mulched with fresh grass

Understanding mulching influences on nitrogen (N) availability is important for developing N management strategies in plantations at the upland sites of the southwestern China. Dynamics of biomass loss and nutrient release of mulching material, N availability in the soil and N mineralization in situ were evaluated for the treatments with different mulch quantity in degraded agricultural soil. The time taken for 95% decomposition of the initial biomass of Cogon grass (Imperata cylindrical L. Beauv. var. major) was 17 months with a half-life (t _1/2) of about 4.8 months. During the first 4 months about 55.2% of N was released, and after 1-year decomposition about 71.6% of N was released from the mulch material. The fresh grass mulch increased the available N in the soil as they decomposed. Compared to no mulch treatment, mulch treatments with 2.5, 5.0 and 7.5 kg m⁻² mulching grass increased available N by about 13.1, 40.8 and 56.4% in the top soil (0–5 cm), and about 23.6, 78.0 and 139.3% in the middle layer (5–20 cm), respectively. The mean annual net N mineralization in the mulched plots had 9.0–40.9% higher cumulative rate than that in no-mulch plots, and the majority of the accumulated N in the incubated soils existed as NO₃–N. There was a positive relationship between the rate of N mineralization and the available N in both the top soil and the middle layer. Mulch improves soil nutrients and this improvement increased with increasing mulching quantity. The increment of net N mineralization was approximately 69, 161 and 322 kg N ha⁻¹ year⁻¹ in the soil of 0–20 cm depth for the 2.5, 5.0 and 7.5 kg m⁻² grass mulch treatments, respectively. The results from this study will provide a basis to optimize mulching techniques for poplar plantations in degraded agricultural soils of southwestern China.     

Short-range spatial variability of soil δ15N natural abundance – effects on symbiotic N2-fixation estimates in pea

The δ¹⁵N natural abundance (‰) of the total soil N pool varies at the landscape level, but knowledge on short-range variability and consequences for the reliability of isotopic methods are poorly understood. The short-range spatial variability of soil δ¹⁵N natural abundance as revealed by the ¹⁵N abundance in spring barley and N₂-fixing pea was measured within the 0.15–4 m scale at flowering and at maturity. The short-range spatial variability of soil δ¹⁵N natural abundance and symbiotic nitrogen fixation were high at both growth stages. Along a 4-m row, the δ¹⁵N natural abundance in barley reference plants varied up to 3.9‰, and sometimes this variability was observed even between plants grown only 30 cm apart. The δ¹⁵N natural abundance in pea varied up to 1.4‰ within the 4-m row. The estimated percentage of nitrogen derived from the atmosphere (%Ndfa) varied from 73–89% at flowering and from 57–95% at maturity. When increasing the sampling area from 0.01 m² (single plants) and up to 0.6 m² (14 plants) the %Ndfa coefficient of variation (CV) declined from 5 to 2% at flowering and from 12 to 2% at maturity. The implications of the short-range variability in δ¹⁵N natural-abundance are that estimates of symbiotic N₂-fixation can be obtained from the natural abundance method if at least half a square meter of crop and reference plants is sampled for the isotopic analysis. In fields with small amounts of representative reference crops (weeds) it might be necessary to sow in reference crop species to secure satisfying N₂-fixation estimates.     

Investigation of intermolecular hydrogen bond interactions in crystalline l-Cysteine by DFT calculations of the oxygen-17, nitrogen-14, and hydrogen-2 EFG tensors and AIM analysis

A systematic computational study is carried out to investigate hydrogen bond (HB) interactions in the real crystalline structures of l-Cysteine at 30 and 298 K by density functional theory (DFT) calculations of electric field gradient (EFG) tensors at the sites of O-17, N-14, and H-2 nuclei. One-molecule (monomer) and nine-molecule (cluster) models of l-Cysteine are created by available crystal coordinates at both temperatures and the EFG tensors are calculated for both models to indicate the effect of HB interactions on the tensors. The calculated EFG tensors at the level of B3LYP and B3PW91 DFT methods and 6-311++G?? and cc-pVTZ basis sets are converted to those experimentally measurable nuclear quadrupole resonance (NQR) parameters i.e. quadrupole coupling constants (qcc) and asymmetry parameters (η_Q). The evaluated NQR parameters reveal that the EFG tensors of ¹⁷O, ¹⁴N, and ²H are influenced and show particular trends from monomer to the target molecule in the cluster due to the contribution of target molecule to classic N–H…O, and non-classic S–H…O and S–H…S types of HB interactions. On the other hand, atoms in molecules (AIM) analyses confirm the presence of HB interactions and rationalize the observed EFG trends. The results indicate different contribution of various nuclei to HB interactions in the cluster where O2 and N1 have major contributions. The EFG tensors as well as AIM analysis at the H6 site show that the N1-H6…O2 HB undergoes a significant change from 30 to 298 K where changes in other N–H…O interactions are almost negligible. There is a good agreement between the calculated ¹⁴N NQR parameters and reported experimental data.