Exchangeable NH Research Articles

Gas-Phase Intercluster Thiyl-Radical Induced C-H Bond Homolysis Selectively Forms Sugar C2-Radical Cations of Methyl D-Glucopyranoside: Isotopic Labeling Studies and Cleavage Reactions.

A suite of isotopologues of methyl D-glucopyranosides is used in conjunction with multistage mass spectrometry experiments to determine the radical site and cleavage reactions of sugar radical cations formed via a recently developed 'bio-inspired' method. In the first stage of CID (MS2), collision-induced dissociation (CID) of a protonated noncovalent complex between the sugar and S-nitrosocysteamine, [H3NCH2CH2SNO + M]+, unleashes a thiyl radical via bond homolysis to give the noncovalent radical cation, [H3NCH2CH2S• + M]+. CID (MS3) of this radical cation complex results in dissociation of the noncovalent complex to generate the sugar radical cation. Replacement of all exchangeable OH and NH protons with deuterons reveals that the sugar radical cation is formed in a process involving abstraction of a hydrogen atom from a C-H bond of the sugar coupled with proton transfer to the sugar, to form [M - H• + D+]. Investigation of this process using individual C-D labeled sugars reveals that the main site of H/D abstraction is the C2 position, since only the C2-deuterium labeled sugar yields a dominant [M - D• + H+] product ion. The fragmentation reactions of the distonic sugar radical cation, [M - H•+ H+], were studied by another stage of CID (MS4). 13C-labeling studies revealed that a series of three related fragment ions each contain the C1-C3 atoms; these arise from cross-ring cleavage reactions of the sugar. Graphical Abstract ᅟ.

Imidazole-Appended Macrocyclic Complexes of Fe(II), Co(II), and Ni(II) as ParaCEST Agents.

The solution chemistry and solid state structures of the Co(II), Fe(II), and Ni(II) complexes of N,N'-bis(imidazole-2-ylmethyl)-4,10-diaza-15-crown-5 (HINO) are reported. The Co(II) and Ni(II) complexes of HINO are the first examples of paraCEST agents (paramagnetic chemical exchange saturation transfer) that feature exchangeable imidazole NH protons. The crystal structures of [Co(HINO)]CoCl4·H2O and [Fe(HINO)](CF3SO3)2 have the metal ions coordinated to four nitrogen and three oxygen donor atoms of the macrocyclic ligand in a distorted pentagonal bipyramidal geometry. In [Ni(HINO)](CF3SO3)2, the nickel ion is bound to only two of the three ether oxygens and three nitrogens to produce a complex with a distorted octahedral geometry. The 1H NMR spectra of the three paramagnetic complexes show resonances characteristic of effective C2 symmetry in solution. CEST peaks attributed to the imidazole NH proton of the pendent group are observed at 32 and 55 ppm away from bulk water for [Co(HINO)]2+ and [Ni(HINO)]2+, respectively, on a 11.7 or 9.4 T NMR spectrometer. For both complexes, an optimal CEST effect was observed at pH 7.2, and the rate constant for proton exchange under these conditions was 1.0 × 103 s-1. [Fe(HINO)]2+ did not produce a CEST peak due to oxidation of the complex in water at neutral pH. The CEST peak of [Co(HINO)]2+ or [Ni(HINO)]2+ is observed in the presence of human serum albumin (HSA). These complexes show enhanced kinetic inertness toward dissociation in acid or in the presence of HSA in comparison to analogous complexes with amide pendent groups.

Transformation of Different Fractions of N and P in Soil

A laboratory experiment was conducted to monitor the changes in different forms of inorganic and organic N and P in an alluvial and an acidic soil fertilized with inorganic N and P either alone or in combination. Results revealed that in both the soils exchangeable NH 4 +and soluble NO 3 - decreased with increase in the period of investigation. Irrespective of soils, addition of inorganic N alone causes faster rate of N loss from the soil than combined application of N and P fertilizers. Combined application of N and P fertilizers accentuates mineralization process of hydrolysable NH4 + and amino acid-N fractions particularly, in acidic Jhargram soil. Irrespective of soils, the highest amount of decrease in active P (saloid-P + Al-P + Fe-P + Ca-P) was recorded in the treatment fertilized with inorganic P alone. Higher amount of decrease in organic P was recorded in alluvial Mohanpur soil than that of acidic Jhargram soil treated with inorganic P alone.

ChemInform Abstract: Macrocyclic Ligands for Fe(II) ParaCEST and Chemical Shift MRI Contrast Agents

AbstractReview: 56 refs.

Effect of pendimethalin and quizalofop on N2-fixing bacteria in relation to availability of nitrogen in a Typic Haplustept soil of West Bengal, India

An experiment was conducted under laboratory conditions to investigate the effect of two systemic herbicides viz., pendimethalin and quizalofop, at their recommended field rates (1.0 kg and 50 g active ingredient ha(- 1), respectively) on the growth and activities of non-symbiotic N(2)-fixing bacteria in relation to mineralization and availability of nitrogen in a Typic Haplustept soil. Both the herbicides, either singly or in a combination, stimulated the growth and activities of N(2)-fixing bacteria resulting in higher mineralization and availability of nitrogen in soil. The single application of quizalofop increased the proliferation of aerobic non-symbiotic N(2)-fixing bacteria to the highest extent while that of pendimethalin exerted maximum stimulation to their N(2)-fixing capacity in soil. Both the herbicides, either alone or in a combination, did not have any significant difference in the stimulation of total nitrogen content and availability of exchangeable NH(4)(+) while the solubility of NO(3)(-) was highly manifested when the herbicides were applied separately in soil.

Low Temperature Dynamic Mapping Reveals Unexpected Order and Disorder in Troponin

Troponin is a pivotal regulatory protein that binds Ca(2+) reversibly to act as the muscle contraction on-off switch. To understand troponin function, the dynamic behavior of the Ca(2+)-saturated cardiac troponin core domain was mapped in detail at 10 °C, using H/D exchange-mass spectrometry. The low temperature conditions of the present study greatly enhanced the dynamic map compared with previous work. Approximately 70% of assessable peptide bond hydrogens were protected from exchange sufficiently for dynamic measurement. This allowed the first characterization by this method of many regions of regulatory importance. Most of the TnI COOH terminus was protected from H/D exchange, implying an intrinsically folded structure. This region is critical to the troponin inhibitory function and has been implicated in thin filament activation. Other new findings include unprotected behavior, suggesting high mobility, for the residues linking the two domains of TnC, as well as for the inhibitory peptide residues preceding the TnI switch helix. These data indicate that, in solution, the regulatory subdomain of cardiac troponin is mobile relative to the remainder of troponin. Relatively dynamic properties were observed for the interacting TnI switch helix and TnC NH(2)-domain, contrasting with stable, highly protected properties for the interacting TnI helix 1 and TnC COOH-domain. Overall, exchange protection via protein folding was relatively weak or for a majority of peptide bond hydrogens. Several regions of TnT and TnI were unfolded even at low temperature, suggesting intrinsic disorder. Finally, change in temperature prominently altered local folding stability, suggesting that troponin is an unusually mobile protein under physiological conditions.

Multi-frequency PARACEST agents based on europium(III)-DOTA-tetraamide ligands.

Magnetic resonance imaging (MRI) is one of the most versatile and powerful diagnostic tools in modern medicine. Recently, a conceptually different approach to contrast enhancement based on chemical exchange saturation transfer (CEST) has emerged that takes advantage of slow-to-intermediate exchange conditions between two or more pools of protons (kex ≤ Δω).[1] While the first reported CEST agents were diamagnetic molecules containing exchangeable NH and OH groups (Δω ≤ 5 ppm), it was later shown that the slow water exchange characteristics of certain paramagnetic Ln3+ complexes of DOTA-tetraamide ligands allows selective saturation of a hyperfine shifted Ln3+-bound water pool (Δω > 50 ppm) for creating CEST contrast.[2] Radio frequency (RF) saturation of highly shifted exchange resonances in paramagnetic systems offer significant advantages over diamagnetic CESTagents with small Δω values.[3]

Composts with wood ash addition: A risk or a chance for ameliorating acid tropical soils?

Within the framework aiming at assessing the potential risk of the use of wood ash amended composts to tropical acid soils, a greenhouse experiment was conducted with two acid soils (Oxisol and Ultisol) collected from Cameroon and amended with three types of compost 3:1 (w/w) prepared with 0, 8 and 16% wood ash prior to composting, respectively. They were planted with soybean ( Glycine max L.) for 100 d. Addition of organic waste composts increased soil pH, SOM, WHC, EC, total C and N. Concentrations of exchangeable Al and NH 4 +–N decreased while those of NO 3 −–N and phosphorous increased. The exchangeable cation (Ca, Mg, Na and K) and micronutrient (Zn and Cu) concentrations increased while concentrations of Fe and Mn decreased. The C/N ratio of treated soils ranged from 12 to 14. Soil pH, concentrations of Fe and Mn were higher; EC and nitrate concentration were lower under ash compost amendment. Eight percent ash amendment to compost yielded best results in many aspects. Concentration of exchangeable Al was most decreased in soils treated with compost without ash. Plant biomass production was not significantly enhanced by compost amendment. This lack of an effect may be attributed to a soil pH (> 7.7) beyond the optimum for soybean (6.2). Summarising the results, composts with ash addition could serve the purpose of ameliorating acid tropical soils, albeit the optimal amendment rates still need to be addressed in future research.

Carbon respiration and nitrogen dynamics in Corsican pine litter amended with aluminium and tannins

We investigated the carbon (C) mineralisation and nitrogen (N) dynamics in litter from a Corsican pine forest in response to individual and combined additions of aluminium (Al), condensed tannin (extracted from fresh Corsican pine needles) and hydrolysable tannin (commercial tannic acid). Production rates of CO 2, NH 4 + and NO 3 − concentrations, tannin concentrations and Al speciation were determined at various time intervals during a 28-day incubation experiment. The addition of Al decreased CO 2 production and shut down nitrification. Exchangeable NH 4 + strongly increased in the Al-amended litter, likely due to (i) decreased microbial uptake of NH 4 +, (ii) the inhibition of nitrification and (iii) competition for soil organic matter (SOM) binding sites by Al. Both tannin species affected C mineralisation and/or N dynamics, be it in different ways. Addition of tannic acid led to a strong increase of the C mineralisation rate and microbial uptake of N, caused by rapid degradation of this labile tannin and subsequent increased microbial nutrient demand. Net immobilisation of N occurred as long as one week after addition. Condensed tannin was not consumed but probably strongly bound to (nitrogenous) SOM compounds, forming recalcitrant complexes and decreasing net N mineralisation. Complexation of Al by tannins in solution before addition to the litter mitigated the Al-induced release of exchangeable NH 4 +. In the case of condensed tannin with complexed Al, this was due to detoxification of Al through complexation. Increased microbial demand for N likely played a major role in decreased NH 4 + accumulation in the samples to which tannic acid with complexed Al was added. Nitrification was shut down despite of the complexation of Al by either condensed tannin or tannic acid.

Biological and physical effects of non-native earthworms on nitrogen cycling in riparian soils

Global nitrogen cycling is being altered by anthropogenic disturbances including invasion by non-native species. European and Asian earthworms have invaded northern temperate forests in North America with dramatic consequences for litter thickness, forest floor plant diversity, and soil nitrogen cycling. Invasive earthworms present at the boundary of terrestrial and aquatic ecosystems (i.e., riparian zones) may alter the flux of nitrogen into adjacent aquatic ecosystems. We examined how nitrogen cycling in riparian soil responds to amendments of invasive earthworms or artificial earthworm burrows. In earthworm-free riparian plots (0.25 m 2), we established treatments of invasive earthworms (60 g fresh mass·m −2), artificial burrows (120 m −2), or control plots and sampled the plots after 30 days. Before and after treatment application we measured major soil characteristics (water-filled pore space, organic matter, and pH), nitrogen pools (exchangeable NH 4 + and NO 3 −), and nitrogen transformation rates (net N-mineralization, net nitrification, and denitrification). Exchangeable NH 4 + and NO 3 − changed through time but did not differ among treatments. Net N-mineralization and net nitrification rates did not change through time and were similar across all treatments. However, denitrification rates in plots with added earthworms were 4 times greater than rates in control and burrow-only plots, which represents a large rapid increase in gaseous nitrogen flux out of these riparian soils. For all response variables, artificial burrows responded similarly to control plots, suggesting that earthworm biological activity (i.e., feeding, excretion, and mucus production) rather than physical effects (i.e., burrowing and soil aeration) drove the changes in nitrogen cycling. Examination of soil nitrogen pool and flux measurements suggest that this increase in denitrification was coupled with NH 4 + consumption by nitrifying bacteria, but future studies are needed to confirm this hypothesis. We conclude that the activity of invasive earthworms in riparian zones can increase the flux of N out of riparian zones, but the hydrologic context of the riparian zone (e.g., pore-water residence time) ultimately controls whether denitrification or nitrate leaching is the dominant flux of N.

Influence of Trypaea australiensis population density on benthic metabolism and nitrogen dynamics in sandy estuarine sediment: A mesocosm simulation

Laboratory mesocosm incubations were undertaken to investigate the influence of natural densities of the thalassinidean shrimp, Trypaea australiensis (marine yabby) on sediment oxygen demand (SOD), inorganic nutrient fluxes, and the N-cycle processes of nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Mesocosms (~ 0.1 m 2 × 55 cm deep) of sieved, natural T. australiensis inhabited sands were continually flushed with fresh seawater and pre-incubated for two weeks prior to being assigned to one of three treatments; control (no additions), low yabby density (40 T. australiensis m − 2 ) or high yabby density (80 T. australiensis m − 2 ). Thereafter, SOD and sediment–water column inorganic nutrient fluxes were determined periodically over a 38 day period. On the final day rates of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) were also determined using the 15N-isotope pairing technique. Yabbies consistently and significantly ( p < 0.001) stimulated SOD over the entire 38 day incubation period (mean values: 4.92, 9.21 and 14.9 mmol m − 2 day − 1 for control, low and high density treatments, respectively). The increased organic matter mineralisation rates greatly enhanced nitrogen regeneration rates in the sediment and fuelled significantly higher effluxes of dissolved inorganic nitrogen with NH 4 + and total DIN effluxes in the low and high density treatments respectively, being 617 and 1534%, and 269 and 565% higher than those in the controls, despite sediment bioavailable (porewater + exchangeable) NH 4 + pools being approximately 2 and 4-fold lower in the low and high density yabby treatment sediments compared to the controls, measured at the end of the 37 day experiment. Mass balance calculations based on the final day nutrient flux and nitrate reduction rate data demonstrated that yabbies stimulated benthic nitrification rates by 31 and 46% in the low and high density treatments. However, somewhat surprisingly T. australiensis population density had no effect on rates of denitrification and DNRA despite the higher rates of nitrification and higher equilibrium water column nitrate concentration. Indeed, nitrate reduction processes became an increasingly unimportant element with increasing yabby density with for example, N 2 generated by coupled nitrification–denitrification representing 11.5, 5.2 and 2.8% of the total inorganic-N recycled to the water column in the control, low density and high density yabby treatments, respectively. Overall, the major influence of T. australiensis in the studied low organic matter content, sandy sediments was to enhance coupling between the benthic and pelagic systems through increased rates of inorganic nitrogen regeneration in the sediment and enhanced export of this nitrogen to the water column. Our results also suggest that the influences of organisms such as T. australiensis which form deep, extensive and complex burrow systems where irrigation rates differ greatly between different burrow sections, may be more complex than those recorded for infauna which form simple U-shaped burrows. Additionally, there may be a strong interaction between faunal effects and the sediment physical and biological environment and thus the same species may have contrasting influences in different sediment types.

Effect of Potassium and C/N Ratios on Conversion of NH +4 in Soils

Two soils, one consisting of 1:1 clay minerals at pH 4.5 and the other containing 2:1 clay minerals at pH 7.0, were used to estimate the conversion of added NH + 4 under different C/N ratios (glucose as the C source) and the addition of potassium. Under lower C/N ratios (0:1 and 5:1), a large part of the added NH + 4 in the acid soil was held in the forms of either exchangeable or water soluble NH + 4 for a relatively long time and under higher C/N ratio (50:1), a large amount of the added NH + 4 was directly immobilized by microorganisms. In the second soil containing appreciable 2:1 type clay minerals a large part of the added NH + 4 at first quickly entered the interlayer of the minerals under both lower and higher C/N ratios. In second condition, however, owing to microbial assimilation stimulated by glucose the newly fixed NH + 4 could be completely released in further incubation because of a large concentration gradient between external NH + 4 and fixed NH + 4 in the mineral interlayer caused by heterotrophic microorganisms, which imply the fixed NH + 4 to be available to plants. The results also showed that if a large amount of K+ with carbon source together was added to soil, the higher K+ concentration of soil solution could impede the release of fixed NH + 4, even if there was a lot of carbon source.

Vertical distribution and interaction of ethylene and methane in temperate volcanic forest soils

Seven soil layers within a depth of 20 cm were sampled from three natural temperate forest stands (e.g. Betula ermanii and Picea jezoensis, and Pinus koraiensis mainly mixed with Tilia amurensis, Fraxinus mandshurica and Quercus mongolica) in northeast China developed from volcanic-materials, to study the vertical distribution and interaction of ethylene (C 2H 4) and methane (CH 4) in temperate volcanic forest soils and ambiguous mechanisms for maximum CH 4 consumption in subsurface soil layers. For atmospheric and elevated CH 4 concentrations, the maximum CH 4 consumption activity was localized in soil layers (2.5–7.5 cm) under spruce and birch below those (2.5–5 cm) of C 2H 4 consumption activity at a same initial carbon concentration. However, there was a same consumption activity of C 2H 4 and CH 4 in the 0–20-cm profile soils under the Korean pine and broadleaf mixed forest, and the 0–2.5-cm soil layers showed a maximum consumption of both. There was a higher exchangeable NH 4 + concentration in the upper soil layers (0–2.5 cm) under spruce and birch than under the Korean pine and broadleaf mixed forest ( P ≤ 0.05). Under waterlogged conditions, ethylene accumulation in the soils under each forest stand decreased with increasing depth, and maximum C 2H 4 concentrations of approximately 15 μl l − 1 in the headspace during a 14-day incubation period were observed in the 0–2.5-cm soil layers under spruce and birch ( P ≤ 0.05). Ethylene concentration of 20 μl l − 1 decreased atmospheric CH 4 consumption in the soils at various depths under the three temperate forests ( P ≤ 0.05). Our observations strongly indicate that cycling of C 2H 4 and NH 4 + accumulation in forest upper soil layers can inhibit the soil CH 4 consumption, thus being main reasons for the localization of maximum methanotrophic activity in subsurface soil layers under spruce and birch.

An inhibitor of urease activity effectively reduces ammonia emissions from soil treated with urea under Mediterranean conditions

Urea is an important source of ammonia (NH 3) emissions to the atmosphere from agricultural soils. Abatement strategies are necessary in order to achieve NH 3 emission targets by reducing those emissions. In this context, a field experiment was carried out on a sunflower crop in spring 2006 with the aim of evaluating the effect of the N-( n-butyl) thiophosphoric triamide (NBPT) in the mitigation of volatilized NH 3 from a urea-fertilised soil. Ammonia emission was quantified, using the integrated horizontal flux (IHF) method, following application of urea with and without the urease inhibitor NBPT. Urea and a mixture of urea and NBPT (0.14%, w/w) were surface-applied at a rate of 170 kg N ha −1 to circular plots (diameter 40 m). The soil was irrigated with 10 mm of water just after the application of urea to dissolve and incorporate it into the upper layer of soil. Over the duration of the measurement period (36 days) three peaks of NH 3 were observed. The first peak was associated with hydrolysis of urea after irrigation and the others with the increase of ammonia in soil solution after changes in atmospheric variables such as wind speed and rainfall. The total NH 3 emission during the whole experiment was 17.3 ± 0.5 kg NH 3–N ha −1 in the case of urea treated soils and 10.0 ± 2.2 kg NH 3–N ha −1 where NBPT was included with the urea (10.1 and 5.9%, respectively, of the applied urea–N). The lower NH 3 emissions from plots fertilised with urea + NBPT, compared with urea alone, were associated with a reduction in urease activity during the first 9 days after inhibitor application. This reduction in enzymatic activity promoted a decrease in the exchangeable NH 4 + pool.

Consistent patterns of 15N distribution through soil profiles in diverse alpine and tundra ecosystems

We studied the vertical patterns of δ 15nitrogen in total N and exchangeable NH 4 +-N through soil profiles in diverse alpine and tundra ecosystems. Soil samples were analyzed from 11 sites located in three mountain areas: NW Caucasus (Russia), the Khibiny Mountains (NW Russia) and Abisko region (N Sweden). Despite differences in the profile patterns of organic matter, nitrogen accumulation and nitrogen availability, we found consistent patterns of 15N distribution through all studied soil profiles. The δ 15N values of total N were in general about zero or positive in the surface horizon and increased with soil depth. In contrast with total N, the δ 15N values of exchangeable NH 4 +-N were in general about zero or negative in the surface horizons and decreased with soil depth. NH 4 +-N was significantly 15N-depleted compared with total N in all mineral horizons, while in the surface organic horizons differences between isotopic composition of total N and NH 4 +-N were mostly not significant. We do not know the exact mechanism responsible for 15N depletion of NH 4 +-N with soil depth and further research needs to evaluate the contributions of natural processes (higher nitrification activity and biological immobilization of “lighter” NH 4 +-N near the soil surface) or artifacts of methodological procedure (contribution of the 15N-enriched microbial N and dissolved organic N near the soil surface). Nevertheless, our finding gives a new possibility to interpret variability in foliar δ 15N values of plant species with different rooting depth in alpine and tundra ecosystems, because plants with deeper root systems can probably consume “lighter” rather than “heavier” NH 4 +-N.

Albumin-binding PARACEST agents

Lanthanide complexes (Eu(3+), Gd(3+) and Yb(3+)) of two different 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid tetraamide derivatives containing two (2) and four (3) O-benzyl-L-serine amide substituents were synthesized and their chemical exchange saturation transfer (CEST) and relaxometric properties were examined in the presence and absence of human serum albumin (HSA). Both Eu2 and Eu3 display a significant CEST effect from a single slowly exchanging Eu(3+)-bound water molecule, making these PARACEST complexes potentially useful as vascular MRI agents. Yb2 also showed a detectable CEST effect from both the Yb(3+)-bound water protons and the exchangeable NH amide protons, making it potentially useful as a vascular pH sensor. Fluorescence displacement studies using reporter molecules indicate that both Gd2 and Gd3 displace dansylsarcosine from site II of HSA with inhibition constants of 32 and 96 microM, respectively, but neither complex significantly displaces warfarin from site I. Water proton relaxation enhancements of 135 and 171% were observed upon binding of Gd2 and Gd3 to HSA, respectively, at 298 K and pH 7.4.

Impact of harvesting and logging slash on nitrogen and carbon dynamics in soils from upland spruce forests in northeastern Ontario

The potential impact of timber harvesting in the boreal forest on aquatic ecosystem water quality and productivity depends in part on the production of nutrients within the soil of the harvested catchment. Nitrogen supplied by organic matter decomposition is of particular interest because of the important role that N plays in biotic processes in surface waters, and in forest nutrition in general. Logging slash quality and input to the forest floor has the potential to influence N availability after harvest on clearcut sites. Net production of organic and inorganic-N and microbial biomass C and N concentrations were determined during a 90-day laboratory incubation at constant temperature and moisture. Incubated soils included F horizon and shallow mineral soil horizons (0–5 cm) from unharvested and full-tree harvested (2 and 12 growing seasons since harvest) boreal forest sites at the Esker Lakes Research Area (ELRA), in northeastern Ontario, Canada. In an ancillary experiment, black spruce foliage was added to unharvested forest floor material after 30 days during a 90-day laboratory incubation to simulate the influence of logging slash from full-tree harvesting on C and N dynamics. Twelve-year old clearcut F horizon material released on average 75 and 5 times more NO 3 - -N and 3 and 2 times as much inorganic-N than soil collected from unharvested and 2-year-old clearcuts, respectively. This increase in NO 3 - -N accumulation during the incubation was accompanied by decreases in both exchangeable NH 4 + -N and microbial biomass C and N levels. Net daily changes in microbial biomass N were significantly related to organic and inorganic-N accumulation or loss within the F horizon. Mineral soil release of inorganic-N was lower than release from the forest floor. Nitrate-nitrogen accumulation was lower, and NH 4 + -N accumulation was higher in mineral soil from unharvested sites when compared to 12-year-old clearcuts. Calculated harvest response ratios indicated that incubated mineral soil from the 12-year-old clearcut sites released significantly greater amounts of NO 3 - -N than 2-year-old clearcuts. Incorporation of black spruce needles into F horizon material reduced the production of organic and inorganic-N and increased microbial biomass N. Laboratory incubations of F horizon and shallow mineral soil from 12-year-old clearcuts suggested that these boreal soils have the capacity for increased inorganic-N production compared to uncut stands several years after harvesting. This has the potential to increase N availability to growing boreal forest plantations and increase N leaching due to greater NO 3 - -N levels in the forest soil.

Retention of Inorganic Nitrogen by Epiphytic Bryophytes in a Tropical Montane Forest1

ABSTRACTWe developed and evaluated a model of the canopy of a tropical montane forest at Monteverde, Costa Rica, to estimate inorganic nitrogen (N) retention by epiphytes from atmospheric deposition. We first estimated net retention of inorganic N by samples of epiphytic bryophytes, epiphyte assemblages, vascular epiphyte foliage, and host tree foliage that we exposed to cloud water and precipitation solutions. Results were then scaled up to the ecosystem level using a multilayered model of the canopy derived from measurements of forest structure and epiphyte mass. The model was driven with hourly meteorological and event‐based atmospheric deposition data, and model predictions were evaluated against measurements of throughfall collected at the site. Model predictions were similar to field measurements for both event‐based and annual hydrologic and inorganic N fluxes in throughfall. Simulation of individual events indicated that epiphytic bryophytes and epiphyte assemblages retained 33–67 percent of the inorganic N deposited in cloud water and precipitation. On an annual basis, the model predicted that epiphytic components retained 3.4 kg N ha/yr, equivalent to 50 percent of the inorganic N in atmospheric deposition (6.8 kg N ha/yr). Our results indicate that epiphytic bryophytes play a major role in N retention and cycling in this canopy by transforming highly mobile inorganic N (ca. 50% of atmospheric deposition is NO−3) to less mobile (exchangeable NH+4) and recalcitrant forms in biomass and remaining litter and humus.

Ammonium interaction with coastal marine sediments: influence of redox conditions on K*

Ammonium adsorption on sediments is an important process in aquatic environments where often about twice as much exchangeable NH 4 + is found adsorbed on sediments as that measured in associated porewaters. Adsorption of NH 4 + on fine-grained terrigenous sediments from the Louisiana Shelf west of the Mississippi River delta and from Lower Laguna Madre (LLM), Texas was studied under reducing and oxic conditions. Sediment from both sites yielded K* values of 2.3 and 4.0 ml g dw −1 under oxidizing and reducing conditions, respectively, even though the NH 4 + concentration was about 10 times greater in Laguna Madre sediments than sediments from the Louisiana Shelf. K* determined under reducing conditions was a better predictor of exchangeable NH 4 + in sediments from the two sites, based on NH 4 + porewater concentrations, than K* determined under oxidizing conditions (the mean square error of reducing K* was ∼1/4 that of oxidizing K*). However, when considering potential NH 4 + release from sediments if they are resuspended into overlying oxic waters, the oxic adsorption coefficient should be used.

The effects of common carp bioturbation on fishpond bottom soil

Bioturbation activity generated by fish has the potential to improve bottom soil quality by increasing oxygen supply to a greater depth in aquaculture ponds bottoms. This study was designed to assess the common carp ( Cyprinus carpio L) potential to improve conditions in pond bottom soils. Enclosures (3×2 m) constructed within a tilapia production pond were stocked with zero, two or seven carp each. Treatments were triplicated (four replicates used for the control treatment). Carp were not fed by a prepared diet during the study. Fluorescent stained quartz particles were added to the bottom soils (as a bioturbation tracer) in the enclosures. Bottom soil cores were taken from the enclosures sectioned to 0–1, 1–2, 2–3 and 3–5 cm layers. Total dissolved sulfide (TDS), soluble Mn, easily oxidized matter (EOM), and exchangeable NH 4-N were determined. Concentrations of all the above measured constituents, 45 days after stocking, along the bottom soil profile in the enclosures that contained fish, were lower than in the enclosures without fish. Bulk density of surface soil in enclosures containing carp was significantly greater than for the control. Tracer analysis results indicated that more particles were displaced down to 3-cm depth in the enclosures with carp as compared to control, suggesting that the carp stirring activity was effective down to approximately 3 cm. Pond bottom soil perturbation increases oxygen transfer to the soil, decrease the concentration of toxic reduced compounds and enable more efficient food web recycling and nutrient release.