Partitioning of 15N-labeled ammonium and nitrate among soil, litter, below- and above-ground biomass of trees and understory in a 15-year-old Picea abies plantation

The partitioning of nitrogen deposition among soil, litter, below- and above-ground biomass of trees and understory vegetation was investigated in a 15-year-old Picea abies (L.) Karst. plantation in the Fichtelgebirge, Germany, by labeling with 62 mg of¹⁵N tracer per square meter in March 1991. Ammonium and nitrate depositions were simulated on five plots each, by labeling with either¹⁵N-NH₄ ⁺ or¹⁵N-NO₃ ^–, and the¹⁵N pulse was followed during two successive growing seasons (1991 and 1992). Total recovery rates of the¹⁵N tracer in the entire stand ranged between 93 and 102% for both nitrogen forms in 1991, and 82% in June 1992. ⁵ N ratios increased rapidly in all compartments of the ecosystem. Roots and soils (to 65 cm depth) showed significant¹⁵N enrichments for both¹⁵N-treatments compared to reference plots. Newly grown spruce tissues were more enriched than older ones, but the most enriched ¹⁵N values were found in the understory vegetation. Although spruce trees were a much larger pool (1860 g biomass/m²) than understory vegetation (Vaccinium myrtillus 333 g/m², Calluna vulgaris 142 g/m², Deschampsia flexuosa 22 g/m²), the ericaceous shrubs and the perennial grass were a much greater sink for the¹⁵N label. Eight months after labeling, 9% of the ammonium and 15% of the nitrate label were found in the understory. P.abies retained only 3% of the¹⁵N-ammonium and 7% of the¹⁵N-nitrate. The main sink for both¹⁵N tracers was the soil, where 87% of the ammonium and 79% of the nitrate tracer were found. The organic soil horizon (5-0 cm depth) contained 63% of the¹⁵N-ammonium and 46% of the¹⁵N -nitrate suggesting strong immobilization by microorganisms of both N forms. Eight months after tracer application, about 16% of both¹⁵N-tracers was found below 25 cm soil depth. This 16% corresponds well to a 20% decrease in the recovery of both¹⁵N tracers after 15 months and indicates a total loss out of the ecosystem. Highly enriched ¹⁵N values were found in fruit bodies of fungi growing in reference lots (no¹⁵N addition), although soils did not show increased ¹⁵N ratios. No transfer of¹⁵N-tracer between fungi and spruce or understory vegetation was apparent yet.     

The effect of harvesting intensity on the fate of applied 15N-ammonium to the organic horizons of a coniferous forest in N. Wales

¹⁵N-ammonium sulphate equivalent to 0.5 kg N/ha was added as a tracer to lysimeters containing the organic horizons of an acid forest soil. The effect of logging debris (brash), vegetation and second rotationPicea sitchensis seedlings on the amount of the¹⁵N found in various soil, vegetation and leachate pools was followed over a period of 60 days. Transformation of¹⁵N-ammonium to nitrate occurred within 24 hours. Although total nitrate leachate losses were high, tracer-derived nitrate represented only 0.4%–4.2% of the applied¹⁵N-ammonium. The atom % excess of the KCI-extractable organic-N pool was initially lower than for the inorganic species but due to the large pool size, consistently represented 3–6% of the applied¹⁵N-ammonium. The similarity of the atom % excess of the ammonium and nitrate pools indicated an autotrophic nitrification pathway.A significant proportion of the¹⁵N-ammonium passed through the microbial biomass which contained between 16 and 48% of the¹⁵N-ammonium 2 days after addition of the¹⁵N-ammonium. This nitrogen was in a readily available form or short-term pool for the first two weeks (with no change in the overall biomass pool), after which the nitrogen appeared to become transformed into more stable compounds representing a long-term pool. Total recovery of the¹⁵N was between 68% and 99% for the different treatments. The presence of brash reduced microbial immobilisation of the¹⁵N-ammonium and total retention in the organic matter. This is suggested to be a consequence of greater nitrification and denitrificatiion rate in organic horizons beneath a brash covering due to different microclimatic conditions.     

Natural 15N abundance of soil N pools and N2O reflect the nitrogen dynamics of forest soils

Natural ¹⁵N abundance measurements of ecosystem nitrogen (N) pools and ¹⁵N pool dilution assays of gross N transformation rates were applied to investigate the potential of δ¹⁵N signatures of soil N pools to reflect the dynamics in the forest soil N cycle. Intact soil cores were collected from pure spruce (Picea abies (L.) Karst.) and mixed spruce-beech (Fagus sylvatica L.) stands on stagnic gleysol in Austria. Soil δ¹⁵N values of both forest sites increased with depth to 50 cm, but then decreased below this zone. δ¹⁵N values of microbial biomass (mixed stand: 4.7 ± 0.8‰, spruce stand: 5.9 ± 0.9‰) and of dissolved organic N (DON; mixed stand: 5.3 ± 1.7‰, spruce stand: 2.6 ± 3.3‰) were not significantly different; these pools were most enriched in ¹⁵N of all soil N pools. Denitrification represented the main N₂O-producing process in the mixed forest stand as we detected a significant ¹⁵N enrichment of its substrate NO₃⁻ (3.6 ± 4.5‰) compared to NH₄⁺ (−4.6 ± 2.6‰) and its product N₂O (−11.8 ± 3.2‰). In a ¹⁵N-labelling experiment in the spruce stand, nitrification contributed more to N₂O production than denitrification. Moreover, in natural abundance measurements the NH₄⁺ pool was slightly ¹⁵N-enriched (−0.4 ± 2.0 ‰) compared to NO₃⁻ (−3.0 ± 0.6 ‰) and N₂O (−2.1 ± 1.1 ‰) in the spruce stand, indicating nitrification and denitrification operated in parallel to produce N₂O. The more positive δ¹⁵N values of N₂O in the spruce stand than in the mixed stand point to extensive microbial N₂O reduction in the spruce stand. Combining natural ¹⁵N abundance and ¹⁵N tracer experiments provided a more complete picture of soil N dynamics than possible with either measurement done separately.     

A field study on the fate of 15N-ammonium to demonstrate nitrification of atmospheric ammonium in an acid forest soil

To demonstrate the contribution of atmospheric ammonium to soil acidification in acid forest soils, a field study with¹³N-ammonium as tracer was performed in an oak-birch forest soil. Monitoring and analysis of soil solutions from various depths on the¹³N-ammonium and¹⁵N-nitrate contents, showed that about 54% of the applied¹⁵N-ammonium was oxidized to nitrate in the forest floor. Over a period of one year about 20% of the¹⁵N remained as organic nitrogen in this layer. The percentage¹⁵N enrichment in ammonium and nitrate were in the same range in all the forest floor percolates, indicating that even in extremely acid forest soils (pH < 4) nitrate formation from ammonium can occur. Clearly, atmospheric ammonium can contribute to soil acidification even at low soil pH.     

A field method of determining NH 4 + and NO 3 - uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species

Models describing plant and ecosystem N cycles require an accurate assessment of root physiological uptake capacity for NH ₄ ⁺ and NO ₃ ^- under field conditions. Traditionally, rates of ion uptake in field-grown plants are determined by using excised root segments incubated for a short period in an assay solution containing N either as a radioactive or stable isotope tracer (e.g., ³⁶ClO₃ as a NH ₄ ⁺ analogue, ¹⁴CH₃NH₃ as an NO ₃ ^- analogue or ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^- ). Although reliable, this method has several drawbacks. For example, in addition to radioactive safety issues, purchase and analysis of radioactive and stable isotopes is relatively expensive and can be a major limitation. More importantly, because excision effectively interrupts exchange of compounds between root and shoot (e.g., carbohydrate supply to root and N transport to shoot), the assay must be conducted quickly to avoid such complications. Here we present a novel field method for simultaneous measurements of NH ₄ ⁺ and NO ₃ ^- uptake kinetics in intact root systems. The application of this method is demonstrated using two tree species; red maple (Acer rubrum) and sugar maple (Acer saccharum) and two crop species soybean (Glycine max) and sorghum (Sorghum bicolor). Plants were grown in open-top chambers at either ambient or elevated levels of atmospheric CO₂ at two separate US national sites involved in CO₂ research. Absolute values of net uptake rates and the kinetic parameters determined by our method were found to be in agreement with the literature reports. Roots of the crop species exhibited a greater uptake capacity for both N forms relative to tree species. Elevated CO₂ did not significantly affect kinetics of N uptake in species tested except in red maple where it increased root uptake capacity, V, for NH ₄ ⁺ . The application, reliability, advantages and disadvantages of the method are discussed in detail. This revised version was published online in June 2006 with corrections to the Cover Date.     

Incorporation of atmospheric 15NO2-nitrogen into free amino acids by Norway spruce Picea abies (L.) Karst.

During spring and autumn 1991, potted 6-yearold spruce trees (Picea abies (L.) Karst.) were fumigated with 60 nl·1^{–1 15}NO₂ for 4 days under controlled conditions in constant light. Current and previous flush needles, the bark and the fine roots were analysed for total ¹⁵N content and incorporation of ¹⁵N into the -amino nitrogen of free amino acids. In addition, in vitro nitrate reductase activity and stomatal conductance of the needles were measured. Nitrate reductase activity was significantly higher in the needles of fumigated trees compared to control trees exposed to filtered air. With an average of 9.1% ¹⁵N, free glutamate was the pool with the most label. Taking into account the time-course of the labelling of this pool, this figure can be taken as an estimate of the minimum contribution of NO₂ to the N nutrition of the needles. ¹⁵N-labelled amino acids were also detected in the bark and the roots, indicating export from the needles.     

The interaction of defoliation and nutrient uptake in Sporobolus kentrophyllus,a short-grass species from the serengeti plains

Summary Sporobolus kentrophyllus, a grazing-tolerant C₄ grass from the southeastern Serengeti Plains, was grown in solution culture to examine the effects of clipping on the uptake, preference and subsequent transport of varying nitrogen forms. Clipping reduced offtake mass, crown mass ane root mass, resulting in a 58% decline in plant mass. Proportional biomass allocation to roots decreased with clipping, while tillering rates increased. Clipping also increased the nitrogen concentrations of all tissues, and plant nitrogen uptake (nitrogen accumulated throughout the experiment per gram root). The ¹⁵N concentrations (% atom excess) of all tissues were higher in clipped compared with unclipped plants, and the average ¹⁵N uptake rate of clipped plants was twice that of unclipped plants. The relative ¹⁵N allocation to aboveground mass, a measure of canopy sink strength, was higher in clipped plants. Plants fed ¹⁵N-ammonium or ¹⁵N-nitrate during the ¹⁵N pulse experiment had greater ¹⁵N tissue concentrations compared with urea-fed plants, and ¹⁵N uptake rates were higher in ammonium-fed and nitrate-fed plants, compared with urea-fed plants. The relative magnitudes of these differences were higher when plants were clipped. Clipped plants had higher uptake rates for potassium, phosphorus and sodium, while differences between clipping treatments for calcium, iron, and magnesium were indistinguishable. Rapid uptake rates for species on the southeastern Serengeti plains, particularly during grazing periods, have important implications for nutrient cycling in this system.     

Interactions between cyanobacterium and fungus during 15N2-incorporation and metabolism in the lichen Peltigera canina

On following N₂-incorporation and subsequent metabolism in the lichen Peltigera canina using ¹⁵N as tracer, it was found, over a 30 min period, that greatest initial labelling was into NH ₄ ⁺ followed by glutamate and the amide-N of glutamine. Labelling of the amino-N of glutamine, aspartate and alanine increased slowly. Pulse-chase experiments using ¹⁵N confirmed this pattern. On inhibiting the GS-GOGAT pathway using l-methionine-dl-sulphoximine and azaserine, ¹⁵N enrichment of glutamate, alanine and aspartate continued although labelling of glutamine was undetectable. From this and enzymic data, NH ₄ ⁺ assimilation in the P. canina thallus appears to proceed via GS-GOGAT in the cyanobacterium and via GDH in the fungus; aminotransferases were present in both partners. The cyanobacterium assimilated 44% of the ¹⁵N₂ fixed; the remainder was liberated almost exclusively as NH ₄ ⁺ and then assimilated by fungal GDH.Abbreviations ADH alanine dehydrogenase - APT aspartate-pyruvate aminotransferase - AOA aminooxyacetate - GDH glutamate dehydrogenase - GOT glutamate-oxaloacetate aminotransferase - GOGAT glutamate synthase - GPT glutamate-pyruvate aminotransferase - GS glutamine synthetase - HEPES 4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid - MSX l-methionine-dl-sulphoximine     

Estimation of biological nitrogen fixation associated withBrachiaria andPaspalum grasses using15N labelled organic matter and fertilizer

Summary Six pasture grasses,Paspalum notatum cv batatais,P. notatum cv pensacola,Brachiaria radicans, B. ruziziensis, B. decumbens andB. humidicola, were grown in concrete cylinders (60 cm diameter) in the field for 31 months. The soil was amended with either a single addition of¹⁵N labelled organic matter or frequent small (2 kg N. ha^–1) additions of¹⁵N enriched (NH₄)₂SO₄. In the labelled fertilizer treatment soil analysis revealed that there was a very drastic change in¹⁵N enrichment in plant-available nitrogen (NO ₃ ^– +NH ₄ ⁺ ) with depth. The different grass cultivars recovered different quantities of applied labelled N, and evidence was obtained to suggest that the roots exploited the soil to different depths thus obtaining different¹⁵N enrichments in soil derived N. This invalidated the application of the isotope dilution technique to estimate the contribution of nitrogen fixation to the grass cultivars in this treatment. In the labelled organic matter treatment the¹⁵N label in the plant-available N declined at a decreasing rate during the experiment until in the last 12 months the decrease was only from 0.274 to 0.222 atom % excess. There was little change in¹⁵N enrichment of available N with depth, hence it was concluded that although the grasses recovered different quantities of labelled N, they all obtained virtually the same¹⁵N enrichment in soil derived N. Data from the final harvests of this treatment indicated thatB. humidicola andB. decumbens obtained 30 and 40% respectively of their nitrogen from N₂ fixation amounting to an input of 30 and 45 kg N.ha^–1 year^–1 respectively.     

Potassium transport in red blood cells of frog Rana temporaria: demonstration of a K−Cl cotransport

Pathways of K⁺ movement across the erythrocyte membrane of frog Rana temporaria were studied using ⁸⁶Rb as a tracer. The K⁺ influx was significantly blocked by 0.1 mmol·l^-1 ouabain (by 30%) and 1 mmol·l^-1 furosemide (by 56%) in the red cells incubated in saline at physiological K⁺ concentration (2.7 mmol·l^-1). Ouabain and furosemide had an additive effect on K⁺ transport in frog red cells. The ouabain-sensitive and furosemide-sensitive components of K⁺ influx saturated as f(K⁺)_e with apparent K _m values for external K _e ⁺ concentration of 0.96±0.11 and 4.6±0.5 mmol·l^-1 and V _max of 0.89±0.04 and 2.8±0.4 mmol·l cells^-1·h^-1, respectively. The residual ouabain-furosemide-resistant component was also a saturable function of K _e ⁺ medium concentration. Total K⁺ influx was significantly reduced when frog erythrocytes were incubated in NO _- ³ medium. Furosemide did not affect K⁺ transport in frog red cells in NO ₃ ^- media. At the same K _e ⁺ concentration the ouabain-furosemide-insensitive K⁺ influx in Cl^- medium was significantly greater than that in NO _- ³ medium. We found no inhibitory effect of 1 mmol·l^-1 furosemide on Na⁺ influx in frog red cells in Cl^- medium. K⁺ loss from the frog erythrocytes in a K⁺-free medium was significantly reduced (mean 58%) after replacement of Cl^- with NO _- ³ . Furosemide (0.5 mmol·l^-1) did not produce any significant reduction in the K⁺ loss in both media. The Cl^--dependent component of K⁺ loss from frog red cells was 5.7±1.2 mmol·l^-1·h^-1. These results indicate that about two-thirds of the total K⁺ influx in frog erythrocytes is mediated by a K–Cl cotransport which is only partially blocked by furosemide.Abbreviations DMSO dimethyl sulphoxide - K _e ⁺ external concentration of K⁺ - K _m apparent Michaelis constant for external - K⁺ K _e ⁺ at V _max/2 - RBC red blood cell(s) - V _max maximal velocity of the unidirectional K⁺ influx - TRIS tris(hydroxymethyl)aminomethane     

Simultaneous CT-13C and VT-15N chemical shift labelling: Application to 3D NOESY-CH3NH and 3D 13C,15N HSQC-NOESY-CH3NH

Based on the HSQC scheme, we have designed a 2D heterocorrelated experiment which combines constant time (CT) ¹³C and variable time (VT) ¹⁵N chemical shift labelling. Although applicable to all carbons, this mode is particularly suitable for simultaneous recording of methyl-carbon and nitrogen chemical shifts at high digital resolution. The methyl carbon magnetisation is in the transverse plane during the whole CT period (1/J_CC=28.6 ms). The magnetisation originating from NH protons is initially stored in the 2H_zN_z state, then prior to the VT chemical shift labelling period is converted into 2H_zN_y coherence. The VT -¹⁵N mode eliminates the effect of ¹ J _N,CO and ^1,2 J _N,CA coupling constants without the need for band-selective carbon pulses. An optional editing procedure is incorporated which eliminates signals from CH₂ groups, thus removing any potential overlap with the CH₃ signals. The CT-¹³CH₃,VT-¹⁵N HSQC building block is used to construct two 3D experiments: 3D NOESY-CH₃NH and 3D ¹³C,¹⁵N HSQC-NOESY-CH₃NH. Combined use of these experiments yields proton and heteronuclear chemical shifts for moieties experiencing NOEs with CH₃ and NH protons. These NOE interactions are resolved as a consequence of the high digital resolution in the carbon and nitrogen chemical shifts of CH₃ and NH groups, respectively. The techniques are illustrated using a double labelled sample of the CH domain from calponin.     

Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

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.     

Comparative study of N uptake and distribution in three lines of common bean (Phaseolus vulgaris L.) at early pod filling stage

Summary The uptake and distribution of¹⁵NH ₄ ⁺ ,¹⁵NO ₃ ⁻ and¹⁵N₂ was studied in greenhouse-grown beans (Phaseolus vulgaris L.) with a commercial cultivar and 2 recombinant inbred backcross lines;¹⁵N was supplied in the nutrient solution at the R3 (50% bloom) stage. Plants were harvested 1, 5 and 10 days after treatment, and were separated into nodules, roots, stems, mature leaflets, immature leaflets, and flowers/fruits. All 3 lines showed rapid increases in the N content of flowers/fruits after the R3 stage. However, the percentage N in these tissues decreased after the R3 stage. One of the recombinant lines showed a greater uptake of NH ₄ ⁺ than the other 2 lines. Rates of¹⁵N₂ fixation and NO ₃ ⁻ uptake were similar for all 3 lines, N₂ fixation estimated from total N content showed the 2 recombinant lines with 24 and 34 percent greater activity than the commercial cultivar. Distribution of¹⁵N at the whole plant level was similar for all 3 lines for a similar N source.¹⁵NO ₃ ⁻ was transported first to leaflets and the label then moved into flowers/fruits. Transport of fixed N₂ was from the nodules to roots, stems and into flowers/fruits; usually less than 10 percent entered the leaflets. This indicates that N₂ fixation furnishes N directly to flowers/fruits with over 50 percent of the fixed N being deposited into flowers/fruits within 5 days after treatment.     

Plant and microbial nitrogen use and turnover: Rapid conversion of nitrate to ammonium in soil with roots

Immobilization of ammonium (NH ₄ ⁺ ) by plants and microbes, a controlling factor of ecosystem nitrogen (N) retention, has usually been measured based on uptake of¹⁵NH ₄ ⁺ solutions injected into soil. To study the influence of roots on N dynamics without stimulating consumption of NH ₄ ⁺ , we estimated gross nitrification in the presence or absence of live roots in an agricultural soil. Tomato (Lycopersicon esculentum var. Peto76) plants were grown in microcosms containing root exclosures. When the plants were 7 weeks old,¹⁵N enriched nitrate (NO ₃ ⁻ ) was applied in the 0–150 mm soil layer. After 24 h, > 30 times more¹⁵NH ₄ ⁺ was found in the soil with roots than in the soil of the root exclosures. At least 18% of the NH ₄ ⁺ -N present at this time in the soil with roots had been converted from NO ₃ ⁻ . We estimated rates of conversion of NO ₃ ⁻ to NH ₄ ⁺ , and rates ofNH ₄ ⁺ immobilization by plants and microbes, by simulating N-flow of¹⁴⁺¹⁵N and¹⁵N in three models representing mechanisms that may be underlying the experimental data: Dissimilatory NO ₃ ⁻ reduction to NH ₄ ⁺ (DNRA), plant N efflux, and microbial biomass nitrogen (MBN) turnover. Compared to NO ₃ ⁻ uptake, plant NH ₄ ⁺ uptake was modest. Ammonium immobilization by plants and microbes was equal to at least 35% of nitrification rates. The rapid recycling of NO ₃ ⁻ to NH ₄ ⁺ via plants and/or microbes contributes to ecosystem N retention and may enable plants growing in agricultural soils to capture more NH ₄ ⁺ than generally assumed.     

Specific 15N, NH correlations for residues in15 N, 13C and fractionally deuterated proteins that immediately follow methyl-containing amino acids

A triple-resonance pulse scheme is described which records¹⁵N, NH correlations of residues that immediately follow amethyl-containing amino acid. The experiment makes use of a¹⁵N, ¹³C and fractionally deuterated proteinsample and selects for CH₂D methyl types. The experiment isthus useful in the early stages of the sequential assignment process as wellas for the confirmation of backbone ¹⁵N, NH chemical shiftassignments at later stages of data analysis. A simple modification of thesequence also allows the measurement of methyl side-chain dynamics. This isparticularly useful for studying side-chain dynamic properties in partiallyunfolded and unfolded proteins where the resolution of aliphatic carbon andproton chemical shifts is limited compared to that of amide nitrogens.     

Biodegradation of Trichloroacetic Acid in Norway Spruce/Soil System

Trichloroacetic acid (TCA) belongs to secondary atmospheric pollutants affecting the forest health. Distribution of [1,2-¹⁴C]TCA-residues and TCA biodegradation were investigated in 4-year-old nursery-grown trees of Norway spruce [Picea abies (L.) Karst.] in the whole plant/soil system. Radioactivity was monitored in needles, wood, roots and soil as well as in the air. During two weeks of exposure TCA was continuously degraded, especially in the soil. Estimates of radioactivity balance showed loss of radioactivity into the atmosphere in the form of ¹⁴CO₂; unincorporated [1,2-¹⁴C]TCA, chloroform, carbon monoxide and methane were not detected at all. TCA degradation to CO₂ was indicated also in the spruce needles. Moreover, it was found that soil litter contained [1,2-¹⁴C]TCA unavailable to microorganisms.     

Effect of ammonium and nitrate on growth and appearance of nitrate reductase and nitrite reductase in dark- and light-grown mustard seedlings

Nitrate-induced and phytochrome-modulated appearance of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of the mustard (Sinapis alba L.) seedling is strongly affected by externally supplied ammonium (NH ₄ ⁺ ). In short-term experiments between 60 and 78 h after sowing it was found that in darkness NH ₄ ⁺ —simultaneously given with NO ₃ ^- —strongly inhibits appearance of nitrate-inducible NR and NIR whereas in continuous far-red light—which operates exclusively via phytochrome without significant chlorophyll formation —NH ₄ ⁺ (simultaneously given with NO ₃ ^- ) strongly stimulates appearance of NR. The NIR levels are not affected. This indicates that NR and NIR levels are regulated differently. In the absence of external NO ₃ ^- appearance of NR is induced by NH₄ in darkness as well as in continuous far-red light whereas NIR levels are not affected. On the other hand, in the absence of external NO ₃ ^- , exogenous NH ₄ ⁺ strongly inhibits growth of the mustard seedling in darkness as well as in continuous far-red light. This effect can be abolished by simultaneously supplying NO ₃ ^- . The adverse effect of NH ₄ ⁺ on growth (NH ₄ ⁺ -toxicity) cannot be attributed to pH-changes in the medium since it was shown that neither the growth responses nor the changes of the enzyme levels are related to pH changes in the medium. Non-specific osmotic effects are not involved either.Abbreviations c continuous - D darkness - FR far-red light - NIR nitrite reductase (EC 1.7.7.1) - NR nitrate reductase (EC 1.6.6.1)     

Chemical influences on 14C and 15N primary production in an arctic lake

Summary Twelve 24 h bioassay experiments were conducted in 1980 and 1981 to evaluate seasonal influences of NO ₃ ^- , NH ₄ ⁺ , PO ₄ ^3- , N+P, vitamins, trace metals, a synthetic chelate and common salts on ¹⁴C and ¹⁵N primary production in Toolik Lake, Alaska. Addition of N+P, NO ₃ ^- or NH ₄ ⁺ significantly increased ¹⁴C primary production over all other treatments on most dates. Only on three occasions did any other treatment have any statistically significant effect on ¹⁴C primary production. ¹⁵NO ₃ ^- and ¹⁵NH ₄ ⁺ primary production were each significantly enhanced by PO ₄ ^3- enrichement relative to all other nutrient applications on seven dates. Significantly depressed ¹⁵NO ₃ ^- primary production consistently resulted from NH ₄ ⁺ addition but enrichment with NO ₃ ^- gave significantly depressed ¹⁵NH ₄ ⁺ primary production in just three experiments. Other treatments significantly influenced ¹⁵N primary production on two occasions only. The general stimulatory influence of N+P, NO ₃ ^- and NH ₄ ⁺ on ¹⁴C primary production as well as the similar effect of PO ₄ ^3- on ¹⁵N primary production had no seasonal pattern. The total data show that nitrogen and phosphorus are the most important chemical regulators of primary production in Toolik Lake.     

Nitrite reduction and carbohydrate metabolism in plastids purified from roots of Pisum sativum L.

Nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-d-old seedlings of two-row barley (Hordeum vulgare L. cv. Daisengold) using the ¹⁵N-incorporation model (A. Gojon et al. (1986) Plant Physiol. 82, 254–260), except that NH ₊ ⁴ was replaced by NO _- ² . N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20°C; the media contained different amounts of ¹⁵N labeling. Experiments were started either immediately after the beginning (expt. 1) or immediately prior to the end (expt. 2) of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. The plants effectively utilized ¹⁵NO _- ³ and accumulated it as reduced ¹⁵N, predominantly in the shoots. Accumulation of reduced ¹⁵N in both experiments was nearly the same at the end of the experiment but the accumulation pattern in roots and shoots during each 12-h period differed greatly depending on time and the light conditions. In expt. 1, the roots accounted for 31% (light), 58% (dark), and 9% (light) of nitrate reduction by the whole plants, while in expt. 2 the contributions of the root were 82% (dark), 20% (light), and 29% (dark), during each of the three 12-h periods. Xylem transport of nitrate drastically decreased in the dark, but that of reduced N rather increased. The downward translocation of reduced ¹⁵N increased while nitrate reduction in the root decreased, whereas upward translocation decreased while nitrate reduction in the shoot increased. We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstances.Symbols Anl accumulation of reduced ¹⁵N from ¹⁵NO _- ³ in ¹⁴NO _- ³ -fed roots of divided root system - Ar accumulation in root of reduced ¹⁵N from ¹⁵NO _- ³ - As accumulation in shoot of reduced ¹⁵N from ¹⁵NO _- ³ - Rr ¹⁵NO _- ³ reduction in root - Rs ¹⁵NO _- ³ reduction in shoot - Tp translocation to root of shoot-reduced ¹⁵N from ¹⁵NO _- ³ in phloem - Tx translocation to shoot of root-reduced ¹⁵N from ¹⁵NO _- ³ in xylem