Fate of 15N-labelled nitrogen fertilizer applied to kiwifruit (Actinidia deliciosa) vines

The fate of ¹⁵N-labelled ammonium fertilizer applied once to six-year-old field-grown kiwifruit (Actinidia deliciosa Hayward) vines was measured over three years. The three main treatments were nitrogen (N) applied singularly at 100 or 200 kg N ha^–1 in early spring (two weeks before bud burst) or split with 100 kg N ha^–1 (unlabelled) in early spring and 100 kg N ha^–1 (¹⁵N-labelled) ten weeks later. All N treatments were applied to vines with a history of either 50 or 200 kg N ha^–1 yr^–1. For three years after ¹⁵n application, components of the vines and soil (0–600 mm depth) were sampled at harvest in late autumn and the N and ¹⁵N contents determined.By the first harvest, all plant uptake of ¹⁵N had occurred and this represented 48–53% of the ¹⁵N applied. There was no significant effect of current N fertilizer treatment or of N history on ¹⁵N recovery by vines. Removal of ¹⁵N in harvested fruit was small at 5–6% in the first year and 8% over 3 years. After 2–3 years, most plant ¹⁵N occurred in the roots and this component declined only slowly over time. In contrast, there was a large temporal decline in ¹⁵N in above-ground plant components due to the annual removal in leaf fall and pruning. An associated experiment showed that when ¹⁵N-labelled prunings and leaves were mulched and returned to the soil, only about 9% was recovered by plants within 2 years. Almost all remaining mulched material had been immobilised into the soil organic N.In all treatments, about 20% of the added ¹⁵N remained in soil at the first harvest. This was almost entirely in organic fractions (<0.4% in inorganic N) and mostly in the surface 150-mm layer. The ¹⁵N content in soil changed little over time (from 20 to 17% between the first and third harvests respectively) and indicated that most of the N had been immobilised into stable humus forms.     

Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study

This study presents a comprehensive analysis ofnitrogen (N) cycling in a second-order forestedstream in southern Michigan that has moderatelyhigh concentrations of ammonium (mean,16 g N/L) and nitrate (17 g N/L). Awhole-stream ¹⁵NH₄ ⁺ addition wasperformed for 6 weeks in June and July, and thetracer ¹⁵N was measured downstream inammonium, nitrate, and detrital and livingbiomass. Ancillary measurements includedbiomass of organic matter, algae, bacteria andfungi, nutrient concentrations, hydrauliccharacteristics, whole-stream metabolism, andnutrient limitation assays. The resultsprovide insights into the heterotrophic natureof woodland streams and reveal the rates atwhich biological processes alter nitrogentransport through stream systems.Ammonium uptake lengths were 766–1349 m anduptake rates were 41–60 g N m^–2min^–1. Nitrate uptake could not bedetected. Nitrification rates were estimatedfrom the downstream increase in¹⁵N-enriched nitrate using a simulationmodel. The ammonium was removed bynitrification (57% of total uptake),heterotrophic bacteria and fungi associatedwith detritus (29%), and epilithic algae(14%). Growth of algae was likely limited bylight rather than nutrients, and dissolvedO₂ revealed that the stream metabolism washeterotrophic overall (P:R = 0.2). Incubationsof detritus in darkened chambers showed thatuptake of ¹⁵N was mostly heterotrophic.Microbial N in detritus and algal N inepilithon appeared to reach isotopic steadystate with the dissolved ammonium, but theisotopic enrichment of the bulk detritus andepilithon did not approach that of ammonium,probably due to a large fraction of organic Nin the bulk samples that was not turning over. The actively cycling fraction of total N inorganic compartments was estimated from theisotopic enrichment, assuming uptake ofammonium but not nitrate, to be 23% forepilithon, 1% for fine benthic organic matter,5% for small woody debris, and 7% for leaves. These percentages agree with independentestimates of epilithic algal biomass, whichwere based on carbon:chlorophyll ratios in bulksamples and in algal fractions separated bydensity-gradient centrifugation in colloidalsilica, and of microbial N in the detritus,which were based on N released by chloroformfumigations.     

The fate of15N enriched throughfall in two coniferous forest stands at different nitrogen deposition levels

The stable isotope¹⁵N was added as (¹⁵NH₄)₂SO₄ to throughfall water for one year, to study the fate of the deposited nitrogen at different levels of N deposition in two N saturated coniferous forests ecosystems in the Netherlands. The fate of the¹⁵N was followed at high-N (44–55 kg N ha^–1 yr^–1) 1) and low-N (4–6 kg N ha^–1 yr^–1) deposition in plots established under transparent roofs build under the canopy in a Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) forest.The applied¹⁵N was detectable in needles and twigs, the soil and soil water leaching below the rooting zone (90 cm depth). Total¹⁵N recovery in major ecosystem compartments was 71–100% during two successive growing seasons after the start of a year-round¹⁵N application to throughfall-N. Nine months after the year-round¹⁵N application, the¹⁵N assimilated into tree biomass was 29–33% of the¹⁵N added in the Douglas fir stand and less than 17% in the Scots pine stand. At the same time total¹⁵N retention in the soil (down to 70 cm) of the high-N plots was about 37% of the deposited¹⁵NH₄-N, whereas 46% and 65% of the¹⁵N was found in the soil of the low-N deposition plots at the Douglas fir and Scots pine stand, respectively. The organic layers accounted for 60% of the¹⁵N retained in the soil. The total N deposition exceeded the demand of the vegetation and microbial immobilization. Total¹⁵N leaching losses within a year (below 90 cm) were 10–20% in the high-N deposition plots in comparison to 2–6% in the lowered nitrogen input plots. Relative retention in the soil and vegetation increased at lower N-input levels.Species differences in uptake and tree health seem to contribute to lower¹⁵N recoveries in the Scots pine trees compared to the Douglas fir trees. The excessive N deposition and resulting N saturation lead to conditions were the health and functioning of biota were negatively influenced. At decreased N deposition, lower leaching losses together with increased soil and plant retention indicated a change in the fate of the¹⁵N deposited. This may have resulted from changes in ecosystem processes, and thus a shift along the continuum of N saturation to N limitation.     

Kinetics and mechanisms of dissolved organic carbon retention in a headwater stream

Freeze-dried aqueous extracts of autumn-shed maple leaves, birch leaves, and spruce needles were added to a third-order reach of Bear Brook, New Hampshire at concentrations similar to those predicted to occur during peak leaf fall. Leachate from each species was rapidly removed from solution. With initial concentrations of added leachate of approximately 5 mgl^–1, dissolved organics (DOC) uptake ranged from 73 to 130 mg m^–2 h^–1 for the first five hours of travel downstream from the point of addition. There was no preferential removal of DOC of low molecular weight, or of monomeric carbohydrates relative to phenolics or unidentified DOC.Stream sediments and organic debris rapidly removed DOC from solution in laboratory experiments. No significant flocculation or microbial assimilation of sugar maple leachate occurred in stream water alone. Stream sediments showed small increases in respiration with addition of leaf leachate, but no increase in respiration occurred upon addition of leachate to organic debris. Abiotic adsorption due to the high concentrations of exchangeable iron and aluminium in stream sediments may be responsible for much of the rapid removal of leaf leachate observed in field experiments. Abiotic processes appear to retain DOC within the stream, thereby allowing subsequent metabolism of dissolved organic carbon by stream microflora.     

Natural 15N abundances of maize and soil amended with urea and composted pig manure

To investigate the effect of inorganic fertilizer and composted manure amendments on the N isotope composition (delta ¹⁵N) of crop and soil, maize (Zea mays L.) was cultivated under greenhouse conditions for 30, 40, 50, 60, and 70 days. Composted pig manure (delta ¹⁵N= +13.9) and urea (-2.3) were applied at 0 and 0 kg N ha^–1 (C0U0), 0 and 150 kg N ha^–1 (C0U2), 150 and 0 kg N ha^–1 (C2U0), and 75 and 75 kg N ha^–1 (C1U1), respectively. The delta ¹⁵N of total soil-N was not affected by both amendments, but delta ¹⁵N of NH⁺ ₄ and NO^– ₃ provided some information on the N isotope fractionation in soil. During the early growth stage, significant differences (P < 0.05) in delta ¹⁵N among maize subjected to different treatments were observed. After 30 days of growth, the delta ¹⁵N values of maize were +6.6 for C0U0, +1.1 for C0U2, +7.7 for C2U0, and +4.5 for C1U1. However, effects of urea and composted manure application on maize delta ¹⁵N progressively decreased with increasing growth period, probably due to isotope fractionation accompanying N losses and increased uptake of soil-derived N by maize. After 70 days of growth, delta ¹⁵N of leaves and grains of maize amended with composted pig manure were significantly (P < 0.05) higher than those with urea. The temporal variations in delta ¹⁵N of maize amended with urea and composted manure indicate that plant delta ¹⁵N is generally not a good tracer for N sources applied to field. Our data can be used in validation of delta ¹⁵N fractionation models in relation to N source inputs.     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

The influence of phosphorus enrichment on lotic bryophytes

• 1 Phosphorus enrichments of 5 weeks' duration were performed in two woodland streams in eastern Tennessee, U.S.A. to determine their effect on the dominant stream bryophyte, Porella pinnata.
• 2 In a second-order reach of Walker Branch, which had an N:P ratio (by atoms) of 3.5:1 in the stream water, the P:C ratio of Porella was not significantly affected by enrichment, but the P:N ratio did increase significantly. In Sludge Creek, which had an N:P ratio of 21.6:1 in the stream water, both P:C and P:N ratios of Porella increased significantly following phosphorus addition. Increased phosphorus ratios may have resulted from either assimilation or adsorption.
• 3 Absolute phosphorus concentrations in Porella tissue were significantly greater prior to enrichment in the control reach of Walker Branch, confounding the effect of phosphorus enrichment. In Sludge Creek, absolute phosphorus content in Porella tissue increased significantly in the treated reach and showed no significant difference in the control reach.
• 4 Although mean primary production increased by approximately 15% following enrichment, the increase was not significantly different from that prior to enrichment.
• 5 Epiphyte structure and abundance were not significantly influenced by enrichment in either stream. It is suggested that grazing pressure by snails may have masked any potential epiphyte response to enrichment.

     

Estimates of the residual nitrogen benefit of groundnut to maize in Northeast Thailand

Four cultivars of groundnut were grown in upland soil in Northeast Thailand to study the residual benefit of the stover to a subsequent maize crop. An N-balance estimate of the total residual N in the maize supplied by the groundnut was made. In addition three independent estimates were made of the residual benefits to maize when the groundnut stover was returned to the land and incorporated. The first estimate (Estimate 1) was an N-balance estimate. A dual labelling approach was used where ¹⁵N-labelled stover was added to unlabelled microplots (Estimate 2) or unlabelled stover was added to ¹⁵N-labelled soil microplots (Estimate 3). The nodulating groundnut cultivars fixed between 59–64% of their nitrogen (as estimated by the ¹⁵N isotope dilution method using non-nodulating groundnut as a non-fixing reference) producing between 100 and 130 kg N ha^-1 in their stover. Although the following maize crop suffered from drought stress, maize grain N and dry weights were up to 80% and 65% greater respectively in the plots where the stover was returned as compared with the plots where the stover was removed. These benefits were comparable with applications of 75 kg N ha^-1 nitrogen in the form of urea. The total residual N estimates of the contribution of the nodulated groundnut to the maize ranged from 16.4–27.5 kg N ha^-1. Estimates of the residual N supplied by the stover and fallen leaves ranged from 11.9–21.3 kg N ha^-1 using the N-balance method (Estimate 1), from 6.3–9.6 kg N ha^-1 with the labelled stover method (Estimate 2) and from 0–11.4 kg N ha^-1 with the labelled soil method. There was closest agreement between the two ¹⁵N based estimates suggesting that apparent added nitrogen interactions in these soils may not be important and that N balance estimates can overestimate the residual N in crops following legumes, even in very poor soils. This work also indicates the considerable ability of local groundnut cultivars to fix atmospheric nitrogen and the potential benefits from returning and incorporating legume residues to the soil in the upland cropping systems of Northeast Thailand. The applicability of the ¹⁵N methodology used here and possible reasons for the discrepancies between estimates 1, 2 and 3 are discussed.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

Leaf 15N abundance of subarctic plants provides field evidence that ericoid,ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen

The natural abundance of the nitrogen isotope 15, ¹⁵N, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf ¹⁵N was between–8.8 and–5.5 at the heath and between–6.1 and –3.3 at the fellfield. The leaf ¹⁵N of non- or arbuscular mycorrhizal species was markedly different, ranging from –4.1 to –0.4 at the heath, and from –3.4 to+2.2 at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a ¹⁵N of –0.7 at the heath and +0.5 at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher ¹⁵N (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO _inf3 ^sup- reductase activity. These species may depend more on soil inorganic N than the ericoids. The ¹⁵N of non- or arbuscular mycorrhizal species indicates that the ¹⁵N of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The ¹⁵N of NH _inf4 ^sup+ -N was +12.3 in winter snow and +1.9 in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH _inf4 ^sup+ , NO _inf3 ^sup- and organic N from the soil, atmospheric N₂, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant ¹⁵N in organic soils.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.     

Effect of light intensity on ammonia assimilation in maize leaves

The effect of light on the metabolism of ammonia was studied by subjecting detached maize leaves to 150 or 1350 mol m^–2 s^–1 PAR during incubation with the leaf base in 2 mM ¹⁵NH₄Cl. After up to 60 min, leaves were extracted. Ammonia, glutamine, glycine, serine, alanine, and aspartate were separated by isothermal distillation and ion exchange chromatography. ¹⁵N enrichments were analyzed by emission spectroscopy. The uptake of ammonium chloride did not influence CO₂ assimilation (8.3 and 17.4 mol m^–1 s^–1 at 150 and 1350 mol m^–2 s^–1 PAR, respectively). Leaves kept at high light intensity contained more serine and less alanine than leaves from low light treatments. Within 1 h of incubation the enrichment of ammonia extracted from leaves rose to approximately 20% ¹⁵N. In the high light regime the amino acids contained up to 15% ¹⁵N, whereas in low light ¹⁵N enrichments were small (up to 6%). The kinetics of ¹⁵N incorporation indicated that NH₃ was firstly assimilated into glutamine and then into glutamate. After 15 min ¹⁵N was also found in glycine, serine and alanine. At high light intensity nearly half of the ¹⁵N was incorporated in glycine. On the other hand, at low light intensity alanine was the predominant ¹⁵N sink. It is concluded that light influences ammonia assimilation at the glutamine synthetase reaction.     

Forests losing large quantities of nitrogen have elevated 15N:14N ratios

Summary Urea (U) and ammonium nitrate (AN) had been applied to a Scots pine (Pinus sylvestris L.) forest in northern Sweden for 18 consecutive years at four doses resulting in total N applications ranging from 0 to 1980 kg ha^–1. The ¹⁵N abundance ( ¹⁵N) of the grass Deschampsia flexuosa (L.) Trin. increased linearly (from –0.7 to 11.0) with application rate in the case of U. The response to AN was in the same direction but smaller. While others have shown that the initial response of nitrogen-limited systems to additions of N is a change of ¹⁵N abundance towards that of added N, this study shows that further and excessive additions leads to a retention of ¹⁵N. Monitoring ¹⁵N abundance over time in dose-response trials of this type thus opens new possibilities to estimate critical loads of N and the point of nitrogen saturation.     

Estimation of symbiotic dinitrogen fixation in alder forest by the method based on natural15N abundance

Annual N₂-fixation in virgin forest ecosystems has been measured using a¹⁵N natural abundance (¹⁵N) procedure. This method was compared to a¹⁵N labelled fertilizer isotopic dilution method. For young alders (5–6 years old), ¹⁵N of leaves gave results in good agreement with the isotopic dilution of fertilizer method. Since ¹⁵N variability was expected according to plant physiology, for alder trees, leaves were collected at various heights after the end of the growing season, and, to take account of isotopic variations coming from derived inputs, ¹⁵N of leaves of a large number of other plants in the same are were measured to give control values. Following this procedure, the ¹⁵N method gave reliable evaluation of the nitrogen supply, by through N₂-fixation, to alders, which were found to maintain high nitrogen fixing capacity in a sequence ranging from first stage of establishment of climactic formation. Moreover, the same method is reported to discriminate various origins ofAlnus glutinosa grown in natural conditions, possibly in relation to the genetic diversity of this species.     

Respiration and annual fungal production associated with decomposing leaf litter in two streams

1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch). 2. Both streams received similar annual leaf litter fall (478–492 g m^?2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m^?2; mean ± SE) than Payne Creek (39 ± 9 g m^?2). 3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g^?1) than those from Payne Creek (54 ± 8 mg g^?1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day^?1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek. 4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop. 5. Annual fungal production was 16 ± 6 g m^?2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m^?2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m^?2 in Payne Creek and 7–27 g m^?2 in Lindsey Spring Branch. 6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.     

Nitrogen biogeochemistry of three hardwood ecosystems in the Adirondack Region of New York

The biogeochemistry of nitrogen (N)was evaluated for three forest ecosystems[Woods Lake (WL), Pancake-Hall Creek (PHC) andHuntington Forest (HF)] in the Adirondackregion of New York, U.S.A. to evaluate theresponse of a range of N atmospheric inputsand experimental N additions. Bulk Ndeposition was higher at sites in the westthan those in the central and easternAdirondacks. These higher atmospheric N inputswere reflected in higher bulk throughfallfluxes of N (WL and PHC, 10.1 and 12.0 kg Nha^–1 yr^–1, respectively) in thewestern Adirondacks than at HF (4.6 kg Nha^–1 yr^–1) in the centralAdirondacks. Nitrogen was added to plots as(NH₄)₂SO₄ at 14 and 28 kg Nha^–1 yr^–1 or as HNO₃ at 14 kg Nha^–1 yr^–1. Litter decompositionrates of Fagus grandifolia and Acerrubrum were substantially higher at WL andPHC compared to HF but were not affected byexperimental N additions. Results usingmineral soil bags showed no effects of Naddition on N and C concentrations in soilorganic matter, but C and N concentrationincreases were less at WL and PHC compared toHF. Soil solution nitrate (NO₃ ^–)concentrations at 15-cm depth in the referenceplots were higher at PHC than at WL and HFwhile at 50-cm concentrations were higher atPHC and WL than at HF. The reference plots atthe two sites (WL and PHC) with the highestatmospheric inputs of N exhibited lower Nretention (53 and 33%, respectively) than HF(68%) in reference plots. The greatestincrease in NO₃ ^– loss in response tothe experimental treatments occurred at HFwhere the HNO₃ additions resulted in thehighest NO₃ ^– concentrations andlowest N retentions. In contrast, at WL andPHC increases in soil water NO₃ ^–were not evident in response to experimental Nadditions. The results suggest that the twosites (WL and PHC) in the western Adirondacksdid not respond to additional N inputsalthough they have experienced elevatedatmospheric N inputs and higher N drainagelosses in reference plots than the HF site inthe central Adirondacks. Some of thesedifferences in site response may have alsobeen a function of stand age of WL and PHCthat were younger (24 and 33 years,respectively) than the HF (age 70).Highest NO₃ ^– fluxes in thereference plots across the sites correspondedto higher ¹⁵N values in soil andplants. An experimental addition experimentat PHC found that the forest floor and themineral soil were the largest sinks forexperimentally added N.     

Anthropogenic N deposition and the fate of 15NO 3 − in a northern hardwood ecosystem

Human activity has substantially increased atmospheric NO ₃ ^– deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO ₃ ^– deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5–7 years). Although microbial immobilization is an initial sink (i.e., within 1–2 days) for anthropogenic NO ₃ ^– in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO ₃ ^– additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of ¹⁵NO ₃ ^– in plots receiving chronic NO ₃ ^– addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m^–2 year^–1) and three plots which receive ambient plus experimental N deposition (3.0 g NO₃ ^–-N m^–2 year^–1). Chronic NO ₃ ^– deposition significantly increased the N concentration and content (g N/m²) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO ₃ ^– deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of ¹⁵N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO ₃ ^– over a 1 year duration. Our results indicate that anthropogenic NO ₃ ^– initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.     

15N abundance of surface soils,roots and mycorrhizas in profiles of European forest soils

¹⁵N natural abundances of soil total N, roots and mycorrhizas were studied in surface soil profiles in coniferous and broadleaved forests along a transect from central to northern Europe. Under conditions of N limitation in Sweden, there was an increase in ¹⁵N of soil total N of up to 9% from the uppermost horizon of the organic mor layer down to the upper 0–5 cm of the mineral soil. The ¹⁵N of roots was only slightly lower than that of soil total N in the upper organic horizon, but further down roots were up to 5% depleted under such conditions. In experimentally N-enriched forest in Sweden, i.e. in plots which have received an average of c. 100 kg N ha^–1 year^–1 for 20 years and which retain less than 50% of this added N in the stand and the soil down to 20 cm depth, and in some forests in central Europe, the increase in ¹⁵N with depth in soil total N was smaller. An increase in ¹⁵N of the surface soil was even observed on experimentally N-enriched plots, although other data suggest that the N fertilizer added was depleted in¹⁵N. In such cases roots could be enriched in¹⁵N relative to soil total N, suggesting that labelling of the surface soil is via the pathway: — available pools of N-plant N-litter N. Under N-limiting conditions roots of different species sampled from the same soil horizon showed similar ¹⁵N. By contrast, in experimentally N-enriched forest ¹⁵N of roots increased in the sequence: ericaceous dwarf shrubs15N enriched compounds in fungal material, which could contribute to explain the observed ¹⁵N profiles if fungal material is enriched, because it is a precursor of stable organic matter and recalcitrant N. This could act in addition to the previous explanation of the isotopically lighter soil surface in forests: plant uptake of ¹⁵N-depleted N and its redeposition onto the soil surface by litter-fall.     

1H, 15N, 13C and 13CO assignments and secondary structure determination of basic fibroblast growth factor using 3D heteronuclear NMR spectroscopy

Summary The assignments of the ¹H, ¹⁵N, ¹³CO and ¹³C resonances of recombinant human basic fibroblast growth factor (FGF-2), a protein comprising 154 residues and with a molecular mass of 17.2 kDa, is presented based on a series of three-dimensional triple-resonance heteronuclear NMR experiments. These studies employ uniformly labeled ¹⁵N- and ¹⁵N-/¹³C-labeled FGF-2 with an isotope incorporation >95% for the protein expressed in E. coli. The sequence-specific backbone assignments were based primarily on the interresidue correlation of C, C and H to the backbone amide ¹H and ¹⁵N of the next residue in the CBCA(CO)NH and HBHA(CO)NH experiments and the intraresidue correlation of C, C and H to the backbone amide ¹H and ¹⁵N in the CBCANH and HNHA experiments. In addition, C and C chemical shift assignments were used to determine amino acid types. Sequential assignments were verified from carbonyl correlations observed in the HNCO and HCACO experiments and C correlations from the carbonyl correlations observed in the HNCO and HCACO experiments and C correlations from the HNCA experiment. Aliphatic side-chain spin systems were assigned primarily from H(CCO)NH and C(CO)NH experiments that correlate all the aliphatic ¹H and ¹³C resonances of a given residue with the amide resonance of the next residue. Additional side-chain assignments were made from HCCH-COSY and HCCH-TOCSY experiments. The secondary structure of FGF-2 is based on NOE data involving the NH, H and H protons as well as ³J_H n _H coupling constants, amide exchange and ¹³C and ¹³C secondary chemical shifts. It is shown that FGF-2 consists of 11 well-defined antiparallel -sheets (residues 30–34, 39–44, 48–53, 62–67, 71–76, 81–85, 91–94, 103–108, 113–118, 123–125 and 148–152) and a helix-like structure (residues 131–136), which are connected primarily by tight turns. This structure differs from the refined X-ray crystal structures of FGF-2, where residues 131–136 were defined as -strand XI. The discovery of the helix-like region in the primary heparin-binding site (residues 128–138) instead of the -strand conformation described in the X-ray structures may have important implications in understanding the nature of heparin-FGF-2 interactions. In addition, two distinct conformations exist in solution for the N-terminal residues 9–28. This is consistent with the X-ray structures of FGF-2, where the first 17–19 residues were ill defined.     

15N natural abundances and N use by tundra plants

Plant species collected from tundra ecosystems located along a north-south transect from central Alaska to the north coast of Alaska showed large and consistent differences in ¹⁵N natural abundances. Foliar ¹⁵N values varied by about 10% among species within each of two moist tussock tundra sites. Differences in ¹⁵N contents among species or plant groups were consistent across moist tussock tundra at several other sites and across five other tundra types at a single site. Ericaceous species had the lowest ¹⁵N values, ranging between about –8 to –6. Foliar ¹⁵N contents increased progressively in birch, willows and sedges to maximum ¹⁵N values of about +2 in sedges. Soil ¹⁵N contents in tundra ecosystems at our two most intensively studied sites increased with depth and ¹⁵N values were usually higher for soils than for plants. Isotopic fractionations during soil N transformations and possibly during plant N uptake could lead to observed differences in ¹⁵N contents among plant species and between plants and soils. Patterns of variation in ¹⁵N content among species indicate that tundra plants acquire nitrogen in extremely nutrient-poor environments by competitive partitioning of the overall N pool. Differences in plant N sources, rooting depth, mycorrhizal associations, forms of N taken up, and other factors controlling plant N uptake are possible causes of variations in ¹⁵N values of tundra plant species.