Total 15N Research Articles

The fate of 15N added to high Arctic tundra to mimic increased inputs of atmospheric nitrogen released from a melting snowpack

AbstractIncreases in the long‐range aerial transport of reactive N species from low to high latitudes will lead to increased accumulation in the Arctic snowpack, followed by release during the early summer thaw. We followed the release of simulated snowpack N, and its subsequent fate over three growing seasons, on two contrasting high Arctic tundra types on Spitsbergen (79°N). Applications of 15N (99 atom%) at 0.1 and 0.5 g N m−2 were made immediately after snowmelt in 2001 as either Na15NO3 or 15NH4Cl. These applications are approximately 1 × and 5 × the yearly atmospheric deposition rates. The vegetation at the principal experimental site was dominated by bryophytes and Salix polaris while at the second site, vegetation included bryophytes, graminoids and lichens. Audits of the applied 15N were undertaken, over two or three growing seasons, by determining the amounts of labeled N in the soil (0–3 and 3–10 cm), soil microbial biomass and different vegetation fractions.Initial partitioning of the 15N at the first sampling time showed that ∼60% of the applied 15N was recovered in soil, litter and plants, regardless of N form or application rate, indicating that rapid immobilization into organic forms had occurred at both sites. Substantial incorporation of the 15N was found in the microbial biomass in the humus layer and in the bryophyte and lichen fractions. After initial partitioning there appeared to be little change in the total 15N recovered over the following two or three seasons in each of the sampled fractions, indicating highly conservative N retention. The most obvious transfer of 15N, following assimilation, was from the microbial biomass into stable forms of humus, with an apparent half‐life of just over 1 year. At the principal site the microbial biomass and vascular plants were found to immobilize the greatest proportion of 15N compared with their total N concentration. In the more diverse tundra of the second site, lichen species and graminoids competed effectively for 15NH4‐N and 15NO3‐N, respectively. Results suggest that Arctic tundra habitats have a considerable capacity to immobilize additional inorganic N released from the snow pack. However, with 40% of the applied 15N apparently lost there is potential for N enrichment in the surrounding fjordal systems during the spring thaw.

Nitrogen saturation in UK moorlands: the critical role of bryophytes and lichens in determining retention of atmospheric N deposition

Summary Anthropogenic nitrogen (N) deposition may have several impacts on upland moorland ecosystems, including changes in vegetation composition, eutrophication and surface water acidification through nitrate leaching, but few studies linking N deposition to key biogeochemical processes have been published. A stable isotope tracer (15N) was used to determine the fate of inorganic N inputs to four moorland catchments across gradients of N deposition and leaching, through 2‐weekly additions to experimental plots on major soil types over 1 year. An apparent decline in total 15N recovery from soils and vegetation as the proportion of leached N deposition increased was not significant at the P = 0·05 level, but a significant relationship was found for recovery in mosses and lichens. Vegetation retained 31–68% of 15N inputs, and 15N recovery increased significantly (P = 0·01) with biomass for all compartments except woody shrubs. Mosses and lichens showed far greater 15N recovery per unit biomass than grasses or ericaceous shrubs. There was no significant variation in the proportion of 15N recovered in higher plants across the N deposition gradient (24–29%). In contrast, the proportion recovered in mosses and lichens declined from 44% to 2% as deposition increased, mirroring a decline in their biomass and showing a highly significant inverse relationship (P = 0·01) with nitrate leaching. The proportion of 15N recovered in litter plus surface soils (33–39%) was remarkably constant across the deposition gradient for a variety of soil types. However, significantly declining recovery per unit biomass in litter (P < 0·05) suggested progressive N saturation of this sink and increasing importance of retention in underlying surface soils as deposition increased. Synthesis and applications. Past studies have demonstrated a decline in mosses and lichens in response to increasing N deposition, but we show here for the first time that reduced N retention might result together with increased nitrate leaching into surface waters. The conservation of bryophyte and lichen flora on moorlands is therefore critical to prevent excessive nitrate leaching and associated surface water acidification and eutrophication. Ensuring management practices such as grazing or burning are at an intensity that does not further degrade the bryophyte and lichen communities may help minimize the impact of N deposition on freshwaters, but the only effective means to reduce the risk of N leaching is a reduction in N emissions.

Effects of N-deficiency and timing of N supply on the recovery and distribution of labeled 15N in contrasting maize hybrids

Little information exists on the pattern of nitrogen (N) uptake, remobilization and N use efficiency (NUE) in Leafy and stay-green (SG) maize (Zea mays L.) genotypes. A pot experiment was conducted under controlled nutrition and growing conditions to determine the response of Leafy and SG maize genotypes to different levels of N-deficiency and timing of N supply. Three contrasting maize hybrids, ‘Pioneer 3905’ (a conventional hybrid with moderate SG characteristics), ‘Pioneer 39F06 Bt’ (with a high score of SG trait) and ‘Maizex LF850-RR’ (with a Leafy trait) were grown in 6 L plastic pots. Five different N treatments [no supply of N until V8 (N1), no supply of N after V8 (N2), no supply of N after silking (N3), no supply of N beyond 3 weeks after silking (N4), and continuous N supply from emergence to physiological maturity (N5; standard check)] were imposed through modified Hoagland solution applied manually. Labeled 15N of 5% 15N2–NH4NO3fertilizer was applied at 3 g per pot at the start of each schedule N treatment. Total amounts of N applied in different treatments were 3.13, 1.32, 1.90, 2.63 and 3.40 g, respectively in N1, N2, N3, N4 and N5. Dry matter, N concentration, 15N (atom% enrichment) and NUE were determined in roots, stalk, leaves and grains at crop maturity. The three contrasting hybrids did not differ in grain yield, total N acquisition, partitioning of 15N and NUE. Restriction of N supply until V8, and from V8 to physiological maturity significantly reduced grain yield and N-uptake in all hybrids. Irrespective of the level of N-deficiency in plant and timing when the labeled fertilizer was applied, the amount of 15N recovered in the matured plant was the same in all N treatments. It has been evident that maize continued to take up N beyond 3 weeks after silking and the later N was applied during the development, the higher proportion of it was partitioned to grains. Of the total 15N uptake, 78% was partitioned to kernels in the N4 treatment compared to only 61% in the control. Our data showed no evidence of differential N uptake, remobilization and NUE in the SG or Leafy hybrids tested, but the timing of N application and level of N-deficiency in plant significantly influenced N uptake, remobilization and N-dynamics in maize.

PARTITIONING OF NITROGEN OVER FIVE GROWING SEASONS IN TALLGRASS PRAIRIE

Annual spring burning of tallgrass prairie increases plant biomass production despite losses of N and lower net N mineralization. To better understand how burning influences the cycling of N in prairie, 15N was injected to soil as NH4+, and the partitioning between plant and soil N pools was followed over five growing seasons in annually burned and unburned prairie. Applied 15N was rapidly immobilized with <2% and 11% of the 15N remaining in inorganic forms six days after application in burned and unburned prairie, respectively. Seventy-seven percent (burned prairie) and 70% (unburned prairie) of the applied 15N was initially recovered as soil organic N (oN), with a majority accounted for in microbial biomass. Plants contained ∼20% of the applied N with the largest portion recovered from roots regardless of burning. At the end of the first growing season, only 55% of the applied 15N was recovered from the unburned prairie, while 85% was recovered from burned prairie. The total 15N content of the plants changed little during the first growing season, but the portion recovered in the rhizomes increased, indicating belowground N storage. Total recovery and distribution of applied N changed little from the end of the first to the end of the second season growing season. Accumulations of 15N within the plants decreased greatly between the second and fifth growing seasons, but N lost from plants was accounted for in oN. Conservation of N by plants and tight cycling of N within the root zone suggest mechanisms by which prairie can be a highly productive ecosystem despite limited N availability. The immobilization of potentially mineralized N within the root zone increased with burning, offsetting the loss of N to fire probably by reducing leaching and denitrification losses.

Dry season groundnut stover management practices determine nitrogen cycling efficiency and subsequent maize yields

It is generally thought that grain legume residues make a substantial net N contribution to soil fertility in crop rotation systems. However, most studies focus on effects of residues on crops immediately sown after the legume crop while in fact in many tropical countries with a prolonged dry season there is a large gap before planting the next crop with potential for nutrient losses. Thus the objectives of this study were* to improve the efficiency of groundnut (Arachis hypogaea L.) stover-N (100 kg N ha −1) recycling by evaluating the effect of dry season stover management, i.e. surface application and immediate incorporation after the legume crop or storage of residues until next cropping in the rainy season. N dynamics (litterbags, mineral N, microbial biomass N, N 2O emissions) were monitored and 15N labelled residues were applied to assess the fate of residue N in the plant–soil (0–100 cm) system during two subsequent maize crops. Recycling groundnut stover improved yield of the subsequent maize (Zea mays L.) crop compared to treatment without stover. A higher N recycling efficiency was observed when residues were incorporated (i.e. 55% total 15N recovery after second maize crop) than when surface applied (43% recovery) at the beginning of the dry season. This was despite the faster nitrogen release of incorporated residues, which led to more mineral N movement to lower soil layers. It appears that a proportion of groundnut stover N released during the dry season was effectively captured by the natural weed population (54–70 kg N ha −1) and subsequently recycled particularly in the incorporation treatment. Despite the presence of weeds major leaching losses occurred during the onset of the rainy season while N 2O emissions were relatively small. There was a good correlation between soil microbial biomass N and first crop maize yield. Incorporation of groundnut residues led to small increases in economic yield, i.e., 3120 versus 3528 kg ha −1 over two cropping cycles in the surface versus incorporation treatments respectively, with corresponding residue 15N uptakes of 4 and 8%, while 15N recovery in water stable aggregates (9–15%) was not significantly different. In contrast, when stover was removed and applied before the first crop, yield benefits were highest with cumulative maize yields of 4350 kg ha −1 and residue utilization of 12%. However, N recycling efficiency was not higher than in the early incorporation treatment due to an asynchrony of N release and maize N demand during the first crop.

Influence of Tree Species on Forest Nitrogen Retention in the Catskill Mountains, New York, USA

This study examines the effect of four tree species on nitrogen (N) retention within forested catchments of the Catskill Mountains, New York (NY). We conducted a 300-day 15N field tracer experiment to determine how N moves through soil, microbial, and plant pools under different tree species and fertilization regimes. Samples were collected from single-species plots of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga canadensis L.), red oak (Quercus rubra L.), and sugar maple (Acer saccharum Marsh). Using paired plots we compared the effects of ambient levels of N inputs (11 kg N/ha/y) to additions of 50 kg N/ha/y that began 1.5 years prior to and continued throughout this experiment. Total plot 15N recovery (litter layer, organic and mineral soil to 12 cm, fine roots, and aboveground biomass) did not vary significantly among tree species, but the distribution of sinks for 15N within the forest ecosystem did vary. Recovery in the forest floor was significantly lower in sugar maple stands compared to the other species. 15Nitrogen recovery was 22% lower in the fertilized plots compared to the ambient plots and red oak stands had the largest drop in 15N recovery as a result of N fertilization. Aboveground biomass became a significantly greater 15N sink with fertilization, although it retained less than 1% of the tracer addition. These results indicate that different forest types vary in the amount of N retention in the forest floor, and that forest N retention may change depending upon N inputs.

Nitrogen use efficiency of monoculture and hedgerow intercropping in the humid tropics

The design of productive and efficient intercropping systems depends on achieving complementarity between component species’ resource capture niches. Spatiotemporal patterns of capture and use of pruning and urea nitrogen (N) by trees and intercrops were elucidated by isotopic tracing, and consequences for nitrogen use efficiency were examined. During the first cropping season after applying urea−15N, maize accounted for most of the plant 15N recovery in Peltophorum dasyrrachis (33.5%) and Gliricidia sepium (22.3%) hedgerow intercropping systems. Maize yield was greatest in monoculture, and maize in monoculture also recovered a greater proportion of urea 15N (42%) than intercropped maize. Nitrogen recovery during active crop growth will not be increased by hedgerow intercropping if hedgerows adversely affect crop growth through competition for other resources. However, hedgerows recovered substantial amounts of 15N during both cropping cycles (e.g. a total of 13–22%), showing evidence of spatio-temporal complementarity with crops in the spatial distribution of roots and the temporal distribution of Nuptake. The degree of complementarity was species-specific, showing the importance of selecting appropriate trees for simultaneous agroforestry. After the first cropping season 17–34% of 15N applied was unaccounted for in the plant-soil system. Urea and prunings N were recovered by hedgerows in similar amounts. By the end of the second (groundnut) cropping cycle, total plant 15N recovery was similar in all cropping systems. Less N was taken up by the maize crop from applications of labelled prunings (5–7%) than from labelled urea (22–34%), but the second crop recovered similar amounts from these two sources, implying that prunings N is more persistent than urea N. More 15N was recovered by the downslope hedgerow than the upslope hedgerow, demonstrating the interception of laterally flowing N by hedgerows.

Decomposer animals (Lumbricidae, Collembola) and organic matter distribution affect the performance of Lolium perenne (Poaceae) and Trifolium repens (Fabaceae)

Decomposer animals stimulate plant growth by indirect effects such as increasing nutrient availability or by modifying microbial communities in the rhizosphere. In grasslands, the spatial distribution of organic matter (OM) rich in nutrients depends on agricultural practice and the bioturbation activities of large detritivores, such as earthworms. We hypothesized that plants of different functional groups with contrasting nutrient uptake and resource allocation strategies differentially benefit from sites in soil with OM accumulation and the presence of decomposer animals. In a greenhouse experiment we investigated effects of spatial distribution of 15N-labelled grass litter, earthworms and collembola on a simple grassland community consisting of Lolium perenne (grass) and Trifolium repens (legume). Litter aggregates (compared to homogeneous litter distribution) increased total shoot biomass, root biomass and 15N uptake by the plants. Earthworms and collembola did not affect total N uptake of T. repens; however, the presence of both increased 15N uptake by T. repens and L. perenne. Earthworms increased shoot biomass of T. repens 1.11-fold and that of L. perenne 2.50 fold. Biomass of L. perenne was at a maximum in the presence of earthworms, collembola and with litter concentrated in a single aggregate. Shoot biomass of T. repens increased in the presence of collembola, with L. perenne generally responding opposingly. The results indicate that the composition of the decomposer community and the distribution of OM in soil affect plant competition and therefore plant community composition.

Evidence of N2O emission and gaseous nitrogen losses through nitrification-denitrification induced by rice plants (Oryza sativa L.)

A greenhouse experiment was conducted with a specially designed apparatus consisting of an upper and lower chamber where the treatment with rice was carried out (treatment 1). The apparatus also had a single chamber where treatment 2, without rice plants, was carried out. The scope of this study was to elucidate the influence of rice plant growth on gaseous N losses as N2 and N2O produced by nitrification-denitrification in a flooded soil fertilized with (NH4)2SO4 (with 56.50 atom% 15N). Gas samples were withdrawn weekly and analyzed for (N2 + N2O)-15N losses by mass spectrometer and for N2O by gas chromatograph. The gaseous (N2 + N2O)-15N losses of the treatment with rice plants were significantly (P =0.01) higher than those of the treatment without rice plants, as were the amounts of N2O emitted. Rice plants facilitate the efflux of N2 and N2O from soil to atmosphere, as about half of the total gaseous 15N loss as N2 and N2O was found in the upper chamber. The proportion of N2O-15N to (N2 + N2O)-15N in the upper chamber was 10.56%, much higher than that of the lower chamber in treatment 1 and the headspace of treatment 2.

Nitrogen-15 Uptake by Pinus contorta Seedlings in Relation to Phenological Stage and Season

This study measured the amount of uptake of labeled nitrogen (15N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after 15N application, the amount of 15N recovered in seedlings as a percentage of the total 15N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total 15N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total 15N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low 15N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.

The role of Azolla cover in improving the nitrogen use efficiency of lowland rice

The development of management techniques to improve the poor N use efficiency by lowland rice (Oryza sativa L.) and reduce the high N losses has been an important focus of agronomic research. The potential of an Azolla cover in combination with urea was assessed under field conditions in Laguna, Philippines. Two on-station field experiments were established in the 1998–1999 dry season and eight on-farm experiments per season were carried out in the 2000–2001 wet and dry seasons. Treatment combinations consisting of N levels applied alone or combined with Azolla were evaluated with respect to floodwater chemistry, 15N recovery, crop growth, and grain yield. A full Azolla cover on the floodwater surface at the time of urea application prevented the rapid and large increase in floodwater pH and floodwater temperature. As a consequence, the partial pressure of ammonia (ρNH3), which is an indicator of potential NH3 volatilization, was significantly depressed. 15N recovery was higher in plots covered with Azolla where the total 15N recovery ranged between 77 and 99%, and the aboveground (grain and straw) recovery by rice ranged between 32 and 61%. The tiller count in Azolla-covered plots was significantly increased by 50% more than the uncovered plots at all urea levels. Consequently, the grain yield was likewise improved. Grain yields from the 16 on-farm trials increased by as much as 40% at lower N rates (40 and 50 kg N ha−1) and by as much as 29% at higher N rates (80 and 100 kg N ha−1). In addition, response of rice to treatments with lower N rates with an Azolla cover was comparable to that obtained with the higher N rates without a cover. Thus, using Azolla as a surface cover in combination with urea can be an alternative management practice worth considering as a means to reduce NH3 volatilization losses and improve N use efficiency.

The Fate and Retention of Organic and Inorganic 15N-Nitrogen in an Old-Growth Forest Soil in Western Oregon

Forests in the American Pacific Northwest receive very little nitrogen (N) through atmospheric deposition; therefore, they can provide insights into how the N cycle functioned in other regions before heavy atmospheric deposition of inorganic N began. Our objectives were to determine (a) if the fate of organic N differed from the fate of inorganic N, (b) the effect that polyphenols have on the fate of organic N, and (c) the effect of season of addition on the fate of N inputs. We traced N added to in situ soil cores as ammonium, organic N, tannin-complexed organic N, and the N2-fixing lichen Lobaria oregana. Total 15N recovery was between 74% and 109% for all N additions. Total 15N recovery did not vary significantly from the first sampling date to the last date. The litter/organic horizon, as a bulk pool, was the largest N retention pool for all forms of N addition. Within the litter/organic horizon, the chloroform-extractable microbial biomass initially accounted for nearly all of the added N from the ammonium additions. On a different time scale, microbial biomass also played a noteworthy role in the retention of N from organic N, tannin-complexed organic N, and Lobaria. Complexing organic matter with tannin appeared to slow N cycling, but it did not significantly change the ultimate distribution of added organic N. Season of N addition had little effect on the retention of added N; however, where differences did occur, spring additions had lower recoveries than autumn additions.

Nitrogen Use Efficiency of 15N‐labeled Poultry Manure

The timing between poultry manure N mineralization and crop N uptake as influenced by manure application time (winter or spring) and bedding material [barley (Hordeum vulgare L.) straw, wood chips, or no bedding] was investigated with 15N‐labeled poultry manure. Reference plots received either a high or low dose of 15N‐labeled mineral fertilizer in spring or were without manure and fertilizer N. The 15N recovery was measured for 1 yr in spring barley and for the two following years in perennial ryegrass (Lolium perenne L.). Barley grain yield and N uptake were significantly higher when poultry manure had been applied in spring compared with application in winter. The barley 15N recovery was 15% for winter‐applied manure and 38% for manure applied in spring. The recovery of manure 15N from spring‐applied poultry manure corresponded to 82% of what was taken up by barley in the fertilizer‐amended soil, where 15N recovery averaged 46%. Bedding in poultry manure had no effect on crop 15N uptake. Ryegrass recoveries of 15N from manure and fertilizer were 4 to 6% in the second year and 1 to 2% in the third year. After 3 yr, 94% of 15N of spring‐applied poultry manure and 88% of mineral fertilizer were recovered in crops plus soil. Total recovery of 15N from winter‐applied poultry manure averaged 56%. The bedding had no influence on total 15N recovery. It is concluded that although poultry manure contains bedding with high C:N ratios which potentially could immobilize N for a period, optimum time for manure application is in the spring.

Nitrogen Use Efficiency of N-labeled Poultry Manure

The timing between poultry manure N mineralization and crop N uptake as influenced by manure application time (winter or spring) and bedding material [barley (Hordeum vulgare L.) straw, wood chips, or no bedding] was investigated with 15N-labeled poultry manure. Reference plots received either a high or low dose of 15N-labeled mineral fertilizer in spring or were without manure and fertilizer N. The 15N recovery was measured for 1 yr in spring barley and for the two following years in perennial ryegrass (Lolium perenne L.). Barley grain yield and N uptake were significantly higher when poultry manure had been applied in spring compared with application in winter. The barley 15N recovery was 15% for winter-applied manure and 38% for manure applied in spring. The recovery of manure 15N from spring-applied poultry manure corresponded to 82% of what was taken up by barley in the fertilizer-amended soil, where 15N recovery averaged 46%. Bedding in poultry manure had no effect on crop 15N uptake. Ryegrass recoveries of 15N from manure and fertilizer were 4 to 6% in the second year and 1 to 2% in the third year. After 3 yr, 94% of 15N of spring-applied poultry manure and 88% of mineral fertilizer were recovered in crops plus soil. Total recovery of 15N from winter-applied poultry manure averaged 56%. The bedding had no influence on total 15N recovery. It is concluded that although poultry manure contains bedding with high C:N ratios which potentially could immobilize N for a period, optimum time for manure application is in the spring.

N uptake, soil retention and loss of soil-applied 15NH415NO3 in young Fuji/M.26 apple trees with different N status

SummaryNitrogen (N) uptake, soil retention and loss of soil-applied N were studied in young apple trees with different N backgrounds. Bench-graft potted Fuji/M.26 (Malus domestica Borkh) trees were fertigated with 5, 10 or 20 mM N twice a week from June to August, and the trees were removed from soil and bare-root stored in a 2°C cold room in December of the first season. In April of the second season, the trees were washed and replanted in containers with a N-free medium (perlite:vermiculite=l:l v), and received 500 ml Hoagland’s nutrient solution without N weekly through the experiment. The trees received 15NH415NO3 at 1g per plant or no N on June 21. Four trees from each treatment were harvested at one, two and four weeks after 15N application, and 15N and total N in plant tissues and soil were analysed. N fertigation rates during the first growing season increased tree growth and N reserve levels, and N content in trees in the second year. New shoot and leaf growth in the following season was positively related to reserve N. 15N uptake increased during the four weeks after 15N application while soil 15N retention decreased. There was no significant difference in the total 15N uptake per tree. However, trees with the lowest N contents at the end of the first growing season had the highest rate of 15N uptake per unit root dry weight. Four weeks after application of 15N, tree uptake of N accounted for about 60% of applied 15N, while about 20% of the N still remained in the soil, and another 20% of the N was lost. Our results suggested that trees with lower N status are more efficient in N uptake from soil.

Resource remobilization in Quercus ilex L. resprouts

We studied the source of the nitrogen used for the growth and resprouting of holm-oak (Quercus ilexL.), and the contribution of nitrogen and carbohydrate root reserves to these processes. Three-year-old plants were grown in a greenhouse with either a sufficient or restricted nitrogen supply for one year. Half the individuals were subjected to shoot excision to provoke resprouting, and a 15N solution was given to these plants and to controls for two months. Nitrogen, Total Non-structural Carbohydrate (TNC), Total Soluble Protein content, and 15N and 13C composition were determined, and histological analyses of woody tissue were performed. Our results show that N-deprived plants used nitrogen from root reserves to support a growth rate similar to that of non-deprived plants. However, deprived plants lost their resprouting capacity in spite of the high TNC accumulation and nitrogen resupply to the soil. After the supply of nitrogen was restored to N-deprived plants, this nutrient mainly accumulated in under-ground organs, which limited the above-ground growth. Resprouting plants first remobilized the nitrogen stored in roots, and thereafter took it up from the solution. The root-crown region did not behave as a specialised reserve organ in three-year-old Quercus ilex L. plants.

Exploring symbiotic nitrogen fixation and assimilation in pea root nodules by in vivo 15N nuclear magnetic resonance spectroscopy and liquid chromatography-mass spectrometry.

Nitrogen (N) fixation and assimilation in pea (Pisum sativum) root nodules were studied by in vivo (15)N nuclear magnetic resonance (NMR) by exposing detached nodules to (15)N(2) via a perfusion medium, while recording a time course of spectra. In vivo (31)P NMR spectroscopy was used to monitor the physiological state of the metabolically active nodules. The nodules were extracted after the NMR studies and analyzed for total soluble amino acid pools and (15)N labeling of individual amino acids by liquid chromatography-mass spectrometry. A substantial pool of free ammonium was observed by (15)N NMR to be present in metabolically active, intact nodules. The ammonium ions were located in an intracellular environment that caused a remarkable change in the in vivo (15)N chemical shift. Alkalinity of the ammonium-containing compartment may explain the unusual chemical shift; thus, the observations could indicate that ammonium is located in the bacteroids. The observed (15)N-labeled amino acids, glutamine/glutamate and asparagine (Asn), apparently reside in a different compartment, presumably the plant cytoplasm, because no changes in the expected in vivo (15)N chemical shifts were observed. Extensive (15)N labeling of Asn was observed by liquid chromatography-mass spectrometry, which is consistent with the generally accepted role of Asn as the end product of primary N assimilation in pea nodules. However, the Asn (15)N amino signal was absent in in vivo (15)N NMR spectra, which could be because of an unfavorable nuclear Overhauser effect. gamma-Aminobutyric acid accumulated in the nodules during incubation, but newly synthesized (15)N gamma-aminobutyric acid seemed to be immobilized in metabolically active pea nodules, which made it NMR invisible.

Nitrogen absorption, translocation and distribution from urea applied in autumn to leaves of young potted apple (Malus domestica) trees.

We studied the absorption, assimilation, translocation and distribution of nitrogen (N) from urea applied in autumn to leaves of 1-year-old potted Fuji/M26 apple (Malus domestica Borkh) trees. In early October, all leaves of each tree were painted with either 3% urea (enriched to 10 atom % with 15N) or water (control trees). Four trees were harvested before the treatment and N and amino acid contents were determined. Four trees from each treatment were harvested at 2, 4, 7, 10, 15 and 20 days after urea or water application. Total N, amino acids and 15N in leaves, bark, xylem, shank and roots were analyzed to determine uptake and mobilization of N from urea. Most uptake of 15N by leaves occurred during the first 2 days following application of urea. The mean rate of absorption during these 2 days was 0.29 g m-2 day-1. Amino acids in leaves, bark and roots increased significantly after urea application compared with control values. The highest concentrations of amino acids in leaves and bark occurred 4 days after application, whereas the highest concentrations of amino acids in roots occurred 10 days after application. Total 15N content in leaves peaked 2 days after urea application and then decreased, whereas 15N content in roots and bark increased throughout the experiment. Total 15N content in xylem and shank was low. Leaves absorbed 35% of the 15N applied as urea, and 63.6% of absorbed 15N was translocated out of leaves within 20 days after urea application. We conclude that N from urea was converted to amino acids in leaves after foliar application in autumn, and roots and bark were the main sinks of N from urea applied to leaves.

Uptake and assimilation of 15 N-ammonium by the symbiotic sea anemones Bartholomea annulata and Aiptasia pallida : conservation versus recycling of nitrogen

The uptake of ammonium and subsequent movement of nitrogen between symbiotic dinoflagellates (zooxanthellae) and host tissue in the sea anemones Bartholomeaannulata (Le Sueur) and Aiptasiapallida (Verrill) were studied in time course experiments using \({}^{15}{\rm NH}_4^ + \) in 1994. Although there was net uptake of ammonium during exposure to the tracer, there was significant, concurrent efflux, and the anemones continued to lose ammonium over 11 days in low-nutrient seawater. There was no clear evidence of translocation of nitrogen from the zooxanthellae to the host in either species. Host tissue in both species was capable of ammonium assimilation based on simultaneous appearance of 15N in host and zooxanthellae, as well as in aposymbiotic A. pallida and symbiont-free body column tissue of B. annulata. The amount of tracer in B. annulata tentacles from a single anemone was significant, with 66% of total 15N atoms present in the host at all times during exposure to \({}^{15}{\rm NH}_4^ + \) and during the subsequent 11-day chase period. The degree of labeling in the tentacles of B. annulata and symbiotic A. pallida was much greater than host tissue elsewhere in B. annulata or in aposymbiotic A. pallida, demonstrating a strong influence of the symbionts on host metabolism. Our data are consistent with the conservation paradigm that photosynthate from the zooxanthellae reduces the host catabolism of nitrogenous compounds for respiration so that there is little ammonium to be recycled. Although there was little support for nitrogen recycling in our studies, there is likely a continuum between the recycling and conservation paradigms that depends on the availability of photosynthate and hence the coupling between carbon and nitrogen cycles of the partners.

Implications of heterogeneity on procedures for estimating plant 15 N recovery in hedgerow intercrop systems

Nitrogen flows in agroforestry systems can be quantified by applying excess 15N to one pool or part of the system and subsequentlymeasuring the quantity of 15N in other pools. Accurate quantifications depend on accurate determination of the mass, percentage N, and percentage 15N enrichment of each pool and past studies have mainly used physically isolated subplots to reduce variability. We thus assessed the within-plot and within-plant variability of 15N recovery by maize and by two hedgerow tree species, Gliricidia sepium and Peltophorum dasyrrachis, following applications of 15N-enriched materials to unbounded plots. We also assessed the potential for estimating total plant 15N recovery from a reduced set of samples, and for estimating N content allometrically from simple measurements. 15N uptake beyond 1 m radius was negligible in maize but recommended minimum inter-plot spacing for Gliricidia sepium and Peltophorum dasyrrachis was 8 m. Within-plant variation was also greater in trees than in maize. Calculating recovery from all crop plant components is not always necessary; the recovery of 15N in maize grain predicted 98.9% of variation in total plant 15N recovery. 89–95% accurate estimates of biomass and N content of maize plants and of regrowing tree shoots can be obtained non-destructively from height or basal shoot diameter respectively. 15N tracing techniques are potentially very useful for studies of competition and complementarity with respect to nitrogen uptake in agroforestry systems but they require unbounded plots and hence particular care in design and sampling procedures.