15N Application Research Articles

Impact of Spraying with Some Micronutrients on Onion (Allium Cepa L.) Yield and Nutrients Uptake with Application of 15N

Impact of Spraying with Some Micronutrients on Onion (Allium Cepa L.) Yield and Nutrients Uptake with Application of 15N

Root Development and Subsoil 15N-labelled N Uptake in Soybean (Glycine max (L.) Merr.)

The aim of this study was to investigate fertiliser-derived N uptake of soybean from different depths of the soil under field conditions. In addition, soybean root growth in sandy and loess soil was evaluated to understand the impact of site and soybean variety characteristics on soybean N uptake under continental conditions in Central Europe. Root analysis to determine rooting depth and root length density (RLD) was carried out using the profile wall method at three growth stages and two soybean cultivars (Glycine max (L.) Merr. cvs. Merlin and Sultana) in three consecutive years at two locations in eastern Germany. Fertiliser-derived N uptake of soybean from the soil surface and the subsoil was determined at 0.3 and 0.6 m depths using 15N-labelled nitrate N. Root studies showed that soybean roots grew up to 1.4 m on sandy and loess soil sites. Root length densities of up to 2.4 cm cm−3 were documented in the topsoil. By means of 15N application, soybean was shown to take up 15% of the surface-applied nitrogen in the dry growing season and 67 % in high rainfall years, between 19 and 77 % of the nitrogen placed at 0.3 m soil depth, and between 2 and 64 % of the nitrogen placed at 0.6 m soil depth by flowering. The field trials showed that soybeans can absorb a high proportion of the nitrogen placed in the subsoil by flowering time. Due to a well-developed root system reaching deep into the soil, soybeans are able to cover their N demand from soil-borne sources and secure yield formation during dry periods by water uptake from the subsoil.

Fate of fertiliser nitrogen in a ryegrass–kikuyu dairy pasture system

Context Dairy pasture production is reliant on fertiliser to supply nitrogen (N); however, fertiliser N-use efficiency (FNUE) is low and N can be lost to the environment. Aims The aim of this study was to track the fate of N fertiliser applied in a pasture system of ryegrass (Lolium multiflorum, temperate grass) oversown into kikuyu (Pennisetum clandestinum, tropical grass). Methods We used 15N-labelled urea to track the residual plant uptake of a one-off application of 15N over three pasture cuts subsequent to the first cut in the kikuyu growing season from February 2018 to April 2018 (Experiment 1), followed by total soil and plant recoveries of 15N over a 12-month period (Experiment 2). Total N treatment rates were 0, 120, 240 and 480 kg N ha−1 year−1, consisting of application events of 40 kg N ha−1. In Experiment 1, 15N was applied only at the first fertilisation, whereas in Experiment 2, 15N-labelled urea was applied at each fertilisation event. Key results In Experiment 1, uptake of residual 15N fertiliser in the pasture biomass was <6%. In Experiment 2, FNUE was 29–32% and unaccounted 15N fertiliser was 22–142 kg N ha−1, across the 120, 240 and 480 kg N ha−1 year−1 treatments. Conclusions Recovery of 15N residual fertiliser did not increase with N rate and was attributed to the mass increase in soil 15N recovery. FNUE in the pasture did not decrease with N rate. Unaccounted 15N increased with N rate. Implications Existing and alternative N and pasture management strategies such as clover and multi-species pasture need to be implemented and explored to reduce the amount of unaccounted N in dairy pasture production.

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

The stable isotope 15N is an extremely useful tool for studying the nitrogen (N) cycle of terrestrial ecosystems. The affordability of isotope-ratio mass spectrometry has increased in the last decades and routine measurements of δ15N with an accuracy better than 1‰ are now easily achieved. Except perhaps for wood, which has a very high C/N ratio, isotope analysis of samples is, thus, no longer the main challenge in measuring the partitioning of 15N used as tracer in ecosystem studies. The central aim of such experiments is to quantitatively determine the fate of N after it enters an ecosystem, mainly as fertilizer, as atmospheric deposition or as plant litter. By measuring how much of this incoming N goes into different ecosystem pools, inferences can be made about the entire N cycle. Sample collection and preparation can be tedious work. Optimizing sampling schemes is thus an important aspect in the application of 15N in ecosystem research and can be helpful for obtaining a high precision of the results with the available manpower and budget. In this contribution, we combine statistical and practical considerations and give recommendations for the design of labeling experiments and also for assessments of natural 15N abundance. In particular, we discuss soil, vegetation and water sampling. We additionally address the most common questions arising during the calculation of tracer partitioning, and we provide some examples of the interpretation of experimental results.

Recovery of 15N Labeled Nitrogen Fertilizer by Fertigated and Drip Irrigated Greenhouse Vegetable Crops

The stable isotope 15N was used to assess the recovery of mineral N fertilizer applied to fertigated and drip-irrigated spring muskmelon and autumn-winter sweet pepper crops grown in greenhouse soil plots. They received 92–96% of mineral N fertilizer as NO3−. 15N-labeled Ca (NO3)2 fertilizer was applied to crops during vegetative growth and fruit production phases. Crops were grown with either conventional management or combined improved N and irrigation management. Improved management for both irrigation and N was based on the combined use of models, to estimate crop requirements, and of monitoring of soil parameters. In sweet pepper, from conventional management, 15N recoveries from the 15N applications made during vegetative growth and fruit production were 66% and 58%, respectively. With improved management in sweet pepper, the corresponding 15N recoveries were 82% and 77%. In muskmelon, 15N recoveries from conventional management from the 15N applications made during vegetative growth and fruit production were 71% and 42%, respectively. With improved management, the corresponding 15N recoveries were 68% and 44%, respectively. The results demonstrated that combined drip irrigation and fertigation systems with frequent irrigation and N fertilizer application can have very high recovery of applied N fertilizer, of 77–82%.

Nitrogen absorption by field-grown tea plants (Camellia sinensis) in winter dormancy and utilization in spring shoots

The information of nitrogen uptake by subtropical, ever-green broad-leaf plants at cold temperatures of winter is very limited. The present field experiment was conducted to investigate whether 15N is taken up by tea (Camellia sinensis L.) plants in winter dormancy in the absence of active shoot growth and utilization in young spring shoots. We applied 15N-labeled urea to soil at five different times i.e. mid-January, early February, mid-February, and early and mid-March. 15N abundance was determined in fibrous roots, twigs and mature leaves after 3, 7 and 15 days after application and in young shoots the following spring. 15N was taken up by fibrous roots and transported to above-ground tissues within 3 days after application under low winter temperatures. Earlier application significantly increased nitrogen derived from 15N-urea (Ndff) and 15N amount in young spring shoots. Ndff values and 15N amount in young spring shoots were described well by quadratic or linear regressions against soil growing degree days (GDD, T ≥ 8 °C, depth 20 cm) between 15N application and harvesting dates (R2 = 0.58–0.90, p < 0.001). Nitrogen was absorbed and translocated in dormant tea plants in the absence of active root and shoot growth throughout the late winter until early spring. Absorbed N was stored and remobilized to support shoot growth the following spring. Soil GDD between N application and harvesting could predict Ndff and 15N amount in young spring shoots.

Post-fire nitrogen uptake and allocation by two resprouting herbaceous species with contrasting belowground traits

Immediately after fire, soil nitrogen availability increases due to ash deposition and the stimulation of mineralization by increased temperature. The coexistence of plant resprouters with contrasting root traits that grow soon after fire suggests different strategies operating for the re-acquisition of space, capturing resources and resisting additional stresses. Brachypodium retusum and Dactylis glomerata are two perennial grass species that co-occur in fire-prone Mediterranean basin ecosystems. B. retusum (Br) is a slow-growing grass with a developed rhizome, whereas D. glomerata (Dg) is a fast-growing grass that resprouts from rootstock buds. We hypothesized that the fast-growing grass would take advantage of the flush in available N after fire, whereas the aboveground growth of the slow-growing grass would rely on rhizome N reserves. The objective of this study was to ascertain the role of the root system characteristics and plant growth traits in relation to soil N uptake and plant N assimilation soon after fire followed by grazing. Therefore, in a pot experiment with plants and soil transplanted from the field, we applied 15N-NH4+ to the surface of the soil, immediately after cutting the aboveground plant biomass to simulate post-fire disturbance. Grazing defoliation was simulated using four successive cuttings. Both species resprouted quickly after the fire-simulation cutting, with the recovery of approximately 15% of the initial aboveground biomass only 5 days afterwards; 20 days later, the biomass reached 50% for Dg and 28% for Br. This regrowth was parallel to a reduction in belowground biomass. Later, the aboveground biomass remained approximately constant after four cuts over 75 days, maintaining the same total N content (close to 2.0 g N m−2 for Dg and 1.0–1.5 g N m−2 for Br). Both species showed high 15N atom % in their leaves after 20 days of 15N application, which slowly decreased over the successive cuts. After 55 days and two cuts, only Br showed a decline in the total plant 15N content. Plant growth and soil N uptake were higher in the non-rhizomatous species (Dg) showing greater root development and higher belowground/aboveground ratio. The rhizome of Br supported early resprouting but this species did not take much advantage of the initial N mineralization after fire. In Dg pots, the 15N-NO3- that leached out was lower than in Br pots. Dg showed higher nitrate reductase activity, in comparison to Br. In agreement with our hypothesis, the results suggest that the fast-growing strategy of Dg takes advantage of increased N availability after fire, whereas the slow growing Br relies on N rhizome reserves, which were slowly replenishing from available soil N. Both species showed high regeneration capacity after fire and high resilience to intensive post-fire grazing pressure.

Nitrogen Flows in Agrocenosis of Winter Wheat Under Conditions of Different Reactions of Typical Chernozem Medium (Based on Application of 15N)

It is shown that an increase in winter wheat on typical chernozem characterized by the acid reaction of the soil medium leads to a decrease in grain yield to 30% and straw weight to 25%. The crop reached the maximum level after introducing urea and manure wastes (the yield gain was 199% in neutral soil and 165% in acidic soil). The improvement of plant nitrogen nutrition by introducing fertilizers led to an increase in the protein content in grain by 1.2 times on both soils; at the same time, the protein content was 1.1 times lower in acidic soil than in the neutral one. The maximum content of protein in winter wheat grain accumulated after both types of fertilizers were applied on neutral soil; the use of manure wastes alone decreased the protein content in grain. It has been revealed that a decrease in the yield of grain and a decrease in the content of crude protein in this grain after acidification of typical chernozem is due to a decrease in the amount of mineral nitrogen in soil and an increase in its gaseous losses. The pattern of nitrogen transformation in soil, as well as the availability of nitrogen for the plants, varied as the reaction of the medium changed. In acidic soil, the consumption of mineral fertilizer nitrogen and manure wastes by the plants decreased by 12–13% and the consumption of soil nitrogen decreased by 45%. The effect of wastes decreased the consumption of urea nitrogen by 49% (neutral soil) and by 69% (acidic soil). The consumption of fertilizer nitrogen by the plants decreased by 12–15% after acidifying typical chernozem and by 12–17% after immobilizing this type of chernozem and increased gaseous losses by 18–81%. It has been concluded that the agrophytocenosis of winter wheat maintains the ecological balance (homeostasis) when wastes from a pig-breeding complex are used and is exposed to stress when urea is applied in acid chernozem. Soil acidification decreased the resistance of agrophytocenosis to the level that is observed when manure wastes were applied and to adaptive depletion when urea was used.

Exogenous short-term silicon application regulates macro-nutrients, endogenous phytohormones, and protein expression in Oryza sativa L.

BackgroundSilicon (Si) has been known to regulate plant growth; however, the underlying mechanisms of short-term exogenous Si application on the regulation of calcium (Ca) and nitrogen (N), endogenous phytohormones, and expression of essential proteins have been little understood.ResultsExogenous Si application significantly increased Si content as compared to the control. Among Si treatments, 1.0 mM Si application showed increased phosphorus content as compared to other Si treatments (0.5, 2.0, and 4.0 mM). However, Ca accumulation was significantly reduced (1.8- to 2.0-fold) at the third-leaf stage in the control, whereas all Si treatments exhibited a dose-dependent increase in Ca as determined by radioisotope 45Ca analysis. Similarly, the radioisotope 15N for nitrogen localization and uptake showed a varying but reduced response (ranging from 1.03–10.8%) to different Si concentrations as compared to 15N application alone. Physiologically active endogenous gibberellin (GA1) was also significantly higher with exogenous Si (1.0 mM) as compared to GA20 and the control plants. A similar response was noted for endogenous jasmonic and salicylic acid synthesis in rice plants with Si application. Proteomic analysis revealed the activation of several essential proteins, such as Fe-S precursor protein, putative thioredoxin, Ser/Thr phosphatase, glucose-6-phosphate isomerase (G6P), and importin alpha-1b (Imp3), with Si application. Among the most-expressed proteins, confirmatory gene expression analysis for G6P and Imp3 showed a similar response to those of the Si treatments.ConclusionsIn conclusion, the current results suggest that short-term exogenous Si can significantly regulate rice plant physiology by influencing Ca, N, endogenous phytohormones, and proteins, and that 1.0 mM Si application is more beneficial to plants than higher concentrations.

Applicability of a "Multi-Stage Pulse Labeling" 15N Approach to Phenotype N Dynamics in Maize Plant Components during the Growing Season.

Highlights This work utilizes “multi-stage pulse labeling” 15N applications, primarily during reproductive growth stages, as a phenotyping strategy to identify maize hybrids with superior N use efficiency (NUE) under low N conditions.Research using labeled isotopic N (15N) can precisely quantify fertilizer nitrogen (N) uptake and organ-specific N allocation in field crops such as maize (Zea mays L.). The overall research objective was to study plant N uptake patterns potentially correlated with N use efficiency (NUE) in field-grown maize hybrids using a “multi-stage pulse labeling” 15N phenotyping strategy with an emphasis on the reproductive period. Five hybrids varying in NUE were compared under zero N fertilizer application (0N) plus a moderate rate of 112 kg N ha−1 (112N) in 2013 (2 locations) and 2014 growing seasons. The equivalent of 3.2 (2013) to 2.1 (2014) kg of 15N ha−1, as labeled Ca(15NO3)2, was injected into soil on both sides of consecutive plants at multiple stages between V14 and R5. Aboveground plant biomass was primarily collected in short-term intervals (4–6 days after each 15N application) in both years, and following a single long-term interval (at R6 after 15N injection at R1) in 2014. Averaged across hybrids and site-years, the moderate N rate (112N) increased absolute 15N uptake at all stages; however, plants in the 0N treatment allocated proportionally more 15N to reproductive organs. Before flowering, short-term recovery of 15N (15Nrec) totaled ~0.30 or 0.40 kg kg−1 of the 15N applied, and ~50% of that accumulated 15Nu was found in leaves and 40% in stems. After flowering, plant 15Nrec totaled ~0.30 kg kg−1 of 15N applied, and an average 30% of accumulated 15Nu was present in leaves, 17% in stems, and the remainder—usually the majority—in ears. At the R5 stage, despite a declining overall rate of 15N uptake per GDD thermal unit, plant 15Nrec represented ~0.25 kg kg−1 of 15N applied, of which ~65% was allocated to kernels. Overall long-term 15Nrec during grain filling was ~0.45 and 0.70 kg kg−1 of total 15N applied at R1 with 0 and 112N, respectively, and most (~77%) 15N uptake was found in kernels. The “multi-stage pulse labeling” technique proved to be a robust phenotyping strategy to differentiate reproductive-stage N uptake/allocation patterns to plant organs and maize efficiencies with newly available fertilizer N.

Transformation of DON in reclaimed water under solar light irradiation leads to decreased haloacetamide formation potential during chloramination

Reclaimed water is usually stored in rivers or lakes before subsequent use. In storage ecosystems, the natural process of solar light irradiation plays a key role in water quality, altering disinfection byproduct formation potential in later use. This study investigated changes in haloacetamide formation potential (HAcAm FP) during subsequent chloramination when reclaimed water was exposed to solar light irradiation. Significant decreases in HAcAm FP were observed for the solar light irradiated reclaimed water, with reductions of 27%–69% for different haloacetamides. Moreover, transformation of dissolved organic nitrogen (DON) to inorganic nitrogen occurred during irradiation. The application of 15N- labeled monochloramine indicated that the nitrogen source of the decreased HAcAms mainly originated from DON, rather than chloramine. Chloramination of the model compound l-asparagine after irradiation demonstrated that the decreased HAcAms could be attributed to the decrease in DON. After solar light irradiation, the brominated HAcAm FP in the presence of bromide was also reduced, while the bromine incorporation factor remained steady. Overall, this study revealed the contribution of natural processes in controlling HAcAm FP during subsequent chloramination, suggesting solar light irradiation is important to water purification during reclaimed water storage.

Spatial patterns nitrogen transfer models of ectomycorrhizal networks in a Mongolian scotch pine plantation

Ectomycorrhizal (EM) networks provide a variety of services to plants and ecosystems include nutrient uptake and transfer, seedling survival, internal cycling of nutrients, plant competition, and so on. To deeply their structure and function in ecosystems, we investigated the spatial patterns and nitrogen (N) transfer of EM networks using 15N labelling technique in a Mongolian scotch pine (Pinus sylvestris var. mongolica Litv.) plantation in Northeastern China. In August 2011, four plots (20 × 20 m) were set up in the plantation. 125 ml 5 at.% 0.15 mol/L 15NH 4 15 NO3 solution was injected into soil at the center of each plot. Before and 2, 6, 30 and 215 days after the 15N application, needles (current year) of each pine were sampled along four 12 m sampling lines. Needle total N and 15N concentrations were analyzed. We observed needle N and 15N concentrations increased significantly over time after 15N application, up to 31 and 0.42%, respectively. There was no correlation between needle N concentration and 15N/14N ratio (R2 = 0.40, n = 5, P = 0.156), while excess needle N concentration and excess needle 15N/14N ratio were positively correlated across different time intervals (R2 = 0.89, n = 4, P < 0.05), but deceased with time interval lengthening. Needle 15N/14N ratio increased with time, but it was not correlated with distance. Needle 15N/14N ratio was negative with distance before and 6th day and 30th day, positive with distance at 2nd day, but the trend was considerably weaker, their slop were close to zero. These results demonstrated that EM networks were ubiquitous and uniformly distributed in the Mongolian scotch pine plantation and a random network. We found N transfer efficiency was very high, absorbed N by EM network was transferred as wide as possible, we observed N uptake of plant had strong bias for 14N and 15N, namely N fractionation. Understanding the structure and function of EM networks in ecosystems may lead to a deeper understanding of ecological stability and evolution, and thus provide new theoretical approaches to improve conservation practices for the management of the Earth’s ecosystems.

Low nitrous oxide production through nitrifier-denitrification in intermittent-feed high-rate nitritation reactors

Nitrous oxide (N2O) production from autotrophic nitrogen conversion processes, especially nitritation systems, can be significant, requires understanding and calls for mitigation. In this study, the rates and pathways of N2O production were quantified in two lab-scale sequencing batch reactors operated with intermittent feeding and demonstrating long-term and high-rate nitritation. The resulting reactor biomass was highly enriched in ammonia-oxidizing bacteria, and converted ∼93 ± 14% of the oxidized ammonium to nitrite. The low DO set-point combined with intermittent feeding was sufficient to maintain high nitritation efficiency and high nitritation rates at 20–26 °C over a period of ∼300 days. Even at the high nitritation efficiencies, net N2O production was low (∼2% of the oxidized ammonium). Net N2O production rates transiently increased with a rise in pH after each feeding, suggesting a potential effect of pH on N2O production. In situ application of 15N labeled substrates revealed nitrifier denitrification as the dominant pathway of N2O production. Our study highlights operational conditions that minimize N2O emission from two-stage autotrophic nitrogen removal systems.

Distinct fates of atmogenic NH4 + and NO3 − in subtropical, N-saturated forest soils

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH4 +) and nitrate (NO3 −). Previous long-term studies indicate efficient turnover of atmogenic NH4 + to NO3 − in weathered, acidic soils of the subtropics, leading to excessive NO3 − leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ 15N tracing experiment in the TieShanPing (TSP) forested catchment, SW China. 15NH4NO3, NH 4 15 NO3 and 15N-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N2O) were monitored for nine days. Incorporation of 15NO3 − into soil organic N was negligible and 80% of the applied label was lost from the top soil (0–15 cm) primarily by leaching within 9 days. In contrast, 15NH4 + was largely retained in soil organic N. However, instant production of 15NO3 − in the 15NH4 + treatment suggested active nitrification. In both the 15NH4 + and 15N-glutamic acid treatments, the 15N enrichment in the NO3 − pool exceeded that in the NH4 + pool one day after 15N application, suggesting preferential nitrification of added 15NH4 + with subsequent dilution of the NH4 + pool and/or immobilization of 15NH4 + followed by heterotrophic nitrification. The cumulative recovery of 15N in N2O after 9 days ranged from 2.5 to 6.0% in the 15NO3 − treatment, confirming the previously reported significant response of N2O emission to N deposition. Source partitioning of 15N2O demonstrated a measurable contribution of nitrification to N2O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH4 + in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO3 − and NH4 +) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO3 − in the soil and (ii) rapid immobilization-mineralization of NH4 + followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N2O emission.

15N allocation into wheat grains (Triticum aestivum L.) influenced by sowing rate and water supply at flowering under a Mediterranean climate

ABSTRACTThis study examined the effects of a reduced wheat sowing rate (250 vs. 500 grains m–2) on grain yield, uptake of 15N into grains, and the incorporation into gluten and non-gluten proteins of wheat under field conditions in the Aegean region. A single 15N application was applied at stem elongation, at flowering, or at both developmental stages. Each 15N treatment included either additional water supply, or no additional water supply at flowering. Sowing rate (either 250 or 500 grains m–2) had no impact on grain yield. Grain yield increased with additional water supply, but at the expense of protein quality, because of a decrease in the protein content of gluten. The 15N content of the gluten and non-gluten proteins at grain maturity was not different among cultivars. 15N applied at both stem elongation and flowering was found in comparable amounts in grains and protein fractions, irrespective of sowing rate.

Sustaining crop productivity while reducing environmental nitrogen losses in the subtropical wheat-maize cropping systems: A comprehensive case study of nitrogen cycling and balance

Balancing the nitrogen (N) budgets of agricultural systems is essential for sustaining yields at lower environmental costs. However, it is still rare to find reports on the total N budgets of agricultural systems including all N fluxes in the literature. Here, we conducted a comprehensive study on the effects of different N fertilizers (control, synthetic fertilizer, 60% synthetic fertilizer N plus 40% pig manure N, pig manure N applied at the same rate of 280kgNha−1 yr−1) on N pools, cycling processes, fluxes and total N balances in a subtropical wheat-maize rotation system in China by monitoring in situ N fluxes combined with field 15N-tracer and 15N isotope-dilution techniques. The warm and wet maize season was associated with significantly larger N losses via gaseous and hydrological pathways than the cooler and drier wheat season. Nitrate leaching and NH3 volatilization were the main N loss pathways, accounting for 78% and 93% of the annual hydrological and gaseous N losses, respectively. The field 15N tracing experiment showed that the wheat system had a high N retention capacity (∼50% of 15N application), although the N residence time was short. In the subsequent maize season, 90% of the residual 15N-labeled fertilizer in the soil that had been applied to the wheat system was utilized by plants or lost to the environment. The combined application of synthetic and organic fertilizers (pig manure) or application of pig manure resulted in significantly higher soil N retention and lower NO3− leaching, while yields remained unaffected. However, the application of manure resulted in larger NH3 volatilization losses compared with the application of synthetic fertilizer alone. Thus, our study suggests that a combination of synthetic and organic N fertilizers is suitable for sustaining agricultural productivity while reducing environmental N losses by fostering interactions between the soil C and N cycles.

Uptake of nitrogen from natural food into fish in differently managed polyculture ponds using 15N as tracer

Food security in Asia depends significantly on small-scale pond aquaculture in which fish feed on external feeds and natural food resources. Little is known about the assimilation of natural food resources into fish. In order to elucidate the food web and nitrogen flows in a traditionally and a semi-intensively managed pond in Vietnam, δ13C and δ15N in pond inputs and fish were measured before and after the application of inorganic 15N at 1.1 and 0.4 % of total nitrogen. The applied 15N was traced through the biomass of stocked fish over 30 days. Under traditional management, grass carp was the dominant species in number and biomass. After 30 days, grass carp assimilated 1.2 % of the applied tracer through natural food resources. Filter feeders assimilated only 0.1–0.3 % of the tracer after 30 days because of the limited productivity in the traditionally managed pond. Nile tilapia showed the highest uptake rates over the 30 days, which demonstrated their ability to feed on a variety of natural food resources. In total, 2.6 % of the applied 15N was assimilated into fish biomass after 30 days. Under semi-intensive management, common carp was the main species numerically and assimilated 1.8 % of the tracer after 30 days. Grass carp was stocked at lower density, but had the highest biomass by the time of the experiment. Grass carp assimilated 0.2 % of the applied tracer suggesting a low contribution of natural food resources to its nutrition. Silver carp benefited most from the higher natural food resources. In total, 4.1 % of applied 15N was assimilated into fish biomass after 30 days. Traditional small-scale aquaculture of a grass carp-dominated polyculture has a lower assimilation efficiency of applied fertilizers into fish biomass than a common carp-dominated polyculture under semi-intensive management.

Denitrification losses from an intensively managed sub-tropical pasture – Impact of soil moisture on the partitioning of N2 and N2O emissions

Intensively managed pastures in subtropical Australia under dairy production are nitrogen (N) loaded agro-ecosystems, with an increased pool of N available for denitrification. The magnitude of denitrification losses and N2:N2O partitioning in these agro-ecosystems is largely unknown, representing a major uncertainty when estimating total N loss and replacement. This study investigated the influence of different soil moisture contents on N2 and N2O emissions from a subtropical dairy pasture in Queensland, Australia. Intact soil cores were incubated over 15 days at 80% and 100% water-filled pore space (WFPS), after the application of 15N labelled nitrate, equivalent to 50 kg N ha−1. This setup enabled the direct quantification of N2 and N2O emissions following fertilisation using the 15N gas flux method. The main product of denitrification in both treatments was N2. N2 emissions exceeded N2O emissions by a factor of 8 ± 1 at 80% WFPS and a factor of 17 ± 2 at 100% WFPS. The total amount of N-N2 lost over the incubation period was 21.27 kg ± 2.10 N2-N ha−1 at 80% WFPS and 25.26 kg ± 2.79 kg ha−1 at 100% WFPS respectively. N2 emissions remained high at 100% WFPS, while related N2O emissions decreased. At 80% WFPS, N2 emissions increased constantly over time while N2O fluxes declined. Consequently, N2/(N2+N2O) product ratios increased over the incubation period in both treatments. N2/(N2+N2O) product ratios responded significantly to soil moisture, confirming WFPS as a key driver of denitrification. The substantial amount of fertiliser lost as N2 reveals the agronomic significance of denitrification as a major pathway of N loss for sub-tropical pastures at high WFPS and may explain the low fertiliser N use efficiency observed for these agro-ecosystems.

Extending long-range heteronuclear NMR connectivities by HSQMBC-COSY and HSQMBC-TOCSY experiments

The detection of long-range heteronuclear correlations presenting J(CH) coupling values smaller than 1–2Hz is a challenge in the structural analysis of small molecules and natural products. HSQMBC-COSY and HSQMBC-TOCSY pulse schemes are evaluated as complementary NMR methods to standard HMBC/HSQMBC experiments. Incorporation of an additional J(HH) transfer step in the basic HSQMBC pulse scheme can favor the sensitive observation of traditionally missing or very weak correlations and, in addition, facilitates the detection of a significant number of still longer-range connectivities to both protonated and non-protonated carbons under optimum sensitivity conditions. A comparative 1H–13C study is performed using strychnine as a model compound and several examples are also provided including 1H–15N applications.

Temperature effects on nitrogen form uptake by seedling roots of three contrasting conifers.

Plant species may show a preference for uptake of particular nitrogen (N) forms, but little is known about how N form preference is influenced by soil temperature. Potential future changes in soil N form availability and plant N form preference in warmer soils might shift competitive interactions among forest tree species. We compared the N uptake and growth of three conifer species from contrasting environments grown at rhizosphere temperatures of 10, 16 or 20 °C and supplied with ammonium (NH4 (+)) or nitrate (NO3 (-)) or a mix of arginine and alanine. Short-term N uptake was assessed using ion-selective microelectrodes and application of (15)N, and long-term uptake was assessed by plant N status. Species exhibited preferences for particular N forms, and these preferences related to the N form most available in native soils. Specifically, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) showed a preference for nitrate (a N form commonly found in warmer areas), Sitka spruce (Picea sitchensis (Bong.) Carr.) preferred ammonium (a N form abundant in cold soils) and Engelmann spruce (Picea engelmannii Parry ex Engelm.) showed a preference for ammonium and organic N (organic N is often abundant in cold soils). Relative N form preference, as indicated by plant growth, changed with temperature in some species, indicating that these species could acclimate to changing rhizosphere temperatures. Understanding how conifers utilize available soil nutrients at different temperatures can help to predict species' future performance as soil temperatures rise.