15N Application Research Articles

Using a Stable Isotope to Label Seeds and Seedlings of an Invasive Shrub, Lonicera maackii

AbstractSeed dispersal is a crucial process in most plant invasions, but is notoriously difficult to study. One technique to identify the maternal source of dispersed seeds and newly established seedlings is labeling with a stable isotope. We tested whether foliar application of 15N-labeled urea would result in sufficient 15N enrichment to discriminate among seeds and seedlings grown from those seeds of the invasive shrub Lonicera maackii (Amur honeysuckle). We subjected mature L. maackii to all combinations of three concentrations of 15N-labeled urea (0.025 g L−1 [0.003 oz gal−1], 0.20 g L−1, and a 0 g L−1 control) and three temporal treatments (one application in August, one application in September, and five applications spaced every three weeks from June through August). Seeds were collected September to November; some of these were analyzed for %15N and others allowed to germinate and grow into seedlings under two treatments (in potting mix in greenhouse and in woodlot soil outdoors). Seedlings were harvested midway through the next growing season. We found that seeds from plants subjected to the three different concentrations had significantly different %15N levels, and there was a significant interaction between concentration and temporal treatment: the highest seed %15N levels were from plants sprayed five times with 15N-labeled urea, and the second highest from plants sprayed once in September. Similar patterns in %15N levels were found in seedlings, except that those from the 0.025 g L−1 spray treatment were only distinguishable from controls for seedlings grown outdoors in woodlot soil. These findings demonstrate that a single foliar application of 15N in early September is sufficient to label both seeds and seedlings of this invasive shrub, enabling one to identify the source of field-collected seeds or seedlings. This provides a tool for studying patterns and processes in seed dispersal of Amur honeysuckle and potentially other invasive plants.

An assessment of N-cycling and sources of N2O during a simulated rain event using natural abundance 15N

In order to accurately predict N2O emissions from agricultural soils and to develop effective management strategies, it is important to understand mechanisms underlying N2O emissions under field conditions. This involves identification of sources of N2O, which is currently methodologically challenging, especially under field conditions. We assessed the suitability of 15N tracers and natural abundance 15N to study N cycling and sources of N2O after a rainfall simulation in an annual cropping system in the Central Valley of California. Our natural abundance 15N approach differed from other studies due to a combination of emphasizing a per-event (e.g. rainfall simulation in this study) assessment of N2O emissions, applying high temporal sampling frequency during this event, determination of 15N of NH4+ and NO3− in addition to N2O, and data analysis using isotope models. In our study, the suitability of 15N tracers to assess N cycling and sources of N2O emissions was limited, likely due to a combination of a fine soil texture, the use of undisturbed soil cores, and a low 15N application rate. Based on natural abundance 15N, we were able to calculate gross NH4+ mineralization, NH4+ immobilization, nitrification and NO3− immobilization rates of 5.37±1.72, 2.70±1.72, 3.01±1.13 and 0.15±0.29μgNg−1soild−1, respectively. Natural abundance 15N was, however, a rather poor predictor of the contribution of nitrification versus denitrification to N2O production. Nevertheless, important trends in N2O reduction rates could be observed, showing a sharp increase from 48% to 78% in reduction of produced N2O between 2hours and 24hours after rainfall simulation, followed by a gradual decrease to 46% of reduction by the fifth day after rainfall simulation. We conclude that the natural abundance 15N approach is very promising to elucidate mechanisms driving N-cycling and N2O emissions during agricultural management or weather events, especially if isotope dynamics are incorporated in site-specific biogeochemical process models.

Density fractions versus size separates: does physical fractionation isolate functional soil compartments?

Abstract. Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. In this study we explore whether physical fractionation techniques isolate soil compartments in a meaningful and functionally relevant way for the investigation of litter-derived nitrogen dynamics at the decadal timescale. We do so by performing aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) on mineral soil samples from two European beech forests a decade after application of 15N labelled litter. Both density and size-based fractionation methods suggested that litter-derived nitrogen became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. However, scientists investigating specific aspects of litter-derived nitrogen dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or protective mechanism. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.

Application of 15N–18O double stable isotope tracer technique in an agricultural nonpoint polluted river of the Yangtze Delta Region

One strategy to combat nitrate (NO3-N) contamination in rivers is to understand its sources. NO3-N sources in the East Tiaoxi River of the Yangtze Delta Region were investigated by applying a (15)N-(18)O dual isotope approach. Water samples were collected from the main channel and from the tributaries. Results show that high total N and NO3-N are present in both the main channel and the major tributaries, and NO3-N was one of the most important N forms in water. Analysis of isotopic compositions (δ (18)O, δD) of water suggests that the river water mainly originated from three tributaries during the sampling period. There was a wide range of δ (15)N-NO3 (-1.4 to 12.4 ‰) and a narrow range of δ (18)O-NO3 (3.7 to 9.0 ‰) in the main channel waters. The δ (15)N and δ (18)O-NO3 values in the upper, middle, and lower channels along the river were shifted as 8.2, 3.5, and 9.5 ‰, and 9.0, 4.2, and 6.0 ‰, respectively. In the tributary South Tiao, the δ (15)N and δ (18)O-NO3 values were as high as 9.5 and 7.0 ‰, while in the tributaries Mid Tiao and North Tiao, NO3-N in most of the samples had relatively low δ (15)N and δ (18)O-NO3 values from 2.3 to 7.5 ‰ and 4.7 to 7.0 ‰, separately. Our results also suggest that the dual isotope approach can help us develop the best management practice for relieving NO3-N pollution in the rivers at the tributary scale.

Use of the 15 N gas flux method to measure the source and level of N2O and N2 emissions from grazed grassland

Understanding the contribution of nitrification and denitrification to production of nitrous oxide (N2O), a potent greenhouse gas, is important in devising effective mitigation strategies to reduce emissions. In this study the 15N gas flux method was used to investigate N2O and N2 emissions following an application of 15N labelled ammonium nitrate (0.71 mol N m−2) to intensive grassland swards (grazed at 2.74 or 2.05 livestock units ha−1 year−1) at a site in Southern Ireland. The 15N labelled fertiliser (NO3 moiety 15N labelled at 60 at. %) was applied to designated soil areas in the field, enclosed by static chambers, in June 2009, September 2009 and March 2010. Fluxes of N2O and N2 were determined over 12 days on each occasion. N2O and N2 emissions were significantly (P < 0.001) lower in March 2010 than in June or September 2009. There was little difference between the two swards grazed at different stocking rates on N2O or N2 emissions. Mean cumulative N2O emissions over 103 h were 212.9, 279.5 and 62.06 mg m−2 for June 2009, September 2009 and March 2010, respectively. Mean cumulative N2 emissions for the three time periods were 818.8, 893.8 and 87 mg m−2, respectively. The N2O mole fraction averaged 0.21 and 0.23 in June 2009 and September 2009, respectively, but increased to 0.41 in March 2010 which may have been due to changes in denitrifier community composition or due to N2O reductase being sensitive to low soil temperatures. The results point to denitrification of nitrate as the major source of N2O at this site which may have implications for choice of fertiliser in moist temperate climates.

NanoSIMS Study of Organic Matter Associated with Soil Aggregates: Advantages, Limitations, and Combination with STXM

Direct observations of processes occurring at the mineral-organic interface are increasingly seen as relevant for the cycling of both natural soil organic matter and organic contaminants in soils and sediments. Advanced analytical tools with the capability to visualize and characterize organic matter at the submicrometer scale, such as Nano Secondary Ion Mass Spectrometry (NanoSIMS) and Scanning Transmission X-ray Microscopy (STXM) coupled to Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), may be combined to locate and characterize mineral-associated organic matter. Taking advantage of samples collected from a decadal (15)N litter labeling experiment in a temperate forest, we demonstrate the potential of NanoSIMS to image intact soil particles and to detect spots of isotopic enrichment even at low levels of (15)N application. We show how microsites of isotopic enrichment detected by NanoSIMS can be speciated by STXM-NEXAFS performed on the same particle. Finally, by showing how (15)N enrichment at one microsite could be linked to the presence of microbial metabolites, we emphasize the potential of this combined imaging and spectroscopic approach to link microenvironment with geochemical process and/or location with ecological function.

Inoculation of &lt;i&gt;Bacillus sphaericus&lt;/i&gt; UPMB-10 to Young Oil Palm and Measurement of Its Uptake of Fixed Nitrogen Using the &lt;sup&gt;15&lt;/sup&gt;N Isotope Dilution Technique

There are increasing applications of diazotrophic rhizobacteria in the sustainable agriculture system. A field experiment on young immature oil palm was conducted to quantify the uptake of N derived from N2 fixation by the diazotroph Bacillus sphaericus strain UPMB-10, using the 15N isotope dilution method. Eight months after 15N application, young immature oil palms that received 67% of standard N fertilizer application together with B. sphaericus inoculation had significantly lower 15N enrichment than uninoculated palms that received similar N fertilizers. The dilution of labeled N served as a marker for the occurrence of biological N2 fixation. The proportion of N uptake that was derived from the atmosphere was estimated as 63% on the whole plant basis. The inoculation process increased the N and dry matter yields of the palm leaflets and rachis significantly. Field planting of young, immature oil palm in soil inoculated with B. sphaericus UPMB-10 might mitigate inorganic fertilizer-N application through supplementation by biological nitrogen fixation. This could be a new and important source of nitrogen biofertilizer in the early phase of oil palm cultivation in the field.

Determination of the uptake and translocation of nitrogen applied at different growth stages of a melon crop ( Cucumis melo L.) using 15N isotope

In order to establish a rational nitrogen (N) fertilisation and reduce groundwater contamination, a clearer understanding of the N distribution through the growing season and its dynamics inside the plant is crucial. In two successive years, a melon crop ( Cucumis melo L. cv. Sancho) was grown under field conditions to determine the uptake of N fertiliser, applied by means of fertigation at different stages of plant growth, and to follow the translocation of N in the plant using 15N-labelled N. In 2006, two experiments were carried out. In the first experiment, labelled 15N fertiliser was supplied at the female-bloom stage and in the second, at the end of fruit ripening. Labelled 15N fertiliser was made from 15NH 4 15NO 3 (10 at.% 15N) and 9.6 kg N ha −1 were applied in each experiment over 6 days (1.6 kg N ha −1 d −1). In 2007, the 15N treatment consisted of applying 20.4 kg N ha −1 as 15NH 4 15NO 3 (10 at.% 15N) in the middle of fruit growth, over 6 days (3.4 kg N ha −1 d −1). In addition, 93 and 95 kg N ha −1 were supplied daily by fertigation as ammonium nitrate in 2006 and 2007, respectively. The results obtained in 2006 suggest that the uptake of N derived from labelled fertiliser by the above-ground parts of the plants was not affected by the time of fertiliser application. At the female-flowering and fruit-ripening stages, the N content derived from 15N-labelled fertiliser was close to 0.435 g m −2 (about 45% of the N applied), while in the middle of fruit growth it was 1.45 g m −2 (71% of the N applied). The N application time affected the amount of N derived from labelled fertiliser that was translocated to the fruits. When the N was supplied later, the N translocation was lower, ranging between 54% at female flowering and 32% at the end of fruit ripening. Approximately 85% of the N translocated came from the leaf when the N was applied at female flowering or in the middle of fruit growth. This value decreased to 72% when the 15N application was at the end of fruit ripening. The ammonium nitrate became available to the plant between 2 and 2.5 weeks after its application. Although the leaf N uptake varied during the crop cycle, the N absorption rate in the whole plant was linear, suggesting that the melon crop could be fertilised with constant daily N amounts until 2–3 weeks before the last harvest.

Isotopically-labelled nitrogen uptake and partitioning in sweet cherry as influenced by timing of fertilizer application

Sweet cherry ( Prunus avium L.) is a fruit of increasing economic importance though it is less significant than other stone fruit species such as peach. Cherry has received little attention concerning nitrogen (N) uptake and dynamics in mature trees. The aim of this work was to determine N uptake and partitioning as influenced by the timing of fertilizer application in 7-year-old sweet cherry trees cultivated in a cold region (Los Antiguos, Santa Cruz, Argentina; 71°38′ W, 46°32′ S). Nitrogen (95 kg ha −1) was applied as ammonium nitrate to a soil with ‘Bing’ sweet cherry trees grafted onto Prunus mahaleb rootstocks. Fertilization was split into two equal applications per treatment, involving either the commercial fertilizer ammonium nitrate or the same fertilizer labelled with 15N isotope (10% atom.). Treatments consisted of one early spring (full bloom, October 2005) or one summer (late January 2006, 15 days after harvest) application of 15N ammonium nitrate to three replicate trees. Fruit were harvested in early January and leaves were collected at both full canopy and leaf fall. All trees were excavated in winter (August, 2006). Trees were partitioned into their components: trunk, branches (current-season shoots, 1-year-old and over-1-year-old branches), buds of the same age, small roots (less than 1 mm thick), large roots, leaves (sampled in February and April), and fruit (collected at harvest). Those components were dried and analysed for total N and 15N content. Total N per tree and N content derived from the fertilizer did not differ between treatments. Summer postharvest 15N application partitioned not only to structural components (trunk and roots) but also to buds and leaves. Uptake efficiency was significantly ( p = 0.0113) higher in the spring than in the summer application (65.7% vs. 37.44%). Nevertheless, 52.5% of N applied in spring was lost due to harvest and summer pruning. This emphasizes the importance of the postharvest N fertilization which increases N accumulation in both reserve organs and buds though, according to our data, it is less efficiently used. The extent of nitrogen uptake, efficiency of use and partitioning in the following growing seasons are still open questions that deserve further research.

Multiseason Recoveries of Organic and Inorganic Nitrogen‐15 in Tropical Cropping Systems

In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A 15N‐recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single 15N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58% (average of 21%) of crop N uptake during the first growing season was derived from fertilizer. On average, 79% of crop N was derived from the soil. When 15N‐labeled residues were applied, in the first growing season 4% of crop N was derived from the residues. Average recoveries of 15N‐labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. Corresponding recoveries in the soil were 38 and 71%. An additional 6% of the fertilizer and 9.1% of the residue was recovered by crops during subsequent growing seasons. There were no significant differences in total 15N recovery (average 54%) between N from fertilizer and N from residue. After five growing seasons, more residue N (40%) than fertilizer N (18%) was recovered in the soil, better sustaining the soil organic matter N content. Long‐term total recoveries of 15N‐labeled fertilizer or residue in the crop and soil were similar. Soil N remained the primary source of N for crops. As higher rainfall and temperature tend to cause higher 15N losses, management practices to improve N use efficiency and reduce losses in wet tropical regions will remain a challenge.

Modeling the flow of 15N after a 15N pulse to study long‐term N dynamics in a semiarid grassland

Many aspects of nitrogen (N) cycling in terrestrial ecosystems remain poorly understood. Progress in studying N cycling has been hindered by a lack of effective measurements that integrate processes such as denitrification, competition for N between plants and microbes, and soil organic matter (SOM) decomposition over large time scales (years rather than hours or days). Here I show how long-term measurements of 15N in plants, microbes, and soil after a one-time addition of 15N ("labeled" N) can provide powerful information about long-term N dynamics in a semiarid grassland. I develop a simple dynamic model and show that labeled-N fractions in plant and microbial-N pools (expressed as a fraction of total N in each pool) can change long after 15N application (> or = 5 years). These 15N dynamics are closely tied to the turnover times of the different N pools. The model accurately simulated the labeled-N fractions in aboveground biomass measured annually during five years after addition of 15N to a semiarid grassland. I also tested the sensitivity of five different processes on labeled-N fractions in aboveground plant biomass. Changing plant/microbial competition for N had very little effect on the labeled-N fraction in aboveground biomass in the short and long-term. Changing microbial activity (N mineralization and immobilization), N loss, or N resorption/re-translocation by plants affected the labeled-N fraction in the short-term, but not in the long-term. Large long-term effects on the labeled-N fraction in aboveground biomass could only be established by changing the size of the active soil-N pool. Therefore, the significantly greater long-term decline in the labeled-N fraction in aboveground biomass observed under elevated CO2 in this grassland system could have resulted from an increased active soil-N pool under elevated CO2 (i.e., destabilization of soil organic matter that was relatively recalcitrant under ambient CO2 conditions). I conclude that short- and long-term labeled-N fractions in plant biomass after a 15N pulse are sensitive to processes such as N mineralization and immobilization, N loss, and soil organic matter (de-)stabilization. Modeling these fractions provides a useful tool to better understand N cycling in terrestrial ecosystems.

Contribution of Nitrogen Absorbed during Ripening Period to Grain Filling in a High-Yielding Rice Variety, Takanari

AbsractHigh-yielding rice varieties require a large accumulation of N in panicles. The objectives of this study were to clarify the change in N allocation during the ripening period (Exp. 1) and to quantify the contribution of N absorbed during the ripening period to panicle N at maturity (Exp. 2) in the high-yielding variety Takanari in comparison with that in Nipponbare as a control. In Exp. 1, 15N-labeled N (15N) was applied at heading to investigate the distribution of newly absorbed N as well as the allocation of plant N. In Exp. 2, split 15N application was performed during the filling period to estimate the above contribution. In Exp. 1, the allocation of plant N and absorbed 15N to the panicles was larger and that to the leaves was smaller in Takanari than in Nipponbare during the ripening period, although Takanari accumulated more N at maturity. The difference in N allocation suggested that the difference in N demand in panicles would be larger than that in N uptake. In Exp. 2, the varietal difference in the grain filling duration was observed: Nipponbare accumulated little N in the panicles after 28 d after heading (DAH), while Takanari accumulated about a quarter of its panicle N during that time. An estimate showed that in Takanari, 13.5% of the panicle N was derived from N absorbed after 28 DAH. These results suggest that the utilization of newly absorbed N until a later period after heading is important for the achievement of high yields.

Leaching of Mineral and Organic Nitrogen from Putting Green Profiles Supporting Various Turfgrasses

Nitrate (NO3−) leached from golf greens has the potential to impair water quality. Dissolved organic N (DON) is also increasingly recognized as a form leached from fertilized soils. A controlled experiment was conducted to determine (i) the significance of DON in total N leaching losses under simulated golf‐green profiles and (ii) the short‐term contribution of fertilizer to leaching of inorganic and organic N forms. Various turfgrasses were grown in lysimeter columns designed to simulate a golf‐green profile. Fertilizer was applied at 25 kg N ha−1 every 14 d for 55 d, and the last application was labeled with 15N. Leachates were analyzed for NO3−‐N, NH4+‐N, and DON. The 15N recovery was assessed in plant, soil, and leachates. In the presence of plants, 10 to 70% (average 40%) of total N leached was accounted for by DON. Application of 15N revealed that one‐half to two‐thirds of NO3−‐N leached in the following 14 d was derived from the fertilizer, whereas the majority of leached DON was derived from soil residual N. Nevertheless, DO15N was present in most leachate samples collected for 14 d after fertilizer application, indicating that only a few days were required to convert mineral fertilizer to leachable organic forms. We conclude that DON may be a significant component of total N leaching losses from putting greens and would account for part of the N losses traditionally attributed to volatilization and denitrification.

Effects of N-application on utilization of 15N and 13C and quality in two wheat cultivars

We studied N uptake and distribution in wheat, and the incorporation of nitrogen and carbon into gluten and non-gluten proteins using a double-labelling approach with 15N and 13C. Doses of N-fertilizer were split and applied at emergence, onset of stem elongation, and heading at rates of 280/140/140 mg N pot−1, respectively simulating 90/45/45 kg N ha−1. Five different combinations of N-fertilizations containing no or 10 % 15N were performed. The recovery of 15N added at the stages emergence, stem elongation or heading were 42, 60, and 64 %. Application of 15N at all three stages yielded in 51 % recovery. Remobilisation of straw N was greater for Golia. The 15N concentration in gluten proteins of Golia show higher values than Gönen. The ratio of 15N gluten/15N non-gluten proteins of Golia were higher, which implies a lower non-gluten protein activity during grain filling. The 13C concentration in gluten and non-gluten proteins did not differ between both cultivars.

Uptake of carbon and nitrogen derived from carbon-13 and nitrogen-15 dual-labeled maize residue compost applied to radish, komatsuna, and chingensai for three consecutive croppings

A pot experiment was conducted to determine the effects of the application of 13C (1.256 atom%) and 15N (1.098 atom%) dual-labeled maize residue compost (MRC) on the nitrogen and carbon uptake by radish, komatsuna, and chingensai as compared with the effect of inorganic fertilizer (IF). The vegetables were grown over three consecutive growing seasons over 4 months; compost was applied at the rate of 24 g kg–1 soil. Nonlabeled nitrogen fertilizer was applied to the compost treatments in the second and third crops to compare the effects of blends of compost with N fertilizer to fertilizer alone. The N uptake and yield of vegetables were significantly higher with the recommended inorganic N treatment. The vegetables took up significantly (P < 0.05) lower amounts of N from MRC than from IFs during the three cultivations. The values of the N uptake derived by fertilizer application to the plant exhibited significant differences among different vegetables. Nitrogen recovered by komatsuna and chingensai from MRC was 7.3 (6.6%), 2.7 (1.8%), and 2.3, (1.7%) in the first, second, and third crops, respectively. Radish, komatsuna, and chingensai recovered significant amounts of C from MRC in the first and second crops, with negligible C recovery in the third crop. The initial loss of fertilizer C in soil at the first crop indicates that the microbial decomposition decoupled substantial amounts of 13C/15N-labeled compounds early in plant development, thus giving the microorganisms a preemptive competitive advantage in the acquisition of easily available 13C/15N-labeled substrates. It is concluded that a combination of compost and inorganic N did not supply sufficient plant-available N to increase vegetables yields or N uptake over those of fertilizer alone. The data suggested that higher productivity of vegetables might be achieved after the accumulation of a certain amount of residual compost N.

Nitrogen rhizodeposition in agricultural crops: Methods, estimates and future prospects

The objective of the present review was to present the current knowledge on nitrogen (N) rhizodeposition, including techniques for 15N labelling of agricultural plants, amounts of N rhizodeposition and its fate in soil. Rhizodeposition is the process of release of organic and inorganic compounds from living plant roots. It is often quantified in terms of carbon (C) and less often as N derived from rhizodeposition (NdfR). Rhizodeposition of N can be estimated by labelling plants with 15N and following its fate in soil. Most methods used for labelling plants with 15N can only be applied after appearance of the first leaf and only allow pulse or multiple pulse labelling. Only the split-root technique and the application of gaseous 15N allow continuous labelling. All methods available at present have their flaccidities mostly due to the fact that the application of N is not following its physiological pathway of assimilation or by using artificial conditions. In the studies reviewed, amounts of N rhizodeposits ranged from 4% to 71% of total assimilated plant N. In legumes the median was 16% and in cereals it was 14%. Rhizodeposits were 15–96% of the below-ground plant biomass (BGP). In legumes the median was 73% and in cereal it was 57%. The high variability of these results shows the need for more investigations on N rhizodeposition looking especially on the factors influencing the amounts released in different plant species under field conditions.

Enxertia e água de irrigação carbonatada no transporte de 15N e na produção do tomateiro

Estudaram-se os efeitos da aplicação de CO2, via água de irrigação, e da enxertia do tomateiro no transporte de 15N e na produção do tomateiro. Os tratamentos foram arranjados em delineamento inteiramente ao acaso no esquema fatorial 2 x 2 (com e sem CO2 na água de irrigação e tomateiro enxertado e pé-franco). A injeção do CO2 na água iniciou-se aos 34 dias após o transplante das mudas (DAT) e se prolongou em todas as irrigações. A aplicação do sulfato de amônio com abundância em átomos de 15N de 3,13% nas plantas destinadas à análise foi feita aos 45 DAT quando as plantas estavam em plena frutificação. Após 14 dias da aplicação do fertilizante (15N ) as plantas foram colhidas, lavadas, secadas e enviadas ao laboratório, para análise do 15N nos seus tecidos. Os resultados demonstraram que o CO2 e a enxertia não alteraram o transporte de 15N na planta. A produção de frutos comerciais foi maior quando se aplicou CO2 na água.

Adducts of rhodium(II) tetraacetate with some nitrogenous organic ligands: Application of natural abundance 15N and 13C CPMAS NMR spectroscopy

Adducts of rhodium(II) tetraacetate with some nitrogenous organic ligands: 1-azabicyclo[2,2,2]octane 1, 1,2-diazabicyclo[2,2,2]octane 2, pyrazine 3, pyrimidine 4, [1,3,5]triazine 5 and 1,3,5,7-tetraazatricyclo[3,3,1,1 3,7]decane 6 have been investigated by means of natural abundance 13C and 15N CPMAS nuclear magnetic resonance (NMR) spectroscopy. 1-Azabicyclo[2,2,2]octane 1 having one nitrogen atom in the molecule produces either the 1:1 or 1:2-adduct depending on the reagent molar ratio; some features of its 13C CPMAS NMR spectra suggest the dimeric structure of the 1:1-adduct. Multifunctional ligands having more than one nitrogen atom in a molecule yield the adducts insoluble in common organic solvents. Elemental analysis and NMR experiments have revealed that 1,2-diazabicyclo[2,2,2]octane, pyrazine, pyrimidine and [1,3,5]triazine produced adducts in the form of 1:1 polymeric chains. 1,3,5,7-tetraazatricyclo[3,3,1,1 3,7]decane yields the adduct containing ligand and metal salt in the molar ratio of 3:4. The 15N chemical shift change caused by the Rh–N bond formation (Δ δ parameter) varies from ca. −9 ppm for aliphatic ligands to ca. −40 ppm for heteroaromatic species. The NMR findings have been supported by theoretical calculation (density functional calculation (DFT), LanLD2Z//B3LYB level) of molecular geometry, energy and chemical shieldings.

Dinitrogen-fixation by three neotropical agroforestry tree species under semi-controlled field conditions

Cultivating dinitrogen-fixing legume trees with crops in agroforestry is a relatively common N management practice in the Neotropics. The objective of this study was to assess the N2 fixation potential of three important Neotropical agroforestry tree species, Erythrina poeppigiana, Erythrina fusca, and Inga edulis, under semi-controlled field conditions. The study was conducted in the humid tropical climate of the Caribbean coastal plain of Costa Rica. In 2002, seedlings of I. edulis and Vochysia guatemalensis were planted in one-meter-deep open-ended plastic cylinders buried in soil within hedgerows of the same species. Overall tree spacing was 1 × 4 m to simulate a typical alley-cropping design. The 15N was applied as (NH4)2SO4 at 10% 15N atom excess 15 days after planting at the rate of 20 kg [N] ha−1. In 2003, seedlings of E. poeppigiana, E. fusca, and V. guatemalensis were planted in the same field using the existing cylinders. The 15N application was repeated at the rate of 20 kg [N] ha−1 15 days after planting and 10 kg [N] ha−1 was added three months after planting. Trees were harvested 9 months after planting in both years. The 15N content of leaves, branches, stems, and roots was determined by mass spectrometry. The percentage of atmospheric N fixed out of total N (%Nf) was calculated based on 15N atom excess in leaves or total biomass. The difference between the two calculation methods was insignificant for all species. Sixty percent of I. edulis trees fixed N2; %Nf was 57% for the N2-fixing trees. Biomass production and N yield were similar in N2-fixing and non-N2-fixing I. edulis. No obvious cause was found for why not all I. edulis trees fixed N2. All E. poeppigiana and E. fusca trees fixed N2; %Nf was ca. 59% and 64%, respectively. These data were extrapolated to typical agroforestry systems using published data on N recycling by the studied species. Inga edulis may recycle ca. 100 kg ha−1 a−1 of N fixed from atmosphere to soil if only 60% of trees fix N2, E. poeppigiana 60–160 kg ha−1 a−1, and E. fuscaca. 80 kg ha−1 a−1.

Variable responses of the depth of tree nitrogen uptake to pruning and competition

Trees in cropped fields may improve nitrogen (N) use efficiency by intercepting leached N, but crop yield will be reduced if the trees compete strongly with crops for N. Ideal trees for intercropping will take up N from deeper soil layers not accessed by the crop species. Spatiotemporal aspects of tree nitrogen capture niches were investigated within a hedgerow intercropping system by placing 15N at three depths and monitoring 15N uptake by trees pruned either 25 or 4 days before application of 15N. Trees with contrasting rooting patterns (Gliricidia sepium L. and Peltophorum dasyrrachis (Miq.) Kurz) were grown in mixed hedgerows and intercropped with maize (Zea mays L.). Neither species showed significant N uptake during the 5-14 days after pruning, even though some shoot regrowth occurred during this time. Mean topsoil (0-5 cm depth) root length density of G. sepium was 520% greater than that of P. dasyrrachis, whereas total root length (0-100 cm depth) of G. sepium was only 450% greater. On average, G. sepium recovered 15 times as much 15N as P. dasyrrachis, following application of 15N at 5 cm depth, but the two species recovered a similar amount following application at 80 cm depth, suggesting that P. dasyrrachis had better niche complementarity with shallow rooting crops. However, both species showed strong plasticity in vertical N uptake pattern in response to competition from establishing maize plants. The species differed in their response: N uptake activity by G. sepium shifted down the soil profile in response to increasing competition from a growing maize crop (uptake from 80 cm depth changed from 4 to 9% of uptake from 5 cm depth), whereas N uptake by P. dasyrrachis became relatively shallow (uptake from 80 cm depth changed from 305 to 25% of uptake from 5 cm depth). Niche avoidance and increased competitiveness within the topsoil represent alternative responses to competition. The response displayed may be related to soil fertility in the species' natural habitats.