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Hadi Susilo, Ying-Chun Peng, Shui-Cheng Lee, Yu-Chun Chen, and Yao-Chien Alex Chang

when studying use of N fertilizer using traditional methods. Nitrogen-14 and nitrogen-15 ( 15 N) are the two stable isotopes of N with atmospheric natural abundances of 99.6337% and 0.3663%, respectively. The latter is an important tracer element in

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Hadi Susilo, Ying-Chun Peng, and Yao-Chien Alex Chang

Schiltz, S. Munier-Jolain, N. Jeudy, C. Burstin, J. Salon, C. 2005 Dynamics of exogenous nitrogen partitioning and nitrogen remobilization from vegetative organs in pea revealed by 15 N in vivo labeling throughout seed filling Plant Physiol. 137 1463 1473

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Hadi Susilo and Yao-Chien Alex Chang

with various nitrogen (N) concentrations during the reproductive stage on 15 N concentration in Phalaenopsis Sogo Yukidian ‘V3’ harvested at the visible bud stage. 15 N was applied 6 weeks before low-temperature forcing and the 15 N label

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John D. Lea-Cox, James P. Syvertsen, and Donald A. Graetz

15Nitrogen uptake, allocation, and leaching losses from soil were quantified during spring, for 4-year-old bearing `Redblush' grapefruit (Citrus × paradisi Macf.) trees on rootstocks that impart contrasting growth rates. Nine trees on either the fast-growing `Volkamer' lemon (VL) (C. volkameriana Ten & Pasq.) or nine on the slower-growing sour orange (SO) (C. aurantium L.) rootstocks were established in drainage lysimeters filled with Candler fine sand and fertilized with 30 split applications of N, totaling 76, 140, or 336 g·year-1 per tree. A single application of double-labeled ammonium nitrate (15NH 15 4NO3, 20% enriched) was applied at each rate to replicate trees, in late April. Leaves, fibrous roots, soil, and leachates were intensively sampled from each treatment over the next 29 days, to determine the fate of the 15NH 15 4NO3 application. Newly developing spring leaves and fruit formed dominant competitive sinks for 15N, accounting for between 40% and 70% of the total 15N taken up by the various treatments. Large fruit loads intercepted up to 20% of this 15N, at the expense of spring flush development, to the detriment of overall tree N status in low-N trees. Nitrogen supply at less than the currently recommended yearly rate of 380 g/tree exceeded the requirements of 4-year-old grapefruit trees on SO rootstock; however, larger trees on VL rootstock took up the majority of 15N from this rate over the 29-day period. Nitrogen-use efficiency declined with increasing N rate, irrespective of rootstock. The residual amounts of 15N remaining in the soil profile under SO trees after this time represented a significant N leaching potential from these sandy soils. Therefore, under these conditions, present N recommendations appear adequate for rootstocks that impart relatively fast growth rates to Citrus trees, but seem excessive for trees on slower-growing rootstock species.

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G.A. Picchioni and Héctor M. Quiroga-Garza

Two greenhouse studies were conducted to trace the fate of fertilizer N in hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy `Tifgreen'], and to estimate total plant N recovery and losses. The first experiment was performed during winter, with artificial light supplementing natural light to provide a photoperiod of 13.6 to 13.8 hours. The second experiment was conducted during summer and fall under only natural light conditions, with a progressively decreasing photoperiod of 13.7 to 11.1 hours. Urea (UR), ammonium sulfate (AS), and ammonium nitrate (AN) were labeled at 2 atom% 15N, and applied at N rates of 100 or 200 kg·ha-1 for 84 days (divided into six equal fractions and applied every 14 days). Fertilizer N source did not affect total dry matter (DM) accumulation by the plant components, but the high N rate increased clipping DM production under the longer photoperiod. Under the decreasing photoperiod, overall DM production was reduced, and clipping DM production was unaffected by increased N rate. Average N concentration of clippings varied between N sources, ranging from a high of 38.6 g·kg-1 DM with AS to a low of 34.7 g·kg-1 for UR. In Expt. 1, the greatest total plant N recovery [clippings, verdure (shoot material remaining after mowing), and thatch plus roots] occurred with AS (78.5%) and the lowest with UR (65.9%). In Expt. 2, these values declined to 53.0% and 38.0%, respectively. Urea fertilization resulted in the greatest N losses as a fraction of the N applied (33.6% to 61.5%) and AS fertilization the lowest (20.7% to 46.3%). In view of the greater N losses, UR may be a less suitable soluble N source for bermudagrass fertilization within the conditions of this study. In addition, late-season N fertilization may result in a significant waste of fertilizer N as bermudagrass progresses into autumnal dormancy when temperature, photoperiod, and irradiance decline and cause reduction in growth and N uptake.

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C.A. Sanchez, H.Y. Ozaki, K. Schuler, and M. Lockhart

Experiments were conducted from 1985 to 1989 to evaluate the response of radishes (Raphanus sativus L.) to N fertilization on Histosols. Three of these experiments used 15N-labeled fertilizer to evaluate the recovery of N by radishes. There was no response to N fertilization in seven of the eight experiments, even though some of them were conducted under conditions of high rainfall. The one experiment in which radish yields increased with N was conducted in a poorly drained, waterlogged field that was atypical of normal radish production fields. Recoveries of fertilizer N in the marketable radish roots averaged 19%. The results of N and 15N analysis showed that although fertilizer N was available for uptake, so was an ample amount of soil mineralized N. These results indicate that under typical growing conditions, radishes produced on Florida Histosols do not respond to N fertilization.

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Philip A. Throop and Eric J. Hanson

Rates of absorption of 15N-enriched ammonium sulfate by young `Bluecrop' highbush blueberries (Vaccinium corymbosum L.) were compared following applications on six dates between late April and September. Ammonium sulfate solutions containing 2.1 g N (10.2 atom % 15N) were dripped directly into the root zone of single bushes. Soil covers and irrigation were used to maintain similar soil moisture conditions during treatment periods. Treated bushes from each application date were excavated after 2 weeks of exposure and separated into roots, stems, and current season's growth (new shoots, leaves, fruit). Tissues were dried, weighed, and analyzed for 15N and 14N by mass spectrometry. Soils were also analyzed for labeled and nonlabeled N. Bushes treated in late May, June, and July absorbed a greater percentage of applied N (6% to 9%) than bushes treated in April, August, or September (1% to 3%). Absorption of N appeared to be affected more by the demand of the plants than soil N availability. Plants absorbed N most efficiently during active growth between late bloom and fruit maturity.

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Wei-Ling Yuan, Shang-yong Yuan, Xiao-hui Deng, Cai-xia Gan, Lei Cui, and Qing-fang Wang

; Song et al., 2014 ): Nitrogen derived from fertilizer (Ndff) in plant (kg N/ha) as the ratio of N uptake by plant to 15 N atom% excess in plant to 15 N atom% excess in fertilizer Nitrogen derived from soil (Ndfs) in plant (kg N/ha) as the difference

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Guihong Bi and Carolyn F. Scagel

-mesh; Thomas Scientific, Swedesboro, NJ). Nitrogen and 15 N measurement and calculations. Concentration of N and 15 N in samples were determined with a Europa Scientific Corporation Roboprep C/N analyzer coupled to a Tracermass Mass