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Oswaldo A. Rubio, Patrick H. Brown, and Steven A. Weinbaum

Leaf N concentrations (% dry wt) appear relatively insensitive to high levels of applied fertilizer N (Weinbaum et al, HortTechnology 1992). This insensitivity may be attributable to growth dilation, lack of additional tree N uptake, a finite capacity of leaves to accumulate additional N or our inhability to resolve a limited increment. Our objective was to asses the relative accumulation of mobile forms of N (NO3, NH4 and amino acids) relative to a total N over a range of fertilizer N application rates in 3 year old, field-grown “Fantasia” nectarine trees. Between 0 and 136 Kg N/Ha/Yr we observed a linear relationship between N supply and all N fractions. Above 136 Kg N/Ha/Yr leaf concentrations of amino acids and total N remined constant, but NO3 and NH4 accumulation continued. These results suggest that leaf concentration of NO3 and NH4 are more sensitive indicators of soil N availability and tree N uptake than was total leaf N concentration.

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Laura Elisa Acuña-Maldonado, Michael W. Smith, Niels O. Maness, Becky S. Cheary, Becky L. Carroll, and Gordon V. Johnson

Nitrogen was applied to mature pecan (Carya illinoinensis Wangenh. C. Koch.) trees annually as a single application at 125 kg·ha-1 N in March or as a split application with 60% (75 kg·ha-1 N) applied in March and the remaining 40% (50 kg·ha-1 N) applied during the first week of October. Nitrogen treatment did not affect yield, and had little effect on the amount of N absorbed. Nitrogen absorption was greater between budbreak and the end of shoot expansion than at other times of the year. Substantial amounts of N were also absorbed between leaf fall and budbreak. Little N was absorbed between the end of shoot expansion and leaf fall, or tree N losses met or exceeded N absorption. Pistillate flowers and fruit accounted for a small portion of the tree's N; ≈0.6% at anthesis and 4% at harvest. The leaves contained ≈25% of the tree's N in May and ≈17% when killed by freezing temperatures in November. Leaves appeared to contribute little to the tree's stored N reserves. Roots ≥1 cm diameter were the largest site of N storage during the winter. Stored N reserves in the perennial parts of the tree averaged 13% of the tree's total N over a three year period. Current year's N absorption was inversely related to the amount of stored N, but was not related to the current or previous year's crop load.

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Timothy L. Righetti, David R. Sandrock, Bernadine Strik, and Anita Azarenko

The ratio of labeled fertilizer nitrogen (N) divided by the amount of total N is a convenient expression for evaluating the amount of N that is derived from fertilizer. The slope of the regression line for the relationship between total N and

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Jinghua Fan, George Hochmuth, Jason Kruse, and Jerry Sartain

autoanalyzer following U.S. Environmental Protection Agency (USEPA) method 351.2 ( USEPA, 1993a ). Total N was calculated as the sum of TKN and NO 3 -N. The concentrations of total P and ortho-P were determined by the molybdenum-blue method after digestion with

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Fang Xiao, Zaiqiang Yang, Haijing Huang, Fei Yang, Liyun Zhu, and Dong Han

cm, height × diameter) filled with 8 kg air-dried soil that passed through a 5-mm sieve. The physicochemical properties of the growing media were determined by Hanlon (1994) : organic matter 17.6 g·kg −1 , total N 0.78 g·kg −1 , available N 21.57 mg

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M. Pilar Bañados, Bernadine C. Strik, David R. Bryla, and Timothy L. Righetti

was oven-dried at 70 °C, to a constant weight, and dry weight was measured. A subsample of each tissue was ground to pass a 40-mesh screen (0.42-mm openings) and measured for total N and 15 N concentrations by mass spectrometry (Isotope Service, Los

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Elvia Hernández-Gómez, Luis A. Valdez-Aguilar, Ana M. Castillo-González, María T. Colinas-León, Donita L. Cartmill, Andrew D. Cartmill, and R. Hugo Lira-Saldívar

the study period. Nutrient solutions. Nutrient solutions with 13 m m total N were prepared with varying proportions of NH 4 + : 0% (control), 25%, 50%, and 75% (the remainder was completed with N in the NO 3 – form) and three concentrations of K: 6

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Heidi J. Johnson, Jed B. Colquhoun, Alvin J. Bussan, and Carrie A.M. Laboski

community to become a PAN form (ammonium or nitrate). Moisture, temperature, and composition of the soil microbe community can affect the rate at which organic N is mineralized. Total N content of organic sources is often not completely mineralized within

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Shichao Wang, Zhujun Chen, Jun Man, and Jianbin Zhou

( ISSAS, 1995 ). To summarize, SOM was measured with the dilution heat K 2 Cr 2 O 7 oxidation volumetric method. Total N was measured with the Kjeldahl method. Available P was determined according to the method of Olsen et al. (1954). Available K was

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Ryan W. Dickson, Paul R. Fisher, and William R. Argo

total N as NH 4 + -N, respectively, with the remainder of N as NO 3 − -N. This ratio would be expected to maintain a stable pH over time when these species were grown with zero alkalinity irrigation water and without residual lime in the substrate