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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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T.K. Hartz and P.R. Johnstone

Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates in-season fertilization. The rate of net nitrogen mineralization (Nmin) from four organic fertilizers (seabird guano, hydrolyzed fish powder, feather meal, and blood meal) containing between 11.7% and 15.8% N was compared in a laboratory incubation. The fertilizers were mixed with soil from a field under organic management and incubated aerobically at constant moisture at 10, 15, 20, and 25 °C. Nmin was determined on samples extracted after 1, 2, 4, and 8 weeks. Rapid Nmin was observed from all fertilizers at all temperatures; within 2 weeks between 47% and 60% of organic N had been mineralized. Temperature had only modest effects, with 8-week Nmin averaging 56% and 66% across fertilizers at 10 and 25 °C, respectively. Across temperatures, 8-week Nmin averaged 60%, 61%, 62%, and 66% for feather meal, seabird guano, fish powder, and blood meal, respectively. Cost per unit of available N (mineralized N + initial inorganic N) varied widely among fertilizers, with feather meal the least and fish powder the most expensive.

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T.K. Hartz, R. Smith, and M. Gaskell

composts mineralized <10% of initial N content in the 4 to 6 months following soil incorporation. A high compost application rate can make a significant contribution to soil N availability, but in practice, application rate is constrained by cost and by the

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Rafael A. Muchanga, Toshiyuki Hirata, and Hajime Araki

factor affecting soil N availability, N uptake, and crop productivity ( Hargrove, 1986 ; Paustian et al., 1992 ; Ranells and Wagger, 1996 ). Thus, given the differences in C and N concentrations, and decomposition speed between livestock compost and HV

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Meng Wei, Aijun Zhang, Hongmin Li, Zhonghou Tang, and Xiaoguang Chen

caffeoylquinic acid, anthocyanins ( Islam et al., 2002 ), as well as beta-carotene compared with other commercial vegetables. Soil N availability is an important component in storage root production of sweetpotato as well as for leafy sweetpotato ( Phillips et al

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Carolyn F. Scagel, Guihong Bi, Leslie H. Fuchigami, and Richard P. Regan

statistical package (1996, Statsoft, Tulsa, OK). Results and Discussion Growth response to nitrogen availability. Increased N availability had little effect on biomass accumulation until after July 2005 for evergreen cultivars (PJM and ER

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M. Lenny Wells

management effects on N availability and provide information needed for improved nutrient management ( Adams and Attiwill, 1986 ; Kolberg et al., 1997 ; Raison et al., 1987 ). There are few studies addressing N availability in pecan orchard soils

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Angela Y.Y. Kong, Cynthia Rosenzweig, and Joshua Arky

plant and/or food materials, and green manures (e.g., cover crop residues). Unless N availability is well synchronized with plant need and uptake, there is a significant potential for the loss of added N from vegetated roofs via leaching after irrigation

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Victoria M. Anderson, Douglas D. Archbold, Robert L. Geneve, Dewayne L. Ingram, and Krista L. Jacobsen

crop production in these systems ( Parr et al., 1990 ), because the bulk of the N must be mineralized from the fertility source by microbial decomposition before plant uptake. Consequently, N availability may be difficult to predict ( Gaskell, 2006

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Kyle E. Bair, Joan R. Davenport, and Robert G. Stevens

that NO 3 -N is a better index of soil N availability. Low PRS NH 4 -N values do, however, suggest a rapid nitrification is taking place. PRS NO 3 -N data ( Table 2 ) for RES and COM vineyards show that during the PB period (only measured in 2004