Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH 4 + ) [as (NH 4 ) 2 SO 4 ], nitrate (NO 3 -) (as NaNO 3 ), or a mixed N source (NH 4 NO 3 ) at 0, 800, or 1,600 mg N plant -1 season -1. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO 3 --fed and NH 4 NO 3 -fed plants compared to observed responses in NH 4 + -fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO 3 --fed than in NH 4 + -fed plants. Exogenous NO 3 -was assimilated in roots or stored, so no difference was observed in NO 3 -levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO 3 -or NH 4 + alone, suggesting NH 4 NO 3 is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.
We evaluated suitability of chemical indices of three media formulations or substrates (A, B, and C) consisting of composted pine bark, coconut coir pith, sphagnum peatmoss, processed bark ash, and perlite in varied proportions for growing northern red oak (Quercus rubra L.) seedlings. These substrates were ranked according to their ability to promote seedling growth. The low-yielding substrate (A) was devoid of pine bark and perlite and the medium-yielding substrate (B) contained no peatmoss or processed bark ash. The high-yielding substrate (C) contained all components. Additionally, we tested plant response to high nitrogen (N) fertilization on each substrate. Media EC, pH, and total dissolved solids measured at transplanting explained 68%, 43%, and 66%, respectively, of the variation in plant dry weight and 39%, 54%, and 46%, respectively, of the variation in shoot height. Vector diagnosis effectively ranked nutritional limitations on seedling growth as N > P > K. High N fertilization highlighted element deficiency in seedlings grown on substrate A, but resulted in element toxicity and antagonistic interactions in plants established on substrates B and C, respectively.
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