PINUS RADIATA SEEDLING GROWTH AND MICRONUTRIENT UPTAKE IN A SAND CULTURE EXPERIMENT, AS AFFECTED BY THE FORM OF NITROGEN

A pot trial was used to investigate the effect of the form of nitrogen (ammonium or nitrate) and the rate of nitrogen addition, at 1, 2.5, or 5 mM (14, 35, and 70 ug/ml respectively), on Pinus radiata D.Don seedlings grown in sand culture. Ammonium-fed seedlings were taller and had heavier shoots and roots than nitrate-fed seedlings, particularly at the highest rate of nitrogen addition. The ammonium-fed seedlings contained greater total amounts of copper and boron and had a higher percentage of copper, boron, and iron in the shoots than nitrate-fed seedlings. The nitrate-fed seedlings contained significantly more iron. Increasing the rate of nitrogen addition from 14 to 35 ug/ml increased shoot weights in the ammonium-fed seedlings, increased shoot height in the nitrate-fed seedlings, and increased shoot:root ratios for both forms of nitrogen. Shoot heights of ammonium-fed seedlings increased when the rate of nitrogen increased from 14 to 70 ug/ml. Within each nitrogen form, the rate of nitrogen addition did not generally affect micronutrient concentrations in the shoots or roots. The concentrations of boron and iron (at all nitrogen rates) and copper (at 70 u.g N/ ml) were higher in the nitrate-fed roots than in the ammonium-fed roots but the concentrations of iron and copper (at all nitrogen rates) in the nitrate-fed shoots were significantly less than ammonium-fed shoots. The nitrate-fed roots were darker in colour and were more branched and finer than ammonium-fed roots. Organic acids, produced during nitrate reductase activity, may have a role in these differences.