Effects of ammonium on elemental nutrition of red spruce and indicator plants grown in acid soil

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Abstract
Decline of high elevation red spruce (Picea rubens Sarg.) forests in the northeastern United States has been related to Ca and Mg deficiencies induced by input of air‐borne nitrogenous nutrients into the forest ecosystem. This research investigated the effects of N nutrition on mineral nutrition of red spruce and radish (Raphanus sativus L.), as an indicator plant, grown in acid forest soil. Red spruce and radishes in the greenhouse were treated with complete nutrient solutions with 15 mM N supplied as 0, 3.75, 7.5, 11.25, or 15 mM NH4 + with the remainder being supplied as NO3. Growth of each species was chlorotic and stunted by increased NH4 + in the nutrient solution. Increasing NH4 + from 0 to 15 mM depressed soil pH by about 1 unit (from 4.5 to an average of 3.5). Accumulation of N, K, Ca, and Mg by each species was restricted as the proportion of NH4 + increased, although the magnitude of these restrictions were small. The restrictions in growth were attributed directly to NH4 + toxicity and not to induction of nutrient deficiencies by NH4 +.