THE INFLUENCE OF NITROGEN APPLICATIONS ON THE RESORPTION OF FOLIAR NUTRIENTS IN SWEETGUM

Resorption of foliar nutrients during senescence is one of the principal ways by which plants enhance nutrient efficiency, particularly of N and P. The objectives of this study were to determine (i) changes in foliar characteristics attributable to N applications and (ii) resorption of nutrients during foliar senescence. Rates of 0, 100, 200, and 400 kg N/ha were applied to a 9-year-old sweetgum (Liquidambar styraciflua L.) plantation in 1981, and resorption was determined the following three growing seasons. Preceding senescence, concentrations of foliar N were doubled, P concentrations declined, and K, Ca, and Mg concentrations were relatively unaffected by N application. Losses from foliage (resorption and (or) leaching) of N, P, K, and Mg, but not Ca, were positively related to the quantity present in the presenescent foliage. Resorption of foliar N, P, K, Ca, and Mg averaged 62, 50, 48, -19, and 12%, respectively; values significantly differed among years for all nutrients. Relative resorption was unaffected by N application rates for all nutrients except K. Results indicate that resorption is an important mechanism for nutrient conservation and efficiency, and under the conditions evaluated in this study, it was influenced more by environmental conditions (i.e., weather) than by nutrient levels.