Nitrogen form and concentration interact to affect the performance of two ecologically distinct Mediterranean forest trees

Abstract

Most studies examining inorganic N formeffects on growth and nutrition of forest trees have beenconducted on single species from boreal or temperateenvironments, while comparative studies with species fromother biomes are scarce. We evaluated the response of twoMediterranean trees of contrasting ecology, Quercus ilex L.and Pinus halepensis Mill., to cultivation with distinctinorganic N forms. Seedlings were fertilized with differentNH4?/NO3- proportion at either 1 or 10 mM N. In bothspecies, N forms had small effects at low N concentration,but at high N concentration they markedly affected theplant performance. A greater proportion of NH4? in thefertilizer at high N caused toxicity as it reduced growth andcaused seedling death, with the effect being greater in Q.ilex than in P. halepensis. An increase in the proportion ofNO3- at high N strongly enhanced growth relative to lowN plants in P. halepensis but had minor effects in Q. ilex.Relatively more NH4? in the fertilizer enhanced plant Pconcentration but reduced K concentration in both species,while the opposite effect occurred with NO3-, and these effects were enhanced under high N concentration. We conclude that species responses to inorganic N forms were related to their ecology. P. halepensis, a pioneer tree, had improved performance with NO3 - at high N concentration and showed strong plasticity to changes in N supply. Q. ilex, a late successional tree, had low responsiveness to N form or concentration.