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

Abstract

Most studies examining inorganic N form effects on growth and nutrition of forest trees have been conducted on single species from boreal or temperate environments, while comparative studies with species from other biomes are scarce. We evaluated the response of two Mediterranean trees of contrasting ecology, Quercus ilex L. and Pinus halepensis Mill., to cultivation with distinct inorganic N forms. Seedlings were fertilized with different NH4 + / NO3proportion at either 1 or 10 mM N. In both species N forms had small effects at low N concentration, but at high N concentration they markedly affected plant performance. A greater proportion of NH4 + in the fertilizer at high N caused toxicity as it reduced growth and caused seedling death, with the effect being greater in Q. ilex than in P. halepensis. An increase in the proportion of NO3at high N strongly enhanced growth relative to low N plants in P. halepensis but had minor effects in Q. ilex. Relatively more NH4 + in the fertilizer enhanced plant P concentration 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 NO3at 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.

An exponential fertilization dose–response model to promote restoration of the Mediterranean oak Quercus ilex

“…While seedlings of many forest tree species suffer damage beyond an EC of 2.5 dS m -2 (Jacobs and Timmer 2005), Q. ilex seedlings can tolerate salinity levels up to 3.0 dS m -2 (Miyamoto et al 2004;Uscola et al 2014), which is notably higher than the values obtained in this experiment. Physiological drought associated with high electrical conductivity is often responsible for toxicity responses in fertilization and it is relatively difficult to separate these causal mechanisms (Jacobs and Timmer 2005).…”

“…Similar values of substrate NO 3 --N concentration and pH without observed toxicity in Q. ilex were also reported by Uscola et al (2014). While seedlings of many forest tree species suffer damage beyond an EC of 2.5 dS m -2 (Jacobs and Timmer 2005), Q. ilex seedlings can tolerate salinity levels up to 3.0 dS m -2 (Miyamoto et al 2004;Uscola et al 2014), which is notably higher than the values obtained in this experiment.…”

“…from other biomes, i.e. 25 mg N applied per seedling in container Quercus rubra L. or 84 mg N applied per seedling in bareroot Quercus alba L. (Birge et al 2006), which may be associated with the low plasticity of holm oak seedlings to resource availability (Sardans et al 2006b;Uscola et al 2014). Nevertheless, Q. ilex had intermediate sufficiency levels compared to other Mediterranean oak species; for instance, Q. faginea reached the sufficiency level at 150 mg N seedling -1 , and Q. coccifera had a lower sufficiency level than Q. ilex, 100 mg N applied per seedling (Villar-Salvador et al 2013).…”

“…Nevertheless, studies conducted with this species show that low N fertilization impairs survival and growth (Villar-Salvador et al 2004a;Cuesta et al 2010b). Alternatively, excess fertilization may induce toxicity, often indicated by decreased plant growth and N content but elevated tissue N concentration, sometimes leading to mortality in the nursery (Uscola et al 2014).…”

Atmospheric concentration and deposition of reactive nitrogen in Spanish forests of Quercus ilex

“…The effects caused by the atmospheric deposition of N are proved to differ between the different type of inputs (dry/wet deposition) and forms (oxidized/reduced N) Uscola et al, 2014). It is, therefore, important to consider the different N forms and their relationship with the different pollutant sources and their respective atmospheric transport and chemical processing in order to provide a useful evaluation of N deposition .…”