Effect of humic substances on nitrogen uptake and assimilation in two species of Pinus

The effects of a humic fraction, with a high molecular weight (Hsp) extracted from a soil with a Fagus sylvatica vegetal cover were compared on ammonium (NH+ 4) and nitrate (NO− 3) uptake in two different coniferous species (Pinus pinaster and Pinus laricio). The activity of malate dehydrogenase (MDH; EC 1.1.1.37), glutamate dehydrogenase (GDH; EC 1.4.1.3) and phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) key enzymes involved in nitrogenmetabolism were tested. The data obtained indicate that the humic fractions, at all concentrations, increased ammonium uptake by both species, while nitrateuptake was unaffected. In P. laricio seedlings this fraction increased only leaf malate dehydrogenase activity, while in P. pinaster seedlings an increase of glutamate dehydrogenase in leaves, and of phosphoenolpyruvate carboxylase activity in both roots and leaves was observed. These results show that humicsubstances stimulate only ammonium uptake and differently influence ammonium assimilation in the two species of Pinus. This diverse behavior might be related to the different environmental conditions where these two species live.