Effects of air humidity and soil nitrogen source on diurnal water flux in hybrid aspen and silver birch

A growing body of evidence suggests that water uptake of plants depends on available N forms. The objective of this study was to investigate tree water flux at different levels of relative humidity (RH) of air, soil N source (ammonium, nitrate), and N contents. Aspen saplings demonstrated lower (pKLEINERDAN0.05) night-time and daytime sap flux densities (F) under high-RH treatment (diurnal RH ~80%) compared with a moderate-RH treatment (RH ~80 and 65% during night and day, respectively). The soil-to-leaf hydraulic conductance (G_T) declined toward the end of the night, whereas the late-night and daytime G_T did not differ between the RH treatments. The soil N source and content did not affect diurnal F and G_T in aspen. In birch saplings, daytime measurements showed the lowest F, G_T, stem hydraulic conductance, specific and leaf-specific conductivity of xylem in the high-RH treatment. No significant differences were observed in F and G_T between the RH treatments in birch at night. In contrast with aspen, the late-night G_T was lower (pKLEINERDAN0.05) than the daytime G_T in both high and moderate-RH treatments. In birch, the daytime F was higher (p<0.05) in nitrate compared to ammonium-fed plants, whereas the night-time F did not depend on the N source. Although G_T was not affected by the N source, the hydraulic conductance of the root system and the whole-plant hydraulic conductance were higher (p<0.05) in nitrate-fed plants. The results suggest that RH and soil N source modify F and hydraulic efficiency of trees, whereas the trees' responses evidently depend on water and N-use strategies of particular species.