Aims Irrigation and fertilization have great potentials to enhance yield in forest plantations. The integrated effect of water and nitrogen management on fine roots morphology and distribution of Populus × euramericana ‘Guariento’, however, remains unclear. The objective of this study was to evaluate the effect of water and nitrogen addition on fine root morphology and distribution in poplar plantations for developing the best water and nitrogen strategy for promoting fine root.
Methods The soil core method was used to quantify the morphology and distribution of fine roots in the 0–60 cm in a poplar plantation with surface dripping irrigation and fertilization technologies. The experiment included nine treatments, which were a combination of three irrigation treatments where dripping irrigation was applied when soil water potential (ψsoil) reached –75, –50, or –25 kPa, and three fertilization treatments at nitrogen additions of 150, 300, or 450 g·tree–1·a–1, respectively). A control plot with non-irrigation and non-fertilization treatment in growing season (CK) was also included in the study.
Important findings The fine roots biomass density, fine root surface area density, average root diameter in all treatments were mainly found at 0–10 cm and 10–20 cm depths, with root biomass density in the 10–20 cm of 1.03 to 1.21 times of that in the 0–10 cm, 1.25 to 1.80 times of that in the 20–30 cm, 1.62 to 22.10 times of that in the 30–40 cm, 2.77 to 54.35 times of that in the 40–50 cm, and 6.48 to 293.09 times of that in the 50–60 cm. The root biomass density in the 10–20 cm accounted for 27%–37% of the total biomass density in the top 60 cm. For root biomass density and average diameter, there were no significant differences between 0–10 cm and 10–20 cm depths, and between 40–50 cm and 50–60 cm depths. Fine roots in the irrigation and fertilization treatments were significantly higher than that of the CK, except the D1F1 treatment (i.e., with low water and low nitrogen level). Additionally, fine roots in the D2F3 treatment (i.e., with intermedia irrigation and high nitrogen level) and the D3F3 treatment (i.e., with high water and high nitrogen level) were significantly higher than those in other treatments, but not significantly different between D2F3 and D3F3. Compared with the CK, the fine roots biomass density in six soil layers were significantly enhanced at 359%, 388%, 328%, 3823%, 4774% and 2866%, respectively, for the treatment with high water and high nitrogen levels. The vertical distributions of fine roots appeared not affected by the interaction of irrigation and nitrogen addition. However, the surface dripping irrigation and fertilization treatments increased fine roots significantly. Finally, we found that the response of fine root growth and distribution was stronger to fertilization than to the irrigation in this poplar plantation.