[Effects of water and nitrogen coupling under drip irrigation on tree growth and soil nitrogen content of Populus ×euramericana cv. 'Guariento'.]

By using surface drip irrigation, a field experiment including nine drip irrigation and fertigation treatments was carried out, with non-irrigation and non-fertilization as control (CK), to evaluate the integrative effects of water and nitrogen management on the increment of diameter, tree height, and stem volume, and the total nitrogen content in the 0-60 cm soil layer in the poplar (Populus ×euramericana cv. 'Guariento') plantations. There were three irrigation levels (irrigation was initiated when soil water potentials at 20 cm soil depth reached -75, -50 and -25 kPa) and three levels of nitrogen addition (150, 300 and 450 g·tree-1·a-1). Surface drip irrigation and fertigation treatments were applied in the gro-wing seasons of 2012 and 2013. The results showed that the combined treatments of water and nitrogen addition significantly increased growth and stem volume of the poplar plantation. In the first year of the experiment, the annual increment of stem volume was 11.54 m3·hm-2·a-1 in high water and high fertilizer treatment (soil water potentials of -25 kPa + nitrogen addition of 450 g·tree-1·a-1), which was 44.1% higher than that in the CK (8.01 m3·hm-2·a-1). In the second year, the annual increment of stem volume was 27.85 m3·hm-2·a-1 in medium water and high fertilizer treatment (soil water potentials of -50 kPa + nitrogen addition of 450 g·tree-1·a-1), being 36.0% higher than that in CK (20.48 m3·hm-2·a-1). The successive combined treatments signi-ficantly increased the total N content in the 0-20 cm soil layer, with total N content of each soil layer in the first and second year of the experiment being 12.3%-59.4% and 71.1%-81.1% higher than CK, respectively. The increments of diameter and tree height were significantly positively correlated with soil total N content. Nitrogen addition and the interactive effects of water and nitrogen addition significantly affected the diameter and height of trees and soil N content, while irrigation showed no significant effect. In conclusion, the combined drip irrigation and fertigation significantly increased growth and stem volume of the poplar plantation by promoting the soil fertility especially nitrogen availability in the topsoil.