Objective  As one of the important fast-growing and high-yielding species in the North China Plain, the triploid Populus tomentosa is an important resource for the timber reservation strategy of China. Drip irrigation and nitrogen fertigation (DIF) technique has been widely used in plantation cultivation. Figuring out the responses of fine root growth, morphology traits and distribution pattern to DIF practices, and identifying the important environmental factors affecting fine root growth are of great significance for the formulation of precise water-nitrogen coupling strategies.

  Method  The plant materials were 2-year-old P. tomentosa plantations on the sandy loam soil, and twelve DIF treatments were a factorial combination of three irrigation levels (−20 kPa (W20), −33 kPa (W33), −45 kPa (W45)) and four fertigation levels (0 (N0), 80 kg/(ha·year) (N80), 150 kg/(ha·year) (N150), 220 kg/(ha·year) (N220)), and the control treatment was with no irrigation and no fertigation. We monitored fine root traits and distribution patterns in 5 treatments, including W20N220, W20N80, W20N0, W45N220 and W45N0. Besides, we did the correlation analysis of soil properties (organic matter, total nitrogen, ammonium nitrogen, nitrate nitrogen, and soil water content) and fine root growth traits.

  Result  (1) DIF treatments and soil depth had significant effects on fine root growth and morphology traits (P < 0.05). W20 associated treatments significantly stimulated fine root growth in 0−20 cm soil layers (P < 0.05), and the fine root accumulated in the shallow soil layers. The specific root length was significantly higher in 0−30 cm soil layers in W45N0 treatment (P < 0.05). (2) Fine root accumulated in the shallow soil layers and decreased with the increment of soil depth in W20 associated treatments, in contrast with the relatively uniform vertical distribution in W45 associated treatments. Fine root biomass accumulated near the trunk and decreased as the horizontal distance increased, and there was no significant difference of fine root biomass in W20N220 and W45N0 treatment in different horizontal locations. (3) Organic matter, ammonium nitrogen and nitrate nitrogen had significant positive correlations with fine root growth trait (P < 0.05), and the order of correlation was organic matter > nitrate nitrogen > ammonium nitrogen.

  Conclusion  W20 associated treatments significantly stimulate fine root growth in the shallow soil layer, and the nitrogen fertigation has less effect on fine root traits; fine root growth and some morphological traits response to the resource heterogeneity in different soil layers for better resource acquisition rather than the mass plasticity in 2-year-old P. tomentosa plantations on the North China Plain. DIF practices should be applied with a little amount of water and nitrogen several times in 0−30 cm soil layers, which are concentrated with fine roots to improve water and nitrogen use efficiency.