Aims Triploid Populus tomentosa, a main clone for pulpwood forestry in the North China Plain, is usually planted using a wide- and narrow-row spacing planting schemes (WNRPS). Our objectives were to make water and fertilizer management recommendations for P. tomentosa under a WNRPS based on root form characteristics, and determine the relationship between the vertical distribution of P. tomentosa fine roots (FR) and soil nutrient factors.
Methods We obtained 2106 soil samples by soil coring down to 150 cm depth around eight sample trees in a five-year-old P. tomentosa plantation located in Gaotang County, Shandong Province, China. The samples were used to measure root distribution, organic matter (OM), available P and alkaline N content.
Important findings FR biomass density (FRBD) in the wide row zone (WRZ) decreased with increasing depth in the 0-30 cm soil, but was distributed evenly below 30 cm depth (p = 0.079). The vertical FRBD profile in the narrow row zone (NRZ) showed a bimodal pattern. In different layers at 10-150 cm depth, FRBD in the NRZ was 17%-148% higher than in the WRZ. With increasing depth in both WRZ and NRZ, coarse root biomass density (CRBD) increased and then decreased. However, the FRBD to CRBD ratio (F/C) varied slightly with depth (p > 0.05), and the mean F/C in the NRZ was 60% higher than that in the WRZ. Horizontally, in both WRZ and NRZ, the FRBD distribution was almost even, while the CRBD and F/C decreased and increased, respectively, with increasing distance from the trunk. On a two dimensional scale, the FRBD distribution within the NRZ was relatively even, but FR were mainly concentrated in surface soil within the WRZ. CRBD was distributed asymmetrically around the trunk. The ranges of 0-20 cm depth and 160-300 cm distance from the tree were the areas with high mean FRBD and F/C in the WRZ, which were 1.8 and 0.1 times larger than the mean values of corresponding variables in the WRZ, respectively. FRBD scaled positively with OM, available P and alkaline N content at 0-30 cm depth, but showed no obvious trend with respect to soil characteristics below 30 cm depth. In conclusion, the differences in P. tomentosa root distribution between WRZ and NRZ were reflected in the one-dimensional vertical FR distribution and two-dimensional FR and coarse roots distribution. OM, available P and alkaline N were key controls on the vertical FR distribution at 0-30 cm depth, but had no effects on FR distribution below 30 cm depth. For P. tomentosa plantations under WNRPS, water should be provided mainly to the NRZ, and slow- and fast-release fertilizer should be supplied to the shallow soil in the area near the wide row center and in the NRZ, respectively.