Objective This paper aims to research the coupling effects of furrow irrigation and nitrogen fertigation on the growth and stand productivity of high-density and short-rotation triploid Populus tomentosa S86 pulp plantations, in order to provide scientific theoretical basis for selecting efficient furrow irrigation-fertilization technology system suitable for the local area.
Method The plant materials were triploid P. tomentosa S86 in Gaotang County, Shandong Province of eastern China. A completely random block experiment design was adopted to monitor three irrigation levels, i.e. irrigation started when the soil water potential reached −20 kPa (W1), −33 kPa (W2) and −45 kPa (W3), and four fertigation levels (N1, 120 kg/(ha·year); N2, 190 kg/(ha·year); N3, 260 kg/(ha·year); N4, 0 kg/(ha·year)), and the control treatment was with no irrigation and no fertigation set (CK).
Result It took a long time for the fast-growing period of DBH growth of triploid P. tomentosa S86. The DBH growth of different stand ages in the growing season conformed to the Logistic function (R2 > 0.99), and the difference of furrow irrigation and fertilization treatments was mainly manifested in June. Nitrogen application in furrow irrigation significantly affected the DBH increment and relative growth rate of 2-year-old P. tomentosa S86 plantations (P < 0.05), but had no significant effect on 3−5-year-old P. tomentosa S86 plantations. The coupling measures of water and fertilizer had a strong promotion effect on the average volume and average productivity of 2−3-year-old trees, but had no significant promotion effect on 4−5-year-old P. tomentosa S86 plantations. The average annual productivity of triploid P. tomentosa S86 stands was 19.65−25.31 m3/(ha·year) in four years, among which, W1N1 had the highest annual productivity, which was significantly higher than CK by 28.41% (P < 0.05). Under the treatment of 5-year-old stand W1N1, the tree growth was optimal, with unit area volume and average productivity reaching 104.39 m3/ha and 27.42 m3/(ha · year), respectively, which were 28.15% and 11.74% higher than CK treatment, respectively.
Conclusion The furrow irrigation and fertilizer coupling measures have different degrees of improvement on the unit area storage and annual productivity of 2−3-year-old triploid P. tomentosa S86 stand, but have no significant effect on the unit area storage and annual productivity of 4−5-year-old P. tomentosa S86 plantation. Among them, W1N1 furrow irrigation and nitrogen application treatment has the best growth. Under similar climatic and soil conditions, maintaining sufficient water (irrigation threshold of −20 kPa) and a lower nitrogen application rate (120 kg/(ha · year) is most conducive to the growth of high-density short-rotation triploid P. tomentosa targeting pulp forests.