| Poplar is one of the most promising short rotation plantation species.China, with nearly 7.5 million ha, is the country with the largest plantation of this species. However, since its productivity was rather low, production still cannot meet the demand. Poplar has higher nutrient requirements than most of the other plantation tree species, especially for N. Thus, N fertilization has been widely used as a critical intensive silvicultural practice in poplar plantations to enhance biomass production. As an alternative of conventional fertilization, potential advantages of N fertilization through subsurface drip irrigation system (SDI) including supply of N fertilizer directly to the center of the root zone and reduced N supply at the soil surface. N fertigation has been widely used in poplar plantations. However, the growth response of poplar to N fertigation is still inaccurately predicted and studies are required to improve site-specific N fertilization regime under fertigation. Thus, in order to make recommendations to N nutrition management strategy under drip irrigation for triploid Populus tomentosa plantation, playing the key role in pulp plantation in the North China Plain, the effects of different N fertigation regimes on tree growth, N uptake, biomass accumulation and its allocation, root distribution, soil nutrition dymanics were investigated in detail.A field experiment was conducted in triploid Populus tomentosa plantation under a subsurface drip irrigation (SDI) system. The experiment include six fertigation treatments, which were a factorial combination of three N application rates (115,230,345 kg ha-1 a-1 or N115, N230, N345 treatments, respectively), two application frequencies (two or four times during the growing season, F2 and F4, respectively) and a control (0 N with irrigation). The results show the fertigation treatments have positive effects on tree growth. The effect of N115 was significantly greater than the higher N application rates (N230 and N345). Trees in the N115F4 treatment achieved greatest growth, with volume and aboveground biomass reaching 30 m3 and 40.2 Mgha-1, respectively. While the control reached only 23.4 m3 and 25.1 Mg ha-1, respectively. Higher application frequencies did not affect biomass accumulation, but did significantly increase N uptake.A quadratic function was used to describe biomass response to N rate and to determine the dose of N fertilization required to maximize aboveground and total biomass under SDI. The optimal N rate (Nopt) estimated by this model was nearly 194.5 kg ha-1 a-1. The 90% confidence interval for Nopt was [247 kg ha-1,142 kg ha-1].N fertigation did not change the dymanics of leaf area index (LAI) and stem growth patterns. But it can improve the value of LAI and increased the stem growth rates.The average LAI in fertigaition treatments was 1.29 m2·m-2, while the control was 1.25 m2·m-2, although the difference was not significant. N fertigation and precipitation had synergistic effects on seasonal stem growth, the positive effects of N fertigation on stem growth declinedwith higher levels of precipitation. The low effectiveness of N fertigation combined with the effect of high precipitation on seasonal growth suggests that drainage practices should be considered when monthly precipitation exceeds 300 mm at this site.N fertigation only resulted in a shift in biomass allocation from roots to leaves, improved soil N availability resulted in a’true’shift toward more leaf per unit root biomass, while both scaled isometrically with increasing biomass. However, biomass allocation to shoot-root and wood-root were significantly affected by both fertigation and increasing biomass. These results imply that the effects of ontogenetic development and response to environmental resource availability on allometry are not mutually exclusive. Suggesting that scientists should consider separating perennial tissues from ephemeral tissues, as well as disentangling the effects of ontogenetic development and treatments on biomass allocation.In the 0-80 cm soil layer, the vertical distribution of fine roots accorded with the exponential decline law. Nitrogen fertigation treatments had a similar root vertical distribution patterns with control. However, the fine roots length density (FRLD) in 0-80 cm soil layer of N345 treatments were higer than that of control. Nitrogen feritigation frequency had significant effect on FRLD, F2 treatments exhibited greater FRLD than F4 treatments. The fine root distribution of triploid Populus tomentosa tended to be shallower with 0-20 cm soil layer. Nearly 67-80% of 0-80 soil layer fine roots occurred in 0-20 cm soil layer. Compared with nitrogen fertigation treatments, control treatment allocation more roots in the shallow soil layer.The N rates had significant effects on the mean fine root diameter (MFRD). The MFRD of N345 showed more thiner than control, this difference were more obvious in the deeply soil layer (60-80 cm).The FRLD showed positive correlation with soil nutrition. The available P was the most powerful factor, which can explain 33.5% variability of FRLD.Under fertigation treatment, increasing N rate significantly improved the nitrate concentration in 0-80 cm soil layer. The dymanics of nitrate concentration of N 155 showed two main peaks, while the other two N rate (N 230 and N 345) showed several peaks, these difference were more pronounced in the deeply soil layer. Higher fertigation frequency could mitigate volatility of dymanics of nitrate, and this effect was more obvious with higher N rates. N115F4 treatment can synchronize the nitrate supply with tree growth, which indicated that triploid Populus tomentosa may favour nitrate than ammonium. In conclusion, under SDI conditions, N application rates greater thanN115 did not result in increased yield, suggestingthat the N application rate commonly applied to Populustomentosaplantations can be markedly reduced. |
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