ObjectiveFast-growing and high-yield poplar forest is the main pulp and timber forest in China. However, lower water and fertilizer use efficiency causes soil degradation, environmental pollution and even decline in forest yield. Therefore, it is urgent to explore appropriate water and fertilizer coupling strategies to achieve sustainable development. In this study, we investigated the effects of different water and fertilizer coupling treatments on the soil nitrogen distribution, root distribution and forest yield, aiming to screen out the optimal water and fertilizer coupling measures for the yield of Populus tomentosa plantation.

MethodIn this experiment, triploid Populus tomentosa was used as the research object. Three irrigation levels: − 20 kPa (I₂₀), − 33 kPa (I₃₃), − 45 kPa (I₄₅), four application N levels: 0 (F₀), 120 kg/(hm²·year) (F₁₂₀), 190 kg/(hm²·year) (F₁₉₀), 260 kg/(hm²·year) (F₂₆₀), and a CK control were set to determine the soil NO₃⁻-N dynamics, absorption root distribution and the biomass yield throughout the year.

Result(1) The amount of N applied, and the amount of irrigation significantly increased the migration, distribution and deep accumulation of NO₃⁻-N. Among them, the deep leaching of NO₃⁻-N treated by I₂₀F₂₆₀ was serious, while the amount of leaching of I₂₀F₁₂₀ was the smallest. (2) The coupling of water and fertilizer can make the root distribution shallow; the supply of N fertilizer can change the distribution of root system, and the irrigation promotes the surface distribution of roots; the response of root growth to N depends on the irrigation level, and I₂₀F₂₆₀ treating the root length density (RLD) was significantly better than other treatments (P < 0.05). (3) NO₃⁻-N content, RLD, and whole plant biomass (TB) of different treatments showed significant correlations with each other. There was a significant parabolic regression relationship between soil NO₃⁻-N content and RLD (P < 0.05). Therefore, soil nutrient difference is an important factor in the difference of fine root distribution. The RLD and TB in 0 – 10 cm and 20 – 40 cm soil layers showed a very significant parabolic regression relationship (P < 0.01). It can be seen that an appropriate amount of RLD in 0 – 10 cm, 20 – 40 cm soil layer is beneficial to increase forest yield. In this study, the annual average biomass of forest trees was highest with I₂₀F₁₂₀ water and fertilizer coupling treatment.

ConclusionThe amount of N applied by I₂₀F₁₂₀ is one quarter of the traditional amount of N applied, and the deep NO₃⁻-N leaching amount is the smallest, which effectively reduces the pollution to the environment; in addition, it can form an appropriate amount of RLD to promote the absorption and utilization of nutrients, and ultimately the formation of higher forest yields and higher water and fertilizer utilization. Therefore, I₂₀F₁₂₀ is recommended as the best water and fertilizer coupling measure.