Quantitative understanding of nutrient uptake by competing plants with different root systems can be a beneficial tool in designing the nutrient management strategies for competing plant environments. A field study was conducted at two sites (Alfalfa and Pasture site) near Meadow Lake, Saskatchewan, Canada to investigate the effect of weed competition on the growth and nitrogen (N) uptake by hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) using the Soil Supply and Nutrient Demand (SSAND) model. Hybrid poplar was grown with and without weeds (mainly dandelion (Taraxacum officinale F.H. Wigg.) and quackgrass (Elymus repens (L.) Gould) for 50, 79 and 100days. Above‐ground and below‐ground growth of hybrid poplar was adversely affected by the presence of weeds at both sites. At the end of the experimental period, hybrid poplar in the control treatment took up 15.8 and 15.7mmolNplant‐1 while seedlings in the weed treatments accumulated only 1.7 and 2.1mmolNplant‐1 at the Pasture and Alfalfa sites, respectively. The SSAND model predictions for hybrid poplar N uptake at the Pasture site were in close agreement with measured N uptake in the control treatment (slope=0.95–0.98). Model predictions for the Alfalfa site underestimated N uptake in the control treatment compared to measured values (slope=0.75–0.77). The SSAND model greatly underestimated N uptake by both hybrid poplar and weeds grown in the weed treatment. Including N mineralization as a model input also did not improve the N uptake predictions; however, including changing soil water content greatly improved the prediction of N uptake by both hybrid poplar and weeds. Results from this study suggest that weed control is an essential practice to successful establishment of hybrid poplar plantations. Furthermore, the SSAND model can be a potentially useful tool for enhancing our understanding of plant nutrient uptake under field conditions where the dynamic nature of soil moisture and mineralization processes influence plant growth.
