Assessing the nutrition of juvenile hybrid poplar using a steady state technique and a mechanistic model

Fiber farming utilizing fast growing trees provides a means to satisfy the increasing demand for hardwood fiber. To maximize growth during the establishment phase, it is important to understand the relationship between maximal growth rate, plant nutrient requirements, and the ability of the soil to supply nutrients. The objectives of this study were: (i) to use a steady state nutrition technique to establish threshold N, P, and K ratios associated with maximal growth and (ii) to use the Barber–Cushman nutrient uptake model as a means of coupling information on the nutrient supplying capacity of the soil with the optimum N:P:K ratio. Growth chamber and greenhouse studies, using Populusnigra×maximowiczii (NM-6), were conducted to determine maximal growth rate, optimal N, P, and K content, uptake kinetics, and soil supply characteristics. Maximal growth rate was 0.12 g g−1 per day at a whole plant ratio of 100N:11P:37K. The Barber–Cushman model was run using soil supply and root growth data from the greenhouse study. Nitrogen, P, and K uptake were simulated for a 105-day period in response to the addition of a slow release 17–6–12 fertilizer at rates equivalent to 0, 75, or 150 kg ha−1 of N. Model predictions of uptake improved as the amount of fertilizer added increased. Uptake estimates at the 150 level were 96, 120, and 98% of observed uptake for N, P, and K, respectively. The model predicted that the supply of N was not adequate to support sustained plant uptake throughout the study period. Plant uptake and soil supply observations confirmed that N uptake occurred primarily in the first half of the growth period and that soil N supply was quickly depleted. Model simulations of P uptake support earlier observations that uptake is not solely a function of supply. A 10-fold increase in simulated supply increased uptake by a factor of 6. Much more needs to be done before a soil supply model like Barber–Cushman can be used as a nutrient management tool in forestry applications. However, establishing optimum levels, ratios, and rates of nutrient addition provides a good starting point for further evaluation.