Nitrogen and phosphorus dynamics for 13-year-old loblolly pine stands receiving complete competition control and annual N fertilizer

Because stand growth and the uptake of nitrogen (N) and phosphorus (P) are proportional under most circumstances, a goal of silvicultural operations should be to increase uptake of nutrients by crop trees. The objectives of this study were to determine the effects of annual fertilization (average of 72 kg N ha−1 y−1 and total of 154 kg P ha−1) and complete elimination of interspecific competition on stand growth, N and P uptake, N and P budgets, and fertilizer retention in 13-year-old of loblolly pine (Pinus taeda L.) stands growing in the Lower Coastal Plain of Georgia, USA. At age 13, fertilization increased average tree height from 14.8 to 19.3 m, average tree diameter from 14.5 to 19.3 cm, and stand basal area from 27.6 to 42.6 m2 ha−1. Competition control increased average tree height from 16.5 to 17.5 m, average tree diameter from 16.3 to 17.6 cm, and stand basal area from 31.9 to 38.3 m2 ha−1. The relative amount of biomass partitioned among the aboveground biomass components was not affected by treatments, but fertilization and complete competition control decreased the biomass of fine roots, a trend opposite that for aboveground biomass. In general, complete competition control had little effect on nutrient concentrations or contents. Annual N uptake at age 13 was approximately 100 and 60 kg ha−1 in the fertilized and nonfertilized stands, respectively. Annual uptake for P in nonfertilized stands averaged 7.5 kg ha−1 while that in fertilized stands averaged 10 kg ha−1. With the exception of stem bark N concentration and foliar P concentration, fertilization increased the N and P concentrations of the aboveground biomass components. Nitrogen contents in the aboveground biomass ranged from less than 160 kg ha−1 in the control stands to almost 410 kg ha−1 in the fertilized stands. Phosphorus contents ranged from approximately 12 kg ha−1 in the control stands to 27 kg ha−1. In contrast to the aboveground biomass, standing fine root N contents were 15.2, 16.4, 8.7, and 7.6 kg ha−1 while standing P contents were 2.22, 1.73, 1.09, and 0.95 kg ha−1 for the control, fertilize only, competition control only, fertilizer + competition control treatments, respectively. The large N and P uptake in these stands facilitated an estimated retention of 90% of N fertilizer and >100% of P fertilizer in the soils, O horizon, and living biomass of the fertilized stands. These results underscore the importance of nutrition to stand growth, raise the bar for the potential productivity of loblolly pine in the southeastern United States, and indicate that it is possible to aggressively fertilize these nutrient poor systems with potentially few offsite impacts.