Foliar elemental composition of loblolly pine (Pinus taeda, L.) forest after long-term exposure to elevated CO2 and nitrogen fertilization

Background/Question/Methods

The relationship between soil nutrient availability, canopy biomass production, and nutrient content will determine the potential for a forest to capture and store carbon.  At the Duke Free Air CO₂ Enrichment site (FACE), long-term exposure to elevated CO₂ has resulted in sustained increases (22-30%) in plant net primary production (NPP) in a loblolly pine forest.  Soil nitrogen (N) availability controlled the spatial variation in NPP among CO₂ treatment plots.  To explore further the role of N on the CO₂ response, each ring in the FACE experiment was divided in half and one-half fertilized annually with 112 kg N ha^-1.  This paper examines the temporal and spatial variation in foliar macronutrients (N, P, Mg, Ca, and K) concentration, content, retranslocation, internal balance, and variation within the crown after 14 years of elevated CO₂ (C) exposure and 5 years of N fertilization.  Two questions were posed: 1. Has long-term exposure to elevated CO₂ induced nutrient deficiency (N and possibly other nutrients), altered foliage and litter nutrient contents, and retranslocation?, and 2.  Will N-only fertilization induce foliar nutrient imbalances with P or other nutrients and if so, do these imbalances affect the CO₂-induced increase in NPP? 

Results/Conclusions

Results indicate that the chemical composition of foliage was largely unaffected by elevated CO₂ while N fertilization increased [N] and decreased [P].  There was a marginal CxN interaction (p=0.08) for [N] in one-year-old foliage, where [N] was increased to a lesser extent under elevated CO₂.  Nitrogen concentration increased with crown height, while concentrations of P, Mg, Ca, and K decreased.  Nutrient retranslocation in one year-old foliage ranged from 46.2-88.2%, 46.8-88.0%, 10.3-60.0%, 58.0-93.6%, and <15% respectively for N, P, Mg, K, and Ca and generally were greater in upper than lower canopy.  In fertilized plots, N retranslocation increased in ambient CO₂ and decreased in elevated CO₂ (CxN: p=0.02) suggesting a reduction in nutrient use efficiency in the elevated and fertilized treatments.  In general, the effect of elevated CO₂ on canopy nutrient dynamics were not large.  N-fertilization increased foliar N and decreased P, but had little effect on other nutrients and there were few CxN interactions.  N-fertilization resulted in significant reductions in P:N and K:N ratios, which could have long-term implications for stand growth.