Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO2 concentration

We examined effects of a first nitrogen (N) fertilizer application on upper-canopy needle morphology and gas exchange in similar to 20-m-tall loblolly pine (Pinus taeda L.) exposed to elevated carbon dioxide concentration ([CO2]) for 9 years. Duke Forest free-air CO2 enrichment (FACE) plots were split and half of each ring fertilized with 112 kg ha(-1) elemental N applied in two applications in March and April 2005. Measurements of needle length (L), mass per unit area (LMA), N concentration (N-1) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (A(a)) and per unit mass (A), and leaf conductance (90 began after the second fertilizer application in existing 1-year-old foliage (F-O) and later in developing current-year first-flush (F-C1) and current-year second-flush (F-C2) foliage. Elevated [CO2] increased A(a) by 43 and 52% in F-O and F-C1, foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or g(L) in F-O foliage; although N, was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, N-1, and A in F-C1, and F-C2 foliage, increasing A(a) by about 20%. These results suggest that, although both needle age classes accumulate N following fertilization, they use it differently-current-year foliage incorporates N into photosynthetic machinery, whereas 1-year-old foliage serves as an N store. There were no significant interaction effects of elevated [CO2] and fertilization on A. Elevated [CO2] increased the intercept of the A:N-1 relationship but did not significantly affect the slope of the relationship in either foliage age class.