Effects of chronic N additions on tissue chemistry, photosynthetic capacity, and carbon sequestration potential of a red pine ( Pinus resinosa Ait.) stand in the NE United States

Abstract

Temperate forests are predicted to play a key role as important sinks for atmospheric carbon dioxide, which could be enhanced by nitrogen (N) deposition. However, experimental evidence suggests that the impact of N deposition on temperate forest productivity may not be as great as originally assumed. We investigated how chronic N addition affects needle morphology, nitrogen metabolism (or partitioning) photosynthetic capacity and foliage productivity. The investigation is based at the Harvard Forest (MA) as part of a now 15-year long N amendment study. Foliar N content in red pine ( Pinus resinosa) of the high N treatment has significantly increased, but this increase was accompanied by a de-coupling of the photosynthesis–N relationship. In addition needle longevity in the high N trees was significantly lower compared to the control trees. Conifers of the high N treatment did not use the surplus of N to optimize the amount of photosynthetically active metabolites. Instead N accumulated as soluble protein (other than Rubisco), amino acids and chlorophyll. Photosynthetic capacity in the control trees was about 50% higher than in the fertilized trees. These results indicate that the increase in leaf N is not accompanied by a greater capacity for carbon assimilation in the high N treatment. Using a simple model (PnET-Day) of canopy photosynthesis and carbon allocation, we assessed the long-term effect of these physiological changes on ecosystem carbon balance. The model results emphasize and reinforce the large difference between rates of carbon accumulation predicted to occur if net photosynthesis remained linearly related to foliar N concentration, and rates measured in the field.