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Is increased nitrogen availability predictive for long-term forest carbon sequestration?
Bauer, Guntram1, Minocha, Rakesh2, Berntson, Glenn3, Aber, John3, Bazzaz, Fakhri1, 1 2 3
ABSTRACT- Temperate forests are predicted to play a key role as important sinks for atmospheric carbon dioxide, which could be enhanced by nitrogen 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 N deposition effects N partitioning, photosynthetic capacity and foliage productivity. Our study is based at the Harvard Forest (MA), where a 12-year addition of N on both coniferous and deciduous forests is underway. The measurements within a Pinus resinosa stand demonstrate that foliar N content has significantly increased in this species, and that this increase is accompanied by a de-coupling of the photosynthesis-N relationship. Conifers of the high N treatment do not use the surplus of N to synthesize more Rubisco, which would allow them to have a higher photosynthetic capacity. Instead this N is being accumulated as soluble protein, arginine 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. Stand level measurements further indicated a decrease in leaf area and stem growth. These results suggest, that long-term input of high dosages of mineral N into this pine stand have not been conducive for a higher net carbon sequestration.
KEY WORDS: nitrogen deposition, photosynthesis, Rubisco, carbon sequestration