Biogeosciences Discuss., 10, 11077-11109, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG.
Nitrogen deposition: how important is it for global terrestrial carbon uptake?
G. Bala1, N. Devaraju1, R. K. Chaturvedi2, K. Caldeira3, and R. Nemani4
1Divecha Center for Climate Change & Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore 560012, India
2Center for Sustainable Technologies, Indian Institute of Science, Bangalore 560012, India
3Department of Global Ecology, Carnegie Institution, 260 Panama Street, Stanford, CA 94305, USA
4NASA Ames Research Center, Moffett Field, CA 94035, USA

Abstract. Global carbon budget studies indicate that the terrestrial ecosystems have remained a~large sink for carbon despite widespread deforestation activities. CO2-fertilization, N deposition and re-growth of mid-latitude forests are believed to be key drivers for land carbon uptake. In this study, we assess the importance of N deposition by performing idealized near-equilibrium simulations using the Community Land Model 4.0 (CLM4). In our equilibrium simulations, only 12–17% of the deposited Nitrogen is assimilated into the ecosystem and the corresponding carbon uptake can be inferred from a C : N ratio of 20:1. We calculate the sensitivity of the terrestrial biosphere for CO2-fertilization, climate warming and N deposition as changes in total ecosystem carbon for unit changes in global mean atmospheric CO2 concentration, global mean temperature and Tera grams of Nitrogen deposition per year, respectively. Based on these sensitivities, it is estimated that about 242 PgC could have been taken up by land due to the CO2 fertilization effect and an additional 175 PgC taken up as a result of the increased N deposition since the pre-industrial period. Because of climate warming, terrestrial ecosystem could have lost about 152 PgC during the same period. Therefore, since preindustrial times terrestrial carbon losses due to warming may have been approximately compensated by effects of increased N deposition, whereas the effect of CO2-fertilization is approximately indicative of the current increase in terrestrial carbon stock. Our simulations also suggest that the sensitivity of carbon storage to increased N deposition decreases beyond current levels, indicating climate warming effects on carbon storage may overwhelm N deposition effects in the future.

Citation: Bala, G., Devaraju, N., Chaturvedi, R. K., Caldeira, K., and Nemani, R.: Nitrogen deposition: how important is it for global terrestrial carbon uptake?, Biogeosciences Discuss., 10, 11077-11109, doi:10.5194/bgd-10-11077-2013, 2013.
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