Abstract
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, the terrestrial ecosystem could have lost about 152 PgC during the same period. Therefore, since pre-industrial times terrestrial carbon losses due to warming may have been more or less 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 that climate warming effects on carbon storage may overwhelm N deposition effects in the future.
Highlights
Though nitrogen is the most abundant element in the atmosphere, most organisms, including plants and animals, cannot use it in its most common form (N2)
Bala et al.: Nitrogen deposition: how important is it for global terrestrial carbon uptake?
Bpraesleanet dtaayld.:epNoistitrioong.en deposition: how important is it for global terrestrial carbon uptake?
Summary
Though nitrogen is the most abundant element in the atmosphere, most organisms, including plants and animals, cannot use it in its most common form (N2). It can only be used in its reactive forms of NOy and NHx, which when deposited on the surface through various processes, is generally referred to as “nitrogen deposition”. The rate is estimated to be ∼ 10.8 TgN yr−1 (Tera grams of nitrogen per year) around 1765 (Galloway et al, 2004; Jain et al, 2009) and around 17.4 TgN yr−1 by 1865 (Galloway et al, 2004).
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