Contributions of nitrogen deposition and forest regrowth to terrestrial carbon uptake

Abstract

BackgroundThe amount of reactive nitrogen deposited on land has doubled globally and become at least five-times higher in Europe, Eastern United States, and South East Asia since 1860 mostly because of increases in fertilizer production and fossil fuel burning. Because vegetation growth in the Northern Hemisphere is typically nitrogen-limited, increased nitrogen deposition could have an attenuating effect on rising atmospheric CO2 by stimulating the vegetation productivity and accumulation of carbon in biomass.ResultsThis study shows that elevated nitrogen deposition would not significantly enhance land carbon uptake unless we consider its effects on re-growing forests. Our results suggest that nitrogen enriched land ecosystems sequestered 0.62–2.33 PgC in the 1980s and 0.75–2.21 PgC in the 1990s depending on the proportion and age of re-growing forests. During these two decades land ecosystems are estimated to have absorbed 13–41% of carbon emitted by fossil fuel burning.ConclusionAlthough land ecosystems and especially forests with lifted nitrogen limitations have the potential to decelerate the rise of CO2 concentrations in the atmosphere, the effect is only significant over a limited period of time. The carbon uptake associated with forest re-growth and amplified by high nitrogen deposition will decrease as soon as the forests reach maturity. Therefore, assessments relying on carbon stored on land from enhanced atmospheric nitrogen deposition to balance fossil fuel emissions may be inaccurate.

Contributions of secondary forest and nitrogen dynamics to terrestrial carbon uptake

“…5), where most of the secondary forest area is increasing with time and nitrogen deposition level is relatively high. While our estimate of global N deposition induced C uptake is comparable to estimates based on 15 N tracer field experiments (0.25 GtC/yr, Nadelhoffer et al, 1999) and results from other process-based models such as Thornton et al ( 2007) (0.24 GtC/yr) and Zaehle et al (2010) (0.2 GtC/yr); they are much lower than Churkina et al (2007) estimates, which suggest that land ecosystems could take up additional 0.75-2.21 GtC/yr in 1990s because of N deposi- tion. While the exact cause of this discrepancy is unclear, a few factors can be identified which may have contributed the difference in estimates.…”

“…While the exact cause of this discrepancy is unclear, a few factors can be identified which may have contributed the difference in estimates. The discrepancy could be related to the fact Churkina et al (2007) did not consider land use changes and assume land cover remains constant during 1980-1999 while in this study land cover changes are driven by historical land use change data including substantial forest regrowth. Additional reason could be the difference in the treatment of N cycle processes and carbon-nitrogen interactions in Churkina et al (2007) and this study.…”

“…The discrepancy could be related to the fact Churkina et al (2007) did not consider land use changes and assume land cover remains constant during 1980-1999 while in this study land cover changes are driven by historical land use change data including substantial forest regrowth. Additional reason could be the difference in the treatment of N cycle processes and carbon-nitrogen interactions in Churkina et al (2007) and this study. For example, in the BIOME-BGC model used in Churkina et al (2007), denitrification loss of N is assumed a fixed proportion of N mineralization.…”

“…Additional reason could be the difference in the treatment of N cycle processes and carbon-nitrogen interactions in Churkina et al (2007) and this study. For example, in the BIOME-BGC model used in Churkina et al (2007), denitrification loss of N is assumed a fixed proportion of N mineralization. While in ISAM-NC, denitrification loss of N is a function of available nitrate N, soil moisture, and heterotrophic respiration .…”

“…Churkina et al (2007) show that N deposition does not lead to significant C sink unless it occurs with the regrowth of forest. Jain et al (2009) While the C sinks associated with regrowth of forest are commonly simulated within terrestrial C cycle models, the impacts of N limitations and N deposition on the C sink associated with regrowing forests have not been considered.…”