Abstract
The terrestrial carbon sink has increased since the turn of this century at a time of increased fossil fuel burning, yet the mechanisms enhancing this sink are not fully understood. Here we assess the hypothesis that regional increases in nitrogen deposition since the early 2000s has alleviated nitrogen limitation and worked in tandem with enhanced CO2 fertilization to increase ecosystem productivity and carbon sequestration, providing a causal link between the parallel increases in emissions and the global land carbon sink. We use the Community Land Model (CLM4.5‐BGC) to estimate the influence of changes in atmospheric CO2, nitrogen deposition, climate, and their interactions to changes in net primary production and net biome production. We focus on two periods, 1901–2016 and 1990–2016, to estimate changes in land carbon fluxes relative to historical and contemporary baselines, respectively. We find that over the historical period, nitrogen deposition (14%) and carbon‐nitrogen synergy (14%) were significant contributors to the current terrestrial carbon sink, suggesting that long‐term increases in nitrogen deposition led to a substantial increase in CO2 fertilization. However, relative to the contemporary baseline, changes in nitrogen deposition and carbon‐nitrogen synergy had no substantial contribution to the 21st century increase in global carbon uptake. Nonetheless, we find that increased nitrogen deposition in East Asia since the early 1990s contributed 50% to the overall increase in net biome production over this region, highlighting the importance of carbon‐nitrogen interactions. Therefore, potential large‐scale changes in nitrogen deposition could have a significant impact on terrestrial carbon cycling and future climate.
Highlights
Fossil fuel CO2 emissions have rapidly increased since the turn of this century, at rates almost doubling those of the previous three decades (Hansen et al, 2013)
Nitrogen limitation (N‐lim) is a key metric in assessments of carbon‐nitrogen coupling and is directly estimated in CLM4.5‐BGC through the ratio of actual GPP to potential GPP (GPP that would occur without nitrogen limitation) at each time step and is a scalar between 0 and 1, with high/low N‐lim values indicating low/high nitrogen limitation
In diagnosing our model results, we evaluate N‐lim along with the Γ factor that expresses the sensitivity of terrestrial carbon storage to atmospheric CO2
Summary
Fossil fuel CO2 emissions have rapidly increased since the turn of this century, at rates almost doubling those of the previous three decades (Hansen et al, 2013). The terrestrial carbon sink (estimated as the residual in the global carbon budget; Le Quéré et al, 2016) does exhibit a sharp increase since the early 2000s. An increasing terrestrial carbon sink since the early 2000s is consistent with independent lines of evidence based on forest inventories (Pan et al, 2011) and process‐based modeling studies (e.g., Le Quéré et al (2018)). Various observation‐based (Clark et al, 2013; Los, 2013; Norby et al, 2005; Terrer et al, 2016) and modeling (Cheng et al, 2017; Keenan et al, 2016; Schimel et al, 2015; Sitch et al, 2015; Zhu et al, 2016) studies have highlighted the role elevated CO2 levels have on photosynthesis and water‐use efficiency in explaining the increase in the terrestrial carbon sink over recent decades. If increased plant carbon uptake via the CO2 fertilization effect alone was the main driver behind the increase in global net carbon uptake since the turn of this century, we would expect a more transient
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
