Have Synergies Between Nitrogen Deposition and Atmospheric CO₂ Driven the Recent Enhancement of the Terrestrial Carbon Sink?

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 CO 2 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 CO 2 , 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 CO 2 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.

Saturation of Global Terrestrial Carbon Sink Under a High Warming Scenario

“…The CO 2 ‐caused sink depends on the balance between carbon uptake by CO 2 fertilization effects and carbon emissions by ecosystem respiration. The CO 2 fertilization effect can be limited by climate factors, and is most pronounced in the tropics, followed by northern mid‐latitudes and boreal latitudes (O'Sullivan et al., 2019; Schimel et al., 2015; Sitch et al., 2015). Ecosystem respiration is mainly regulated by temperature, water availability, microbial activity and carbon substrate (Davidson et al., 1998; Migliavacca et al., 2011).…”

“…Houghton (2010) reported that inventory‐ or satellite‐based estimates of carbon emissions from global land use change could be from 0.9 to 2.2 Pg C yr −1 in the period 1990–1999. Nitrogen deposition was estimated to contribute 14% to current terrestrial carbon sink while its synergistic effect with CO 2 contributed another 14% from 1901 to 2016 (O'Sullivan et al., 2019). But debates exist to the extent to which nitrogen deposition can enhance terrestrial carbon sequestration as minor (e.g., Nadelhoffer et al., 1999) to significant (e.g., Janssens et al., 2010) impacts are reported.…”

A review of the major drivers of the terrestrial carbon uptake: model-based assessments, consensus, and uncertainties

“…The terrestrial biosphere is a sink for carbon in recent decades due to mainly the increases in net primary production (NPP=GPP − R h ), driven by the increases in atmospheric CO 2 (Friedlingstein et al 2010, Le Quéré et al 2015, Sitch et al 2015, O'Sullivan et al 2019. Modeling studies also project changes in the land carbon uptake in future scenarios in response to the changes in climate forcings (e.g.…”

Resistant Soil Microbial Communities Show Signs of Increasing Phosphorus Limitation in Two Temperate Forests After Long-Term Nitrogen Addition

“…In temperate and boreal forests, reactive N inputs have been shown to alter understory biomass (Gundale et al, 2014), accelerate soil acidification and base cation loss (Högberg et al, 2006;Tian and Niu, 2015;Forstner et al, 2019), and increase N leaching (Moldan and Wright, 2011;Schleppi et al, 2017). At the same time, atmospheric N inputs can promote plant CO 2 fixation and thus counteract global warming (Townsend et al, 1996;Norby et al, 2010;Fernández-Martínez et al, 2014;Wang et al, 2017;O'Sullivan et al, 2019).…”