Predicting long‐term carbon sequestration in response to CO₂ enrichment: How and why do current ecosystem models differ?

Abstract

Large uncertainty exists in model projections of the land carbon (C) sink response to increasing atmospheric CO 2 . Free-Air CO 2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO 2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO 2 over decades to centuries, we used a suite of seven models to simulate the Duke and Oak Ridge FACE experiments extended for 300 years of CO 2 enrichment. We also determine key modeling assumptions that drive divergent projections of terrestrial C uptake and evaluate whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial C pools resulting from CO 2 enrichment, though there was substantial variability in quasi-equilibrium C sequestration and rates of change. In two of two models that assume that plant nitrogen (N) uptake is solely a function of soil N supply, the net primary production response to elevated CO 2 became progressively N limited. In four of five models that assume that N uptake is a function of both soil N supply and plant N demand, elevated CO 2 led to reduced ecosystem N losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots which reduced the vegetation turnover rate and increased C sequestration. In addition, self-thinning assumptions had a substantial impact on C sequestration in two models. Accurate representation of N process dynamics (in particular N uptake), allocation, and forest self-thinning is key to minimizing uncertainty in projections of future C sequestration in response to elevated atmospheric CO 2 .

Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review

“…Regarding the soil, they have shown that C stock increases when nitrogen is available and does not vary when nitrogen is limiting, despite increased input via litter production (Hungate et al 2009). These data were compared with large-scale model outputs for different output variables (Walker et al 2015).…”

A representation of the phosphorus cycle for ORCHIDEE (revision 4520)

“…ORCHIDEE-CN is a re-implementation of the nitrogen cycle from a discontinued version of ORCHIDEE (which became OCN, Zaehle and Friend, 2010;Zaehle et al, 2011) in a recent version of ORCHIDEE (r3623). The nitrogen cycle in OCN is well evaluated (De Kauwe et al, 2014;Zaehle et al, 2014;Walker et al, 2015;Meyerholt and Zaehle, 2015) and identical to the one in ORCHIDEE-CN except for the parametrization of the relationship between leaf nitrogen concentration and maximum carboxylation capacity of photosynthesis (V cmax ) as ORCHIDEE (r3623) uses a different carbon assimilation scheme than originally used in Zaehle and Friend (2010). V cmax is directly derived from the leaf nitrogen concentration at the respective canopy level following Kattge et al (2009): Figure 1.…”

Projections of soil carbon using the combination of the CNOP-P method and GCMs from CMIP5 under RCP4.5 in north-south transect of eastern China

“…In those evaluations, the global averaged soil carbon would increase by approximately 200 Pg C; however, large uncertainties were found among different scenarios and models. Walker et al (2015) found that soil carbon increased with changes in CO 2 concentrations over 300 years based on the Duke and Oak Ridge Free-Air CO 2 Enrichment (FACE) experiments using multiply land models.…”

“…As the measure of balance between input from plant production and output of decomposition, soil carbon has a key role in regulation of the global carbon cycle and in climate-change policy and carbon management Sun et al 2010;Tan et al 2010;Tian et al 2015;Walker et al 2015). Therefore, accurate estimates or predictions of the variation in soil carbon are required to determine whether a soil is a source or a sink for carbon (Ni 2013).…”