Abstract
AbstractCarbon (C) enters into the terrestrial ecosystems via photosynthesis and cycles through the system together with other essential nutrients (i.e., nitrogen [N] and phosphorus [P]). Such a strong coupling of C, N, and P leads to the theoretical prediction that limited nutrient availability will limit photosynthesis rate, plant growth, and future terrestrial C dynamics. However, the lack of reliable information about plant tissue stoichiometric constraints remains a challenge for quantifying nutrient limitations on projected global C cycling. In this study, we harmonized observed plant tissue C:N:P stoichiometry from more than 6,000 plant species with the commonly used plant functional type framework in global land models. Using observed C:N:P stoichiometry and the flexibility of these ratios as emergent plant traits, we show that observationally constrained fixed plant stoichiometry does not improve model estimates of present‐day C dynamics compared with unconstrained stoichiometry. However, adopting stoichiometric flexibility significantly improves model predictions of C fluxes and stocks. The 21st century simulations with RCP8.5 CO2 concentrations show that stoichiometric flexibility, rather than baseline stoichiometric ratios, is the dominant controller of plant productivity and ecosystem C accumulation in modeled responses to CO2 fertilization. The enhanced nutrient limitations and plant P use efficiency mainly explain this result. This study is consistent with the previous consensus that nutrient availability will limit xfuture land carbon sequestration but challenges the idea that imbalances between C and nutrient supplies and fixed stoichiometry limit future land C sinks. We show here that it is necessary to represent nutrient stoichiometric flexibility in models to accurately project future terrestrial ecosystem carbon sequestration.
Highlights
As an important functional trait, plant tissue‐level stoichiometric ratios define relative abundances of carbon (C) and other necessary chemical elements (e.g., N and P) in different plant tissues (Watanabe et al, 2007), such as leaves, fine roots, sapwood, and heartwood
Using observed C:N:P stoichiometry and the flexibility of these ratios as emergent plant traits, we show that observationally constrained fixed plant stoichiometry does not improve model estimates of present‐day C dynamics compared with unconstrained stoichiometry
To use the highly variable plant stoichiometry data collected from observations to inform ELMv1‐equilibrium chemistry approximation (ECA) parameterization, we synthesized the observations into 14 natural plant functional types (PFTs) according to structural, phenological, physiological, and climatic features (Table S1)
Summary
As an important functional trait, plant tissue‐level stoichiometric ratios define relative abundances of carbon (C) and other necessary chemical elements (e.g., N and P) in different plant tissues (Watanabe et al, 2007), such as leaves, fine roots, sapwood (live wood), and heartwood (dead wood). According to the stoichiometry homeostasis hypothesis (Sterner & Elser, 2002), plants strive to maintain critical tissue stoichiometric ratios for growth and function, even though external element supplies may dramatically change across space and time To maintain this homeostasis, plant C assimilation can be reduced, for example, when soil nutrient supply is reduced (Agren & Weih, 2012; Harpole et al, 2011). Plant C assimilation can be reduced, for example, when soil nutrient supply is reduced (Agren & Weih, 2012; Harpole et al, 2011) In this case, the “immediate” reduction of plant biomass production could be due to two reasons: (1) direct functional control of nutrients on biochemical photosynthesis reactions and/or (2) biomass construction limitations. The reduction of C productivity could occur when nutrient uptake from soil does not keep pace with C uptake from photosynthesis (Hungate et al, 2003)
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