Increased plant growth under elevated CO2 slows the pace of climate warming and underlies projections of terrestrial carbon (C) and climate dynamics. However, this important ecosystem service may be diminished by concurrent changes to vegetation carbon to nitrogen (C:N) ratios. Despite clear observational evidence of increasing foliar C:N under elevated CO2 , our understanding of potential ecological consequences of foliar stoichiometric flexibility is incomplete. Here, we illustrate that when we incorporated CO2-driven increases in foliar stoichiometry into the Community Land Model the projected land C sink decreased two-fold by the end of the century compared to simulations with fixed foliar chemistry. Further, CO2-driven increases in foliar C:N profoundly altered Earth’s hydrologic cycle, reducing evapotranspiration and increasing runoff; and reduced belowground N cycling rates. These findings underscore the urgency of further research to examine both the direct and indirect effects of changing foliar stoichiometry on soil N cycling and plant productivity.

In the past few decades, there has been an evolution in our understanding of soil organic matter (SOM) dynamics from one of inherent biochemical recalcitrance to one deriving from plant-microbe-mineral interactions. This shift in understanding has been driven, in part, by influential conceptual frameworks which put forth hypotheses about SOM dynamics. Here, we summarize several focal conceptual frameworks and derive from them six controls related to SOM formation, (de)stabilization, and loss. These include: (1) physical inaccessibility; (2) mineral stabilization; (3) abiotic environmental limitation; (4) biochemical reactivity and diversity; (5) biodegradability of plant inputs; and (6) microbial properties. We then review the empirical evidence for these controls, their model representation, and outstanding knowledge gaps. We find relatively strong empirical support and model representation of abiotic environmental limitation but disparities between data and models for biochemical reactivity and diversity, mineral stabilization, and biodegradability of plant inputs, particularly with respect to SOM destabilization for the latter two controls. More empirical research on physical inaccessibility and microbial properties is needed to deepen our understanding of these critical SOM controls and improve their model representation. The SOM controls are highly interactive and also present some inconsistencies which may be reconciled by considering methodological limitations or temporal and spatial variation. Future conceptual frameworks must simultaneously refine our understanding of these six SOM controls at various spatial and temporal scales and within a hierarchical structure, while incorporating emerging insights. This will advance our ability to accurately predict SOM dynamics.

Increased plant growth under elevated CO2 slows the pace of climate warming and underlies projections of terrestrial carbon (C) and climate dynamics. However, this important ecosystem service may be diminished by concurrent changes to vegetation carbon to nitrogen (C:N) ratios. Despite clear observational evidence of increasing foliar C:N under elevated CO2 , our understanding of potential ecological consequences of foliar stoichiometric flexibility is incomplete. Here, we show that incorporating CO2-driven foliar stoichiometry into the Community Land Model reduced the projected land C sink two-fold by the end of the century compared to simulations with fixed foliar chemistry. Further, flexible foliar C:N profoundly altered Earth’s hydrologic cycle, reducing evapotranspiration and increasing runoff. Belowground N cycling rates were reduced in the flexible scenario, highlighting the urgency of further research examining both the direct and indirect effects of changing foliar stoichiometry on soil N cycling and plant productivity.

Nutrient limitation is widespread in terrestrial ecosystems. Accordingly, representations of nitrogen (N) limitation in land models typically dampen rates of terrestrial carbon (C) accrual, compared with C-only simulations. These previous findings, however, rely on soil biogeochemical models that implicitly represent microbial activity and physiology. Here we present results from a biogeochemical model testbed that allows us to investigate how an explicit vs. implicit representation of soil microbial activity, as represented in the MIcrobial-MIneral Carbon Stabilization (MIMICS) and Carnegie–Ames–Stanford Approach (CASA) soil biogeochemical models, respectively, influence plant productivity and terrestrial C and N fluxes at initialization and over the historical period. When forced with common boundary conditions, larger soil C pools simulated by the MIMICS model reflect longer inferred soil organic matter (SOM) turnover times than those simulated by CASA. At steady state, terrestrial ecosystems experience greater N limitation when using the MIMICS-CN model, which also increases the inferred SOM turnover time. Over the historical period, however, higher rates of N mineralization were fueled by warming-induced acceleration of SOM decomposition over high latitude ecosystems in the MIMICS-CN simulation reduce this N limitation, resulting in faster rates of vegetation C accrual. Moreover, as SOM stoichiometry is an emergent property of MIMICS-CN, we highlight opportunities to deepen understanding of sources of persistent SOM and explore its potential sensitivity to environmental change. Our findings underscore the need to improve understanding and representation of plant and microbial resource allocation and competition in land models that represent coupled biogeochemical cycles under global change scenarios.