Abstract. Elevated carbon dioxide (CO2) can increase plant growth, but the
magnitude of this CO2 fertilization effect is modified by soil
nutrient availability. Predicting how nutrient availability affects plant
responses to elevated CO2 is a key consideration for ecosystem
models, and many modeling groups have moved to, or are moving towards,
incorporating nutrient limitation in their models. The choice of assumptions
to represent nutrient cycling processes has a major impact on model
predictions, but it can be difficult to attribute outcomes to specific
assumptions in complex ecosystem simulation models. Here we revisit the
quasi-equilibrium analytical framework introduced by Comins and
McMurtrie (1993) and explore the consequences of specific model assumptions
for ecosystem net primary productivity (NPP). We review the literature applying this framework to plant–soil
models and then analyze the effect of several new assumptions on predicted
plant responses to elevated CO2. Examination of alternative
assumptions for plant nitrogen uptake showed that a linear function of the
mineral nitrogen pool or a linear function of the mineral nitrogen pool with
an additional saturating function of root biomass yield similar CO2
responses at longer timescales (>5 years), suggesting that the added
complexity may not be needed when these are the timescales of interest. In
contrast, a saturating function of the mineral nitrogen pool with linear
dependency on root biomass yields no soil nutrient feedback on the
very-long-term (>500 years), near-equilibrium timescale, meaning that one
should expect the model to predict a full CO2 fertilization effect
on production. Secondly, we show that incorporating a priming effect on slow
soil organic matter decomposition attenuates the nutrient feedback effect on
production, leading to a strong medium-term (5–50 years) CO2
response. Models incorporating this priming effect should thus predict a
strong and persistent CO2 fertilization effect over time. Thirdly,
we demonstrate that using a “potential NPP” approach to represent nutrient
limitation of growth yields a relatively small CO2 fertilization
effect across all timescales. Overall, our results highlight the
fact that the quasi-equilibrium
analytical framework is effective for evaluating both the consequences and
mechanisms through which different model assumptions affect predictions. To
help constrain predictions of the future terrestrial carbon sink, we
recommend the use of this framework to analyze likely outcomes of new model
assumptions before introducing them to complex model structures.