The quasi-equilibrium framework revisited: analyzing long-term CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; enrichment responses in plant–soil models

Mingkai Jiang,David S Ellsworth,Sönke Zaehle,Belinda E Medlyn,Anthony P Walker,Silvia Caldararu,Martin G De Kauwe

doi:10.5194/gmd-12-2069-2019

Abstract

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.

Highlights

Predicting how plants respond to atmospheric carbon dioxide (CO2) enrichment under nutrient limitation is fundamental for an accurate estimate of the global terrestrial carbon (C) budget in response to climate change
By constructing a quasi-equilibrium framework based on the structure of the Generic Decomposition And Yield (G’DAY) model (Comins and McMurtrie, 1993), we evaluate the effects on plant responses to eCO2 of some recently developed model assumptions incorporated into ecosystem models, for example the Community Land Model (CLM) (Oleson et al, 2004), the Community Atmosphere– Biosphere Land Exchange (CABLE) model (Kowalczyk et al, 2006), the Lund–Potsdam–Jena (LPJ) model (Smith et al, 2001), the JSBACH model (Goll et al, 2017b), and the O-CN model (Zaehle et al, 2010)
We tested alternative model assumptions for three processes that affect plant carbon–nitrogen cycling: (1) Sect. 3.2.1 evaluates different ways of representing plant N uptake, namely plant N uptake as a fixed fraction of mineral N pools, as a saturating function of the mineral N pool linearly depending on root biomass (Zaehle and Friend, 2010), or as a saturating function of root biomass linearly depending on the mineral N pool (McMurtrie et al, 2012); (2) Sect. 3.2.2 tests the effect the potential net primary productivity (NPP) approach that downregulates potential NPP to represent N limitation (Oleson et al, 2004); and (3) Sect. 3.2.3 evaluates root exudation and its effect on the soil organic matter decomposition rate

Summary

Introduction

Predicting how plants respond to atmospheric carbon dioxide (CO2) enrichment (eCO2) under nutrient limitation is fundamental for an accurate estimate of the global terrestrial carbon (C) budget in response to climate change. It has been shown that models incorporating different assumptions predict very different vegetation responses to eCO2 (Lovenduski and Bonan, 2017; Medlyn et al, 2015). Careful examination of model outputs has provided insight into the reasons for the different model predictions (De Kauwe et al, 2014; Medlyn et al, 2016; Walker et al, 2014, 2015; Zaehle et al, 2014), but it is generally difficult to attribute outcomes to specific assumptions in these plant–soil models that differ in structural complexity and process feedbacks (Lovenduski and Bonan, 2017; Medlyn et al, 2015; Thomas et al, 2015)

Methods

Results

Conclusion