Model uncertainty obscures major driver of soil carbon

Posted by Design Studio

26 June 2024

Challenge 2: Soil Carbon


Soil organic carbon (SOC) plays a critical role in mitigating climate change, as it stores vast amounts of carbon that would otherwise be in the atmosphere. A recent study by Tao et al. posits which assimilates vast information on soil carbon profile into a process-model highlights that microbial carbon use efficiency (CUE) is the predominant factor influencing global SOC storage, with little influence of soil carbon inputs. However, in our analysis, we express concerns about the robustness and comprehensiveness of their approach, suggesting that it is too early to reject the long-standing paradigm that soil carbon inputs is among the major drivers.


1. Empirical Analysis and Model Robustness:

Tao et al. analyzed 132 paired CUE and SOC measurements, employing a linear mixed effect model. They reported that CUE, along with mean annual temperature (MAT) and soil depth, explained 55% of the variation in SOC. However, they did not consider carbon (C) inputs to the soil in their model. By incorporating net primary production (NPP) as a proxy for C inputs into a similar model, we found that NPP explained a more substantial portion of SOC variation (71%), indicating that the role of CUE may be overstated when C inputs are excluded.


2. Model Structure and Sensitivity Analysis:

The process model used by Tao et al. may inherently bias results by also not adequately representing C inputs. They assumed a linear relationship between microbial turnover and microbial biomass, potentially underestimating the sensitivity of SOC to changes in C inputs. By incorporating a density-dependent turnover function, our analysis suggests a significantly higher sensitivity of SOC to C inputs (see Figure), challenging the conclusions of Tao et al.



Our critique highlights the importance of considering multiple drivers of SOC dynamics beyond microbial CUE. In CALIPSO we will investigate the combined influences of plant, soil microbe, and environmental factors on SOC storage with the aim to better constrain the soil carbon loss pathways in Earth System Models.  


Sensitivity of the CUE-SOC relationship to the inclusion of density-dependent microbial turnover in process-based soil models. Predicted soil organic carbon (SOC) stocks at steady-state from the MIMICS (a-c) and Millennial (d-f) microbial-explicit SOC models using a range of density-dependent microbial turnover exponent (β) values, net primary productivity (NPP), and microbial carbon use efficiency (CUE).


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