Microbial C–N Limitation : Revisiting the Threshold Elemental Ratio

The Threshold Elemental Ratio of carbon and nitrogen (TER_C:N ; Sterner & Elser, 2002), is defined as the C:N ratio of resources at which microorganisms are co-limited by C and N, under the assumption that no other element limits growth (Soong et al., 2020). Below the TER_C:N, microorganisms are carbon-limited, whereas above it they become nitrogen-limited. Various methods are used to estimate this TER_C:N, but experimental studies suggest that the predicted value of TER_C:N should be higher (Mori, 2020; Sinsabaugh et al., 2009; Sinsabaugh et al., 2016; Soong et al., 2020; Cui et al., 2021). 

We propose a model based on simple stoichiometric principles that shows that the expression of the TER_C:N proposed in Sterner & Elser (2002), actually gives an estimate of the C:N ratio of organic matter at which microorganisms start to immobilize nitrogen. We propose to call this C:N ratio TIR_C:N (Threshold Immobilisation Ratio) and to keep TER_C:N for the C:N ratio at which microorganisms switch limitations. Our work predicts that as long as mineral nitrogen remains available (mineralisation > immobilisation and/or fertilization), soil microorganisms remain carbon-limited, even if the C:N ratio of organic matter exceeds the TIR_C:N (TIR_C:N < TER_C:N). Our results give a new expression of the TER_C:N based on ecological stoichiometry and predict that above this ratio, nitrogen inputs are expected to accelerate organic matter decomposition, while below it they tend to slow down decomposition.