Computing the mean residence time of soil carbon fractions using stable isotopes: impacts of the model framework

<jats:p>Soils contain the largest carbon (C) reservoir on Earth, but the mean residence time (MRT) of soil C is often poorly estimated, despite its importance for assessing the efficiency with which soils may serve as a sink for atmospheric C. The objective of this study was to evaluate how the structure of simple models of soil C dynamics affects the MRT determined from isotope‐mixing experiments, using data from field studies with either artificial labelling (FACE) or C3/C4 vegetation change. We first highlighted theoretically how non‐steady‐state conditions and the model structure (one single, two successive, or two parallel C pools) can have an impact on the MRT assessment. We then tested these different model structures against published data on the dynamics of different soil organic matter separates and their constituents.</jats:p> <jats:p>Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single‐pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady‐state conditions an apparent temporal change of MRT computed from a single‐pool model is thus a clear indicator that a two‐pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over‐estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid.</jats:p>