Temporal dynamics of particulate and mineral-associated carbon reveal three timescales of response to experimental manipulation

Abstract

Abstract. In the last decade, the conceptual framework that characterizes soil organic carbon (SOC) into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions has gained traction in studies of C dynamics. This SOC characterization is useful for developing empirical studies and for parsimonious model parameterizations. However, rigorous testing of model structures incorporating the POC-MAOC framework is still lacking, particularly tests evaluating whether this framework can adequately reproduce simultaneous measurements of changes in C pool contents and respiration fluxes. We conducted an incubation experiment using control and litter-addition treatments, measuring changes in SOC fraction contents and respiration fluxes throughout the incubation. Then, we applied an inverse modelling approach to compare the performance of 2-pool (POC-MAOC) and 3-pool models (which also included a faster-cycling litter C pool) to reproduce the observed data. We then calculated the C ages and transit times to explore the predicted C persistence. Finally, we performed simulations to evaluate the effects of different model structures and parameterizations on SOC persistence. For both treatments, we observed that 2-pool models were unable to simultaneously reproduce the changes in C pool contents and respiration, while the 3-pool models adequately predicted both variables and yielded lower C ages and transit times. 2-pool models collapsed POC dynamics operating across different timescales into a single one, failing to capture the distinct respiration phases and gradual C pool changes. Instead, 3-pool models distributed these processes among compartments: the Litter C pool captured fast-cycling dynamics, allowing POC and MAOC to better represent intermediate- and long-term dynamics, respectively. The fact that 3-pool models outperformed 2-pool models -even in control soils- indicates that, in our soils, POC is a heterogeneous pool that cannot be adequately represented as a single compartment. We also found that both model structure and changes in key parameters affected C persistence estimations: models that included shorter pathways to MAOC, or allowed faster transfers into more persistent pools, predicted higher C age and transit time, showing how model structure shapes SOC contents and persistence estimates. This study highlights that the POC-MAOC framework, which frames SOC dynamics using only two time scales, may not always be sufficient to fully characterize SOC processes. Rather than advocating for a specific model configuration, we argue that the conceptual simplification of soil C into POC and MAOC might fail to capture the multiple timescale responses frequently observed in experimental studies. Furthermore, as transfer rates play a key role in determining SOC persistence, it is important to better understand and quantify how C is transferred toward MAOC and how these processes can be represented in models.