The Roots Awaken: Development and implementation of a root autotrophic respiration module for soil CO2 flux partitioning in a cropping system process-based model

As agricultural soils are both sources and sinks of C, evaluating their C balance by properly accounting for the individual components of the C fluxes and their environmental drivers is required. Soil CO2 emissions (Rs) result from the combination of autotrophic respiration (Ra, from root growth and maintenance) and heterotrophic respiration (Rh, from soil organic matter decomposition). In this study, we developed a new module for the process-based ARMOSA (Analysis of cRopping systems for Management Optimization and Sustainable Agriculture) model to explicitly simulate root autotrophic respiration of wheat (Triticum durum Desf.) and processing tomato (Lycopersicon esculentum Mill.), with the aim of improving the quantification of soil C balance in cropping systems. Root respiration is simulated as influenced by the interactions between crop growth, root senescence and management practices. The new module was developed and tested using daily measured soil CO2 fluxes collected in two Mediterranean sites (moist and dry) during multi-year crop rotations, using automated stations equipped with eight non-steady-state measurement chambers. According to the results, root respiration is a crop-specific process that is driven by soil conditions, as highlighted by the sensitivity analysis of the new module, which identified the crop maintenance respiration potential and the optimal soil temperature for growth and maintenance respiration as the top-ranked parameters. The new module application within the framework of ARMOSA model enabled the partitioning of soil CO2 fluxes, allowing the estimation of the Ra/Rs ratio for wheat and tomato (ranging respectively from 9 to 90 % and from 1 to 87 % for wheat and tomato) as influenced by crop development. This improved the simulation of soil Rh throughout the entire crop rotations. The new module is able to quantify the actual contribution of a crop to the C balance under varying management and pedoclimatic conditions.