Measure, Model and Verify (MMV) for soil carbon sequestration strategies under climate change in Australian pasture systems by combining DayCent and Eddy Covariance flux towers

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Grasslands store approximately one-third of the global terrestrial carbon (C) stocks. However, intensified grassland management over the last decades has resulted in soil degradation and subsequent soil organic C (SOC) losses as well as enhanced greenhouse gas emissions. Restoring grassland soils with adequate management practices offers huge opportunities for climate change mitigation with the potential to globally sequester ~150 megatons of CO2 eq per year in the soil. Emerging C credit markets further stress the importance of effective grassland management practices to restore SOC stocks. Despite that several improved management practices have been tested, their efficacy on soil C sequestration largely varies depending on environmental conditions. Soil C sequestration potential of grassland management practices under climate change scenarios is therefore highly uncertain. To this end, biogeochemical models, such as DayCent, offer a powerful tool to investigate the efficacy of grassland management practices, simulating the complex interaction between management and environmental conditions. Furthermore, Eddy Covariance (EC) flux towers provide opportunities to calibrate and validate the model’s C cycling with its high-frequency C balance measurements accounting for high spatial heterogeneity in pasture systems. In this study, DayCent was calibrated for pasture systems in the Brigalow belt region in Australia using EC flux tower data. The model was then validated with SOC data and used to project SOC stocks under combinations of different management practices and climate change scenarios. The calibrated parameters on soil organic matter decomposition reflected the deeper soil depth boundary down to 30 cm and the higher ratio of mineral-associated organic matter observed in Australian pasture systems. The calibrated DayCent model showed the potential to sequester C for the long term under climate change scenarios by introducing deep rooting legume and time-controlled grazing, restoring the degraded pasture soils due to historic intensive management. These simulated C sequestration estimates strongly correlated with C inputs and thus were limited by rather rainfall, grass productivity or grazing management than clay content. This study suggests Measure, Model and Verify (MMV) approach to estimate and project soil C sequestration for evaluation of SOC methodologies by pasture management within a shorter period than soil sampling – measure the baseline SOC, model the C dynamics with the calibrated DayCent and verify the projected soil C sequestration with EC flux tower data.