Department Seminar: Adam Pellegrini

Title: Savanna Carbon in the Anthropocene: Disturbances, Soils, and the Limits of Top-Down Thinking

Abstract: Savannas and grasslands hold 28% of global topsoil carbon while accounting for 79% of global burned area, yet how fire regulates this carbon remains contested. The dominant framework — fire removes biomass, reduces inputs to soil, and depletes soil carbon — is increasingly shown to be conceptually incomplete. Here I test an emerging, bottom-up perspective in which fire restructures plant and soil processes in ways that actively stabilize soil organic matter through organo-mineral associations and fine-root production, making emissions of non-CO2 greenhouse gases important. Drawing on long-term fire and herbivory manipulation experiments across North America and Africa, I show that belowground net primary productivity is largely resilient to shifting disturbance regimes, with roots compensating for aboveground losses through a system-level buffering capacity of plant primary productivity that increases with water availability. On the other hand, carbon stocks were regulated by edaphic factors predictive of physicochemical stabilization between organic matter and minerals, such that wetter systems counterintuitively stored less carbon because trees increased soil carbon turnover when fire and herbivores were excluded. Scaling these dynamics to a park-wide greenhouse gas budget for Kruger National Park reveals a further surprise: prescribed burning delivers real climate mitigation, but primarily through reductions in CH4 and N2O emission factors rather than ecosystem carbon sequestration — and herbivore enteric methane emerges as a flux of comparable magnitude to net fire emissions. Together, these results suggest that effective nature-based solutions in savannas require accounting for the full GHG budget, crediting trace-gas reductions, and recognizing that the soil carbon engine is largely self-stabilizing through plant adaptations and soil mineral protection of organic matter.