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Drivers of Ecosystem-level Coupled Carbon and Nitrogen Pumps in Savannas

Speaker: Dr. Mark Ritchie, Syracuse University

Tropical savannas and grasslands experience multiple processes, such as fire, herbivory and soil leaching, that lead to losses of carbon (C) and nitrogen (N) across the landscape. Yet many of these habitats support high secondary production of diverse animal assemblages. The mechanisms by which these C and N losses are presumably required for the observed persistence of animal assemblages over evolutionary time remains uncertain, given the low atmospheric N deposition, termite activity and high soil respiration rates common to tropical savannas and grasslands.

Here the hypothesis is tested that nitrogen fixation associated with grass roots coupled with migratory behavior of coevolved herbivores drives N and C “pumps” that replace ecosystem level losses in these habitats. In Serengeti National Park, exploratory assays reveal that three dominant grasses can fix substantial atmospheric N2 (8-35 kg N/ha annually) sufficient to replace N volatilization from fires and herbivore dung and urine, and herbivore net transport of N from uplands to woodland and riparian habitats.

Measurements of community relative growth rate of aboveground biomass suggest this nitrogen fixation allows grasses to regrow rapidly following periodic grazing events characteristic of migratory grazing animals, even from low leaf biomass. Such regrowth allows grasses to complete multiple growth and C-accumulation cycles per growing season and, coupled with dung deposition thus increase net annual carbon sequestration in plant biomass and soil organic matter relative to ungrazed conditions. Such a process is consistent with observed 30-60% higher seasonal aboveground production and soil organic carbon stocks, as well as sustained root mass-specific N-fixation rates, associated with grazing in a long-term Serengeti herbivore exclosure experiment.

When these two “pumps” are missing from tropical savannas, as observed in northern Kenya pastoral systems with high densities of non-migratory grazers that have lost N-fixing perennial grass species, severely reduced aboveground production and soil carbon can result.  These results suggest that migratory grazing coupled with nitrogen fixation may act as coupled, mutually supporting N and C pumps that sustain primary and secondary production in iconic tropical grassland and savanna ecosystems.