Optimal Fire Regimes for Soil Carbon Storage in Tropical Savannas of Northern Australia

Abstract

Modification of fire regimes in tropical savannas can have significant impacts on the global carbon (C) cycle, and therefore, on the climate system. In Australian tropical savannas, there has been recent, large-scale implementation of fire management that aims to decrease Kyoto-compliant non-CO2 greenhouse gas emissions by reducing late dry season intense fires through strategic early dry season burning. However, there is no accounting for changes to soil C stocks resulting from changes to savanna fire management, although impacts on these pools may be considerable. We present a hypothesis that soil C storage is greatest under low intensity fires with an intermediate fire return interval. Simulations using the CENTURY Soil Organic Matter Model confirmed this hypothesis with greatest soil C storage under a fire regime of one low intensity fire every 5 years. Key areas of uncertainty for CENTURY model simulations include fine root dynamics, charcoal production and nitrogen (N) cycling, and better understanding of these processes could improve model predictions. Soil C stocks measured in the field after 5 years of annual, 3 year and unburned fire treatments were not significantly different (range 41–58 t ha−1), but further CENTURY modelling suggests that changes in fire management will take up to 100 years to have a detectable impact (+4 t ha−1) on soil C stocks. However, implementation of fire management that reduces fire frequency and intensity within the large area of intact savanna landscapes in northern Australia could result in emissions savings of 0.17 t CO2-e ha−1 y−1, four times greater than reductions of non-CO2 emissions.