Australia savannas: seasonal Carbon sinks and sources

Through estimations of above-and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO 2 efflux, a carbon balance on seasonal and yearly timescales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204 53 t C ha-1 , with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0-1 m) was 151 33 t C ha-1 , accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53 20 t C ha-1 , 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production (GPP) was 20.8 t C ha-1 , of which 27% occurred in above-ground components and 73% below-ground components. Net primary production (NPP) was 11 t C ha-1 y-1 , of which 8.0 t C ha-1 (73%) was contributed by below-ground net primary production and 3.0 t C ha-1 production (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 t C ha-1 y-1. The wet season plays a critical role in controlling carbon flux on a yearly basis. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occurs during the 5-6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a source during the dry season. Annual net ecosystem production (NEP) was 3.8 t C ha-1 y-1. The loss of carbon due to fire is significant and the 3 long-term sequestration potential (NBP) is approximately 40% of the value for NEP at 1.5 t C ha-1 y-1 , although there is a degree of uncertainty associated with this estimate.