The pulsed response of soil respiration to precipitation in an African savanna ecosystem: a coupled measurement and modeling approach

Abstract:

Savannas cover 60% of the African continent and play an essential role in the global carbon (C) cycle. To better characterize the physical controls over soil respiration in these settings, half-hourly observations of volumetric soil-water content, temperature, and the concentration of carbon dioxide (CO₂) at different soil depths were continually measured from 2005 to 2007 under trees (“sub-canopy”) and between trees (“inter-canopy”) in a savanna vegetation near Skukuza, Kruger National Park, South Africa. The measured soil climate and CO₂ concentration data were assimilated into a process-based model that estimates the CO₂ production and flux with coupled dynamics of dissolved organic C (DOC) and microbial biomass C. Our results show that temporal and spatial variations in CO₂ flux were strongly influenced by precipitation and vegetation cover, with two times greater CO₂ flux in the sub-canopy plots (~2421 g CO₂ m^-2 yr^-1) than in the inter-canopy plots (~1290 g CO₂ m^-2 yr^-1). Precipitation influenced soil respiration by changing soil temperature and moisture; however, our modeling analysis suggests that the pulsed response of soil respiration to precipitation [known as “Birch effect (BE)”] is a key control on soil fluxes at this site. At this site, BE contributed to approximately 50% and 65% of heterotrophic respiration or 20% and 39% of soil respiration in the sub-canopy and inter-canopy plots, respectively. These results suggest that pulsed response of respiration to precipitation is an important component of the C cycle of savannas and should be considered in both measurement and modeling studies of carbon exchange in similar ecosystems.