Soil respiration (Rsoil) is a major Carbon (C) efflux to the atmosphere and has often been modeled following passive soil temperature and moisture dependence functions. In temperate systems, this paradigm has been challenged, but in tropical grasslands little is known about the sensitivity of Rsoil and its autotrophic (Raut) and heterotrophic (Rhet) components to temperature, moisture, or plants activity. Due to paucity of data, the complex interactions between the components of Rsoil and predicted changes in temperature and plant productivity and possible C-climate feedbacks in tropical grasslands. The majority of the C in grasslands is stored belowground and disturbances in Rsoil can the C-sink capacity of these ecosystems under climate warming. Hence, we need to understand the controls of Rsoil in tropical grasslands thus we measured isotopic composition of respired CO2 of an established tropical grassland in Brazil dominated by the legumeStylosanthes capitata Vogel and by the grass Panicum maximum Jacq., using a T-FACE (temperature free-air controlled enhancement) system to increase air temperature by 2°C above ambient. We hypothesized that plants' diurnal cycle activities would drive Rsoil and that warming would stimulate Rsoil components rates increasing ecosystem C loss.
Results/Conclusions
Warming did not affect Rsoil, or Raut and Rhet components. However, there was a marked diel effect in Raut and Rhet in which the Raut proportion of total Rsoil was higher during the day (~90%) and reduced at nighttime (~50%). Raut maximum rates were during daytime (3.5 mmol m-2s-1) while Rhet maximum rates were during nighttime (2.0 mmol m-2s-1). Autotrophic soil carbon efflux was dominated by C4 respiration, 80% on average, and the patterns in soil respired CO2 seemed to be controlled by the photosynthetic activity of the C4 plant. There were no evidences that elevated temperature in tropical pasture systems increased the decomposition of carbon from the soil. We conclude Rsoil low sensitivity to warming in tropical grasslands may not increase ecosystem C losses under predicted warming as long as moisture levels are adequate. We also conclude that Rhet in tropical grasslands appears to be substrate- and not temperature-limited and is more affected by recently assimilated C rather than decomposing older soil organic matter. The great dominance of C4 plants in tropical grasslands will presumably maintain high ecosystem productivity increasing ecosystem C-sink strength under warming.