Evaluating autotrophic and heterotrophic respiration in an oak grass savanna using flux-gradient measurements of soil respiration with new infra-red CO2 sensors

Two sets of solid-state infrared CO2 sensors were used in the flux-gradient mode to measure soil respiration in an oak grass savanna. We deployed one profile system under a tree and another in the open grassland to quantify the absolute and relative contributions of heterotrophic and autotrophic respiration to soil respiration. During the summer this is possible because the grass is dead, so the only source of respiration from the open space is from the decomposition of dead plant material by microbes. In contrast, respiration measured in the soil under the tree represents the respiratory contributions from roots, mycorhizzae and microbes.

During the summer dry season, soil respiration rates under the tree were 4 to 6 times greater than those from the open grassland. Diurnal rates of respiration were modulated by changes in soil temperature and by photosynthesis; daily changes in light and vapor pressure deficits suppressed afternoon photosynthesis and this was later reflected in a subsequent decline in soil respiration. The flux-gradient system also enabled us to quantify the effects of rain on soil respiration. Summer rain storms caused a rapid activation of microbes and their production deep in the soil forces CO2 transfer to be bi-directional and at high rates until the soil dried.