Soil Respiration and Carbon Sequestration of an Oak- grass Savanna in California: Roles of temperature, soil moisture, rain events and photosynthesis

We quantified the spatial-temporal roles of photosynthesis, temperature and soil moisture on soil respiration of an oak savanna with a combination of field and laboratory measurements. Canopy photosynthesis was quantified from eddy covariance measurements over and under the woodland. Spatial gradients in soil respiration were surveyed with a portable soil respiration chamber. Contributions to soil respiration by autotrophs and heterotrophs were assessed using profiles of CO 2 sensors inserted in the soil under a tree and in the open grassland. At a given spot, soil respiration was a function of soil moisture and temperature and labile carbon pools associated with decomposing organic matter and recent photosynthetic production. Spatial/temporal variations in soil respiration reflect changes in these controlling factors. Spatial transect studies, for example, showed that soil respiration decreased by 60% as one radiates from a tree to the open grassland. Soil respiration measured under a tree crown reflected the sum of rhizosphere respiration and heterotrophic respiration while soil respiration measured in an open area represented heterotrophic respiration when the grass was dead during the summer. Basal rates of soil respiration diminished over the summer as the soil dried and it stressed photosynthetic and microbial activity. Summer rains, on the other hand, were found to stimulate the activity of latent microbes immediately, thereby pulsing soil respiration by more than a factor of 10 until the upper soil layer dries. Continuous measurements of soil respiration revealed that soil respiration under the tree was decoupled with soil temperature. Soil respiration was strongly correlated with tree photosynthesis, but with a time lag of 7 to 12 hours. These results indicate that photosynthesis drives soil respiration in addition to soil temperature and moisture. Measurements of soil respiration were combined with remote sensing information to upscale soil CO 2 efflux measurements. Cumulative soil respiration was 394 gC m -2 y -1 in the open area and 616 gC m -2 y -1 under trees producing a site-average of 488 gC m -2 y -1 .