Spatial and temporal variability of soil microbial decomposition-derived CO2 efflux at two contrasting Mediterranean ecosystems

We developed two parallel experiments to study which environmental factors control the strong spatial and temporal variability inherent to soil microbial decomposition-derived CO2 efflux. These experiments were carried out at two representative ecosystems of the Californian landscape, an oak-savanna located in the Sacramento valley and a ponderosa pine plantation located in the foothills of the Sierra Nevada. To study the microbial community responses under field conditions, a first set of experiments aimed to partition soil CO2 flux in its autotrophic and heterotrophic components at each location. Soil respiration was continuously assessed at both sites using a soil CO2 gradient measurement system and periodically assessed using non-steady-state through-flow chamber methodology. The spatial distribution of annual grasses and trees in the oak savanna landscape enabled the partition of autotrophic and heterotrophic components of the soil CO2 flux during periods when the annuals were dead. The second approach to partition the flux involved assessing the α13C signal of soil CO2 flux. Trenching methodology was used in the pine ponderosa stand as a third way to partition the flux. A second parallel set of experiments aimed to study the spatial and temporal variation of the microbial decomposition response under controlled conditions. Intact soil cores were collected at both sites at two different seasons, summer and fall. Two water treatments (current field soil moisture and field capacity) were exposed to temperature changes that simulated a typical diurnal cycle while soil CO2 flux was measured at each temperature. Total C and N, labile/recalcitrant organic C evolution was also assessed. Our results suggest that besides temperature, soil moisture and soil organic matter quality, other factors, such as seasonality of fresh organic matter inputs to soil or ecosystem-related differences in microbial community may strongly affect microbial decomposition-derived respiration.