Spatial and temporal variability of soil CO2 efflux in three proximate temperate forest ecosystems
The magnitude of CO2 flux from soil (Fsoil) varies with primary productivity and environmental drivers of respiration, soil temperature (Tsoil) and moisture, all of which vary temporally and spatially. To quantify the sources of Fsoil variability, we first compared Fsoil of three proximate forests within 30 km of one another, ranging in age, composition, soil, and environment and, thus, productivity. We collected data with automated soil respiration chambers during a 10-year period in a mid-rotation Pinus taeda plantation (PP), for three-years in a mature P. taeda stand (OP), and for five-years in a mature, mixed-species hardwood (HW) stand; PP and HW were on clay-loam soil and OP on a sandy soil. Among stands, Fsoil sensitivity to Tsoil was lowest in OP and highest in PP, reflected in mean annual Fsoil (±standard deviation) of 1033 ± 226 (OP), 1206 ± 99 (HW), and 1383 ± 152 (PP) g C m²; both Fsoil sensitivity to Tsoil and annual Fsoil increased with leaf litterfall. For the second portion of our study, we established an additional three plots at PP for a six-year period to examine within-stand variability. Within PP, sensitivity of Fsoil to Tsoil was similar, yet higher leaf area was correlated with a combination of lower soil temperature and below-ground carbon flux, resulting in lower Fsoil. Temporally, diurnal to seasonal Fsoil followed Tsoil whereas annual values were driven by soil moisture. Spatially, among the three stands Fsoil increased with leaf production, whereas within a stand (PP) Fsoil decreased with increasing leaf production.