Soil-surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California

Soil-surface CO2 efflux and its spatial and temporal variations were examined in an 8-y-old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements Of Soil CO2 efflux, soil temperatures and moisture were conducted on two 20 x 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO2 efflux in the plantation was 4.43 mu mol m(-2) s(-1) in the growing season and 3.12 mu mol m(-2) s(-1) in the nongrowing season. These values are in the upper part of the range of published soil-surface CO2 efflux data. The annual maximum and minimum CO2 efflux were 5.87 and 1.67 mu mol m-2 s-1, respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation Of CO2 efflux was relatively small (< 20%) with the minimum appearing around 09.00 hours and the maximum around 14.00 hours. Using daytime measurements Of Soil CO2 efflux tends to overestimate the daily mean Soil CO2 efflux by 4-6%. The measurements taken between 09.00 and 11.00 hours (local time) seem to better represent the daily mean with a reduced sampling error of 0.9-1.5%. The spatial variation of soil CO2 efflux among the 18 sampling points was high, with a coefficient of variation of approximately 30%. Most (84%) of the spatial variation was explained by fine root biomass, microbial biomass, and soil physical and chemical properties. Although soil temperature and moisture explained most of the temporal variations (76-95%) Of Soil CO2 efflux, the two variables together explained less than 34% of the spatial variation. Microbial biomass, fine root biomass, soil nitrogen content, organic matter content, and magnesium content were significantly and positively correlated with soil CO2 efflux, whereas bulk density and pH value were negatively correlated with CO2 efflux. The relationship between soil CO2 efflux and soil temperature was significantly controlled by soil moisture with a Q(10) value of 1.4 when soil moisture was <14% and 1.8 when soil moisture was >14%. Understanding the spatial and temporal variations is essential to accurately assessment of carbon budget at whole ecosystem and landscape scales. Thus, this study bears important implications for the study of large-scale ecosystem dynamics, particularly in response to climatic variations and management regimes.