Concentrations and controls of soil carbon dioxide in sandy soil in the North Carolina coastal plain

Abstract

Forty-four measurements of soil CO 2 were conducted from October 1988 to October 1989 at 1–2 week intervals in the Leon Sand of the North Carolina Coastal Plain at Wilmington. Concentrations were highest in the summer (mean soil CO 2= 0.74% on 31 Aug) and lowest in the winter (mean soil CO 2= 0.20% on 26 Jan). CO 2 concentrations increased with depth in the root zone from d = 30 cm to d = 90 cm, but consistently low values were encountered at d = 120 cm beneath a hardpan layer. Statistical correlations were highest between mean soil log pCO 2 and soil temperature (r = 0.77). Daily minimum air temperature was also a limiting factor (r = 0.67). Correlations were weaker for log pCO 2 and potential evapotranspiration (r = 0.63) as well as actual evapotranspiration (r = 0.52). Soil moisture content, deficit, and surplus were not correlated with soil log pCO 2. Precipitation was negatively correlated with mean log pCO 2 (r = −0.83 winter, r = −0.54 summer), indicating that rainfall had a “flushing” effect on soil CO 2 which was more pronounced during the cold season. However, increases in soil CO 2 content during times of high soil moisture were also observed at times. Hourly measurements were conducted for a 24 hour period on 7 to 8 July, 1989 at a Leon sand site with no hardpan. Concentrations measured at 15, 30, 60, 90, and 120 cm increased with depth. Highest mean soil CO 2 level (0.52%) was measured at 0600 EST, lowest at 1900 EST (0.41%). Correlations with soil temperature were negative, especially at shallow depths (r = −0.64 at d = 15 cm), suggesting that drying of the soil was a limiting factor. Wind speed was not correlated with soil CO 2. In sandy substrates, soil CO 2 levels are largely controlled by temperature fluctuations and changes in soil moisture content. However, local factors such as proximity of individual roots, and vertical differences in diffusivity, also play an important role. In the present study, the greatest weathering potential was found in the months of July and September, when high soil CO 2 concentrations co-incided with high totals of soil surplus water.