Changes in CO₂ concentrations within shallow soils originating from solar radiation and rainwater in Central Amazonia

Abstract

Soil CO₂ concentrations were measured at an agricultural land and at a primary forest in Central Amazonia. During dry season with chronic sunny days, the concentrations and soil water contents decreased at sunset when the soil temperature at a depth of 10 cm peaked, and they increased from midnight to daybreak when the soil temperature decreased. These findings indicate that the phase change between soil water and water vapor caused by soil temperatures brought about the diurnal changes in concentrations. But the heat transfer was so weak at a depth of 80 cm that the concentrations did not change. Further, the diurnal variation was not clear at a soil depth of 10 cm at the forest, because the solar radiation hardly reached the forest floor, and no fluctuation was found at the soil depth of 80 cm. During rainy season, regardless of the place, the percolation of rainwater pushed up the concentration as well as soil water content at the depth of 10 cm, and after the water passed downward, the increases gradually moved down to the 80 cm layer. Therefore, diurnal variations in soil CO₂ originating from soil temperatures did not appear.