Seasonal and interannual changes of HT deposition velocities in pasture soils

Two chamber types were used to investigate tritiated hydrogen (HT) deposition velocity in pasture soils in Rokkasho Village, Japan. A three-year experiment using an aerated chamber with atmospheric HT was conducted. The HT deposition velocity correlated with soil temperature (r = 0.87), which peaked in summer when the soil temperature was high, with a maximum of 0.28 mm s-1. Changes in the aboveground plant biomass caused by harvesting three times in summer did not influence the velocity, and interannual increases in root biomass also did not correlate with the velocity. Experiments using a closed chamber with D2 gas injection were conducted in five pastures. The hydrogen deposition velocity correlated negatively with the volumetric water content. In addition, the soil temperature, soil moisture, and hydrogen deposition velocity obtained from the two experiments were fitted to an equation expressing soil conductance. It was suggested that the hydrogen deposition rate exhibits seasonal variation.