Effect of NO3− on the fate of 99TcO4− in the soil-plant system

This work was undertaken to study the effect of NO3− fertilization on the uptake of 99TcO4− by plants. Rye-grass (Lolium perenne) was grown in a growth chamber on a silty loam soil to which were added increasing quantities of ammonium nitrate (21, 50 and 100 mg N kg−1). Soil samples were then amended with 99TcO4− at a constant level of 29.6 kBq kg−1 dry weight. Total initial NO3− content was 43.3, 57.8 and 82.8 mg N kg−1, and total NH4+ content was 24.7, 39.2 and 64.2 mg N kg−1. Three cuts were made at 4-week intervals, and aerial biomass was analysed for total 99Tc and N. Results showed that total uptake of 99Tc was decreased by increasing mineral N additions at the first cut, as mineral nitrogen was not a limiting factor for plant growth at this stage. Technetium-99 content in plants varied from 64% of the total applied 99Tc with the lowest NO3− content to 31% with the highest. However, at the third cut, cumulative 99Tc uptake reached the same level for the three treatments (80–83%) with no further effect of initial N applications, as NO3− was depleted in the soil. Therefore, 99TcO4− was diluted in the pool of soil NO3− and was absorbed proportionally to nitrate by rye-grass.