Short-Term Iodine Dynamics in Soil Solution

Assessing the reactions of iodine (I) in soil is critical to evaluate radioiodine exposure and understand soil-to-crop transfer rates. Our mechanistic understanding has been constrained by method limitations in assessing the dynamic interactions of iodine between soil solution and soil solid phase over short periods (hours). We use microdialysis to passively extract soil solution spiked with radioiodine (I-129(-) and (IO3-)-I-129) to monitor short-term (<= 40 h) in situ fixation and speciation changes. We observed greater instantaneous adsorption of (IO3-)-I-129 compared to I-129(-) in all soils and the complete reduction of (IO3-)-I-129 to I-129(-) within 5 h of addition. Loss of( 129)I from solution was extremely rapid; the average half-lives of I-129(-) and (IO3-)-I-129 in soil solution were 4.06 and 10.03 h, respectively. We detected the presence of soluble organically bound iodine (org-I-1(29)) with a low molecular weight (MW) range (0.5-5 kDa) in all soils and a slower (20-40 h) time-dependent formation of larger MW org-I compounds (12-18 kDa) in some samples. This study highlights the very short window of immediate availability in which I from rainfall or irrigation remains in soil solution and available to crops, thus presenting significant challenges to phytofortification strategies in soil-based production systems.