Iodine bioavailability in acidic soils of Northern Ireland

Abstract

Iodine is an essential trace element for humans and grazing animals and is often deficient. Our aim was to investigate the role of soil properties in retaining and 'fixing' iodine in soils and thereby controlling its phyto-availability to grass. Soils were spiked with labelled 129 IO3and rye grass (Lolium perenne L.) was grown to measure iodine uptake by grass as a function of yield, soil properties and continuous 127 I inputs from irrigation water. Iodine-129 added at the start of the uptake trial was rapidly fixed (t½ c. 40 hr) into non-labile humus-bound forms in soil. The 129 I/ 127 I isotopic ratio in grass, compared to the ratio in soil, declined over time confirming progressive 129 I fixation into the soil solid phase. The rate of fixation was controlled by soil properties. A model describing iodine dynamics and uptake accounted for c. 75% of the variation in iodine concentration in grass. For most of the soils studied, the main source of iodine in herbage probably arises from the transient availability of periodic rainfall inputs rather than from soil sources. This is expected to improve biofortification strategies. Highlights Vegetation I concentration may be controlled by recent I inputs rather than (re)supply from soil  Iodine is rapidly fixed into non-labile humus-bound forms in soils (t½ c. 40 hr)  Rate of iodine fixation is controlled by soil properties  Irrigation water inputs of iodine will likely provide the most effective biofortification strategy.

Agronomic iodine biofortification of leafy vegetables grown in Vertisols, Oxisols and Alfisols

“…These concentrations are tenfold greater than observed by Watts et al ( 2015 ) for field-grown vegetables in Malawi that were collected from non-calcareous (0.07 mg kg −1 A. retroflexus , 0.05 mg kg −1 B. napus) and calcareous soils (0.16 mg kg −1 A. retroflexus , 0.15 mg kg −1 B. napus ). This may indicate that the irrigation water was a significant source of I to the control plants, as also found by Bowley et al ( 2019 ) for an 129 I -labelled pot experiment. Assuming an adult consumed 30 g DW d −1 (Joy et al 2015 ) of the control vegetables grown in this study, they would ingest between 0.005 and 0.007 mg day −1 of I compared to a daily requirement of 0.15 mg (WHO 2007 ).…”

“…A total of 88 µg pot −1 was added to the irrigated soil over five harvests (Table 3 ), which is small in comparison with the total soil I in each pot that was in the range 4130–5880 µg pot −1 (Table 1 ). However, I in irrigation water is much more available to plants than soil I (Bowley et al 2019 ).…”

“…In temperate regions, I applied to soils is rapidly fixed into inert humus-bound forms and adsorbed onto Fe and Al hydrous oxides; these processes therefore restrict I availability to plants (Bowley et al 2019 ; Shetaya et al 2012 ). Following rainfall or fertiliser I input, only a narrow window of opportunity therefore exists for uptake by plant roots before I becomes unavailable.…”

Multiple geochemical factors may cause iodine and selenium deficiency in Gilgit-Baltistan, Pakistan

“…In contrast, oceans are major reservoirs of I (Fuge & Johnson, 2015;Manousou et al, 2019;Medrano-Macías et al, 2016) and volatilisation from ocean water and movement through the atmosphere plays an essential role in I cycling through the environment and the biosphere (Johnson, 2003;Medrano-Macías et al, 2016). Thus, I input from the atmosphere, as dry or wet precipitation, often contributes greatly to soil and plant I (Bowley et al, 2019;Fuge & Johnson, 1986;Jensen et al, 2019;Johnson, 2003). It is recognised that I concentrations are greater in coastal areas compared to inland and mountainous regions located away from coasts (Bowley et al, 2016;Fuge & Johnson, 2015;Humphrey et al, 2018).…”

“…Apart from inputs, the concentration of I in soil is also affected by several factors affecting the retention capacity of the soil; these include climate, topography and soil characteristics such as organic matter concentration and pH (Bowley et al, 2019;Fuge & Johnson, 2015;Humphrey et al, 2018Humphrey et al, , 2020Mohiuddin et al, 2019). Therefore, a soil with a large I concentration does not necessarily produce I-rich plants because of factors affecting the availability of soil I (Bowley et al, 2019;Fuge & Johnson, 1986;Mohiuddin et al, 2019). Fresh waters generally have low I concentrations (Fuge & Johnson, 2015;Johnson, 2003) unless rivers run through I-rich sedimentary rocks (Fuge, 1989;Moran et al, 2002), whereas groundwaters typically have higher I concentrations; values of up to 1890 lg L -1 have been reported in some areas (Li et al, 2013;Qian et al, 2017).…”