Does long-term application of fertilizers enhance the micronutrient density in soil and crop?-Evidence from a field trial conducted on a 47-year-old rice paddy

To date, how optimized, long-term fertilizer application affects the concentration of micronutrients in the milling fractions of rice grains (bran or edible grain part) with details of their dietary exposure upon consumption has not been much explored. In this study, we report results from a 47-year-old field experiment conducted in rice paddy. Soil and rice grain samples were collected from a 47-year-old long-term fertilization field trial (treatments included NPK, compost, NPK + compost, and no fertilizer/control) conducted at Miryang, South Korea. Density of total micronutrients namely As, B, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, V, and Zn in soil and rice milling fractions were quantified using ICP-MS and inductively coupled plasma optical emission spectrometry (ICPOES). Concentrations of the exchangeable and water-soluble micronutrients in the soil were also determined by ICP-MS and ICPOES. Potentially bioavailable soil micronutrient levels, micronutrient transfer from soil to rice grain, and dietary intake level of micronutrients by rice consumption were calculated. The correlations between soil properties and crop micronutrient availability were analyzed. In general, long-term fertilization (NPK + compost > NPK > compost) yielded a significant effect on grain As, B, Co, Cr, Cu, Fe, Mn, Mo, Ni, Se, V, and Zn fortification and promoted bioavailable soil micronutrient density. The highest contributors to micronutrient intake could be rice grains from long-term NPK + compost fertilized plots. However, estimated levels were still lower than the recommended dietary intake levels for micronutrients established by FAO/WHO. From our study, it is important to note that 80% of total micronutrient content (Fe, Mn, Zn, Co, Ni, Se, and V) were concentrated in rice bran. This has important implications for human nutrition and health. Furthermore, correlation analysis indicates that available soil P could be considered a predictor of crop micronutrient intake. Optimized long-term application of compost with NPK fertilizers to soils is a potential grain fortification strategy. Long-term fertilization will not reasonably benefit individuals who consume only white rice, because bran with enriched micronutrients will be removed by milling. We suggest replacing the consumption of white rice by brown rice so that individuals have a better micronutrient intake.