Evaluation of Cr, Mn, Fe, Zn and Se in humans is challenged by the potentially high within-individual variability of these elements in biological specimens, which are poorly characterised. This study aimed to evaluate their within-day, between-day and between-month variability in spot samples, first-morning voids and 24-h collections. A total of 529 spot urine samples (including eighty-eight first-morning voids and 24-h collections) were collected from eleven Chinese adult men on days 0, 1, 2, 3, 4, 30, 60 and 90 and analysed for these five elements using inductively coupled plasma-MS. Intraclass correlation coefficients (ICC) were utilised to characterise the reproducibility, and their sensitivity and specificity were analysed to assess how well a single measurement classified individuals’ 3-month average exposures. Serial measurements of Zn in spot samples exhibited fair to good reproducibility (creatinine-adjusted ICC = 0·47) over five consecutive days, which became poor when the samples were gathered months apart (creatinine-adjusted ICC = 0·33). The reproducibility of Cr, Mn, Fe and Se in spot samples was poor over periods ranging from days to months (creatinine-adjusted ICC = 0·01–0·12). Two spot samples were sufficient for classifying 60 % of the men who truly had the highest (top 33 %) 3-month average Zn concentrations; for Cr, Mn, Fe and Se, however, at least three specimens were required to achieve similar sensitivities. In conclusion, urinary Cr, Mn, Fe, Zn and Se concentrations showed a strong within-individual variability, and a single measurement is not enough to efficiently characterise individuals’ long-term exposures.

In a 400-day laboratory experiment, soil was amended with rape, sunflower and soybean residues to monitor the quantitative changes in the main inorganic nutrients. Total N, available P, exchangeable K+, Ca2+, and Mg2+ in all the amended samples increased significantly. Generally, the increase in the amounts of these nutrients was maintained until the end of the incubation period, suggesting that the mineralization of the three crop residues enhanced soil fertility. In amended soil samples, disappeared within 14 days, while available N was released as after 60 days in soybean-treated and after 120 days in rape- and sunflower-treated soil, respectively. Water-soluble P was completely lacking in each treatment because of microbial immobilization and adsorption or precipitation processes in soil.