Effects of Air Drying or Sample Storage on Soil-Solution Properties of Biosolids-Amended Soil

Accurate measurements of heavy metal soil-solution concentrations in soils are important for studies of metal bioavailability. The effect of air-drying or storage of field-moist soil samples at 4°C for up to 64 d on copper (Cu), nickel (Ni), and zinc (Zn), macronutrient cation and anions, and soluble organic carbon (SOC) concentrations in soil-solutions was determined using soils sampled from experimental field plots amended 3 y previously with Cu-, Ni-, or Zn-spiked biosolids. The soil at the field site was a Templeton fine sandy loam (Udic Ustochrept), and each of the three metals was applied at 4 different rates (12 plots in total). All data were subjected to analysis of variance, and comparisons between post-sampling treatments for major nutrient cations and anions and SOC were based on mean values from all 12 field plots. The Cu, Ni, and Zn treatments were not replicated in the field, and post-sampling treatment comparisons for soil-solution Cu, Ni and Zn were based on the means of triplicate determinations of single samples. The concentrations of cations (Ca2+, Mg2+, K+, Na+) and anions ( Cl−, ) in soil-solution were increased (by between 38 and 259%) by air-drying and, apart from also increased with storage time. In contrast, Cu, Ni, Zn, and SOC concentrations were decreased by air-drying (up to a 70% decrease in the case of Zn). Copper and Ni solution concentrations also decreased with increased storage time, as did the SOC concentration, while there was no change in Zn concentration. The results suggest that to obtain soil-solution that reflects in situ conditions, extraction should be carried out on field-moist samples as soon as possible after sampling.