Evaluating drying-induced biases in soil biochemical properties: sensitivity patterns and texture controls

Soil pretreatment has a profound influence on measured properties and nutrient assessments, which in turn affect the accuracy of nutrient evaluation and ecological interpretation. In this study, we investigated the effects of air-drying and freeze-drying on organic matter (OM), pH, total nitrogen (TN), mineral nitrogen (NO₃⁻, NH₄⁺), available phosphorus (AP) and potassium (AK), available micronutrients (A_Fe, A_Mn, A_Cu, A_Zn), and nitrogen transformation rates (Ra, Rn, Rm) across land use types and soil textures in the Loess Plateau. Both drying treatments significantly increased OM, micronutrient availability, and nitrogen transformation rates—particularly under freeze-drying—highlighting the high sensitivity of these variables to drying pretreatment. Therefore, fresh or minimally disturbed samples are recommended for accurate measurement of these sensitive variables. Conversely, TN, AP, AK, and NH₄⁺ exhibited relatively minor alterations, indicating that conventional air-drying methods can still yield accurate measurements for these parameters. Furthermore, drying effects were strongly modulated by soil texture: silt- and clay-rich soils showed greater increases in OM and micronutrient availability, while sandy soils experienced more severe pH reductions. Clay-rich soils also suppressed Rm after freeze-drying. These findings provide empirical evidence that soil texture strongly modulates drying effects on key soil properties. Accordingly, pretreatment strategies should be tailored not only to the analytical targets but also to soil texture to reduce analytical bias, particularly in large-scale, cross-site, or long-term monitoring programs.