Influence of drying vs. freezing of archived soil samples on soil organic matter fractions

Archived soil samples are a valuable tool for any long-term soil research. We analysed total carbon (C) and nitrogen (N) content and soil organic matter fractions in 38 archived soil samples that were stored for up to 21 years and compared air-dried storage to frozen storage conditions. Samples include top- and upper subsoils, different soil texture and land use with C contents between 4.3 and 174 mg g⁻¹. The results from this study reveal no changes in total C and N contents with storage time up to 21 years or type of storage (freezing vs. air drying). The analyses of soil physical fractions also revealed no significant differences between air-dried stored and frozen stored samples for most samples. However, we found indications, that freezing of soil material might lead to changes in the mineral fractions for soils containing high amounts of water. Therefore, and as archiving soils in a frozen state is more expensive than storing air-dried samples, we recommend the use of air-dried samples for C quality analyses of archived soil samples.