How air-drying and rewetting modify soil organic matter characteristics: An assessment to improve data interpretation and inference

Air-drying and wetting of air-dried soil samples with water (i.e., rewetting) are widely used sample treatments in soil analyses. It is recognized that both air-drying and rewetting of soil samples affect the characteristics of organic matter (OM), but systematic evaluations are scarce. In this review, we synthesize what is known in the scientific literature concerning the types and magnitudes of effects resulting from air-drying and rewetting with respect to i) characteristics of aggregate-associated and water-extractable OM, ii) soil microbiota, and iii) decomposition of OM. Air-drying of soil samples results in the formation of new and/or stronger OM-mineral interactions as well as increased hydrophobicity and mineral surface acidity. The formation of new and enhancement of existing OM-mineral interactions may lead to an increase in perceived aggregate stability, potentially affecting estimates of amount and persistence of OM associated with soil aggregates. Compared to field moist samples, air-dried samples had 8–41% higher relative dry mass proportions in the 2–0.25 mm aggregate size fraction. Pronounced changes in the amount and composition of the water-extractable OM and soil microbiota are also detected during the course of air-drying and rewetting with the potential to affect the conclusions derived from OM decomposition experiments. Air-dried soil samples were found to have 2–10 times higher amounts of water extractable organic carbon and a decrease between 3% and 69% in the microbial biomass carbon (using the substrate-induced respiration technique) compared to field moist samples. The magnitude of air-drying and rewetting derived effects on sample characteristics appears to be site and soil type specific.