Optimal soil storage methods for enzyme activity assays: a meta-analysis

The lack of standardized soil storage methods for enzyme assays has restricted cross-comparison of soil enzyme activities in the literature, and reported effects of soil sample storage on enzyme activities are often study-specific and conflicting. We conducted a systematic literature review and a meta-analysis to (1) identify data gaps in evaluations of storage effects on soil enzyme activities and (2) quantify enzyme activity responses to soil sample storage method and duration. Evaluated storage methods were (i) cold (field moist soils at 2 to 7°C), (ii) freeze (field moist soils at -5 to -35°C) or (iii) air-dry (air-drying and storing at room temperature, assumed to be ≈24°C) relative to activities assayed ≤ 24 h of soil sampling (field fresh) and/or in soils under cold storage. Twenty-two research articles (1917-2025) evaluated 106 soils for effects of three soil storage methods on activities of β-glucosidase (BG), phosphomonoesterase (PME, assayed at pH 4.0-6.5), N -acetyl-β-glucosaminidase (NAG), and urease (URE). Most soils were acidic (86%), and were Oxisols (34%) or Mollisols (19%). Soil storage decreased BG (9-55%) and PME (7-53%) activities relative to activities in field fresh soils, with the least decreases with cold storage, and NAG activities (33-68%), with the least decreases with cold and freeze storage. Greatest decreases occurred with air-drying for BG, PME and NAG relative to activities in field fresh soils or under cold storage. Only cold storage (8%) decreased URE activity. Enzyme activities were independent of storage duration except for BG activity under cold storage. The decreases in activities due to storage were mostly inconsistent across soil pH, clay, OC, and orders, and varied by assay methods. Based on least decreases in activities that were consistent across soil properties and types, the most appropriate soil storage methods appeared to be cold storage ≤ 3 d for chromogenically assayed BG activity and freeze storage for fluorogenically assayed BG and PME activities. • Soil refrigeration, freezing, and air-drying decreased BG, PME, and NAG activities • Urease activity decreased with refrigeration, unaffected by freezing and air-drying • Storage effects unrelated to pH, clay, and organic C, but varied by assay method • ≤ 3 d of soil refrigeration most suitable for chromogenic assays of BG activities • Soil freezing most suitable for fluorogenic assays of BG and PME activities