Sample storage effects can matter less than spatiotemporal variation in soil phosphomonoesterase activity: A case study

Abstract Measured activities of soil enzymes such as phosphomonoesterase (PHO) can be affected by when and where soils are sampled, and how soils are stored prior to analysis. While these factors have been evaluated individually, simultaneous assessment is needed to determine the relative importance of sampling decisions and sample storage for measured soil PHO activities. We therefore evaluated the relative effects of intraseasonal sampling time, within‐plot location, and storage methods on soil PHO activities in an Argiudoll under perennial grassland and annual cropland. Soil PHO activity (µmol para ‐nitrophenol g −1 h −1 ) varied 1.1–4.6 (coefficient of variation = 31%) under cropland and 3.6–16.0 (26%) under grassland across sampling times, within‐plot locations, and storage methods. The effect of storage method (field‐fresh, refrigeration, or air‐drying) was minor relative to sampling time in cropland and within‐grid heterogeneity in grassland. Inconsistent effects of air‐drying and refrigeration on magnitude and spatiotemporal variability of PHO activities in our study suggest air‐drying may be acceptable given its practical advantages. Our work suggests prioritization of when and where soils are sampled over storage method when designing studies involving PHO activity assays.