Soil refrigeration and air-drying preserve sensitivity of aminopeptidase activities to long-term nitrogen fertilization

Evaluating soil storage impacts on aminopeptidase activity can inform the use of aminopeptidase activity in soil N cycling assessments. We evaluated the impact of soil refrigeration (4 °C) and air-drying relative to field-fresh (4 °C for ≤ 3 d) on the activities and sensitivity of eight amino acid-specific aminopeptidase activities across a 42 y N fertilization gradient (0–269 kg N ha − 1 ) over 1.5, 6, 12 and 24 mo storage. Soil storage generally decreased (7–95%) or did not impact aminopeptidase activities, but in some cases increased activity (7–18%). Activity decreases were greatest for glutamic acid, arginine, and lysine aminopeptidases. Proline and leucine aminopeptidase activities were least impacted by soil storage. In field-fresh soils, only arginine and lysine aminopeptidase activities differed across the N fertilization gradient: ≈50% lower at the second highest (202 kg N ha − 1 ) relative to the lowest (67 kg N ha − 1 ) positive fertilization rate, a trend preserved for up to 12 mo in refrigerated soils and up to 24 mo in air-dried soils. Aminopeptidase activities that were similar across the N fertilization gradient in field-fresh soils also remained similar in stored soils, or responded similarly to N fertilization as arginine and lysine aminopeptidase activities. Most aminopeptidase activities were similar between refrigerated and air-dried soils for at least 6 mo or up to 24 mo. Though soil storage impacts on activities were aminopeptidase-specific, both storage methods preserved the sensitivity of aminopeptidase activities to long-term field N fertilization treatments.