Are Extracellular Enzyme Activities a Useful Indicator of Nutrient Availability in Semi-arid Soils?

Agricultural stakeholders crave better ways to monitor and assess soil nutrient availability. Potential extracellular enzyme activities (EEA) were initially proposed as a biochemical indicator of soil fertility several decades ago due to their fundamental role in the transformation of nutrients by decomposing organic matter. However, both positive and negative correlations between EEA and soluble nutrient concentrations have been found in previous studies, leading to uncertainty about how to interpret the relationship between these measurements. To determine whether commonly assayed extracellular enzymes can serve as a useful indicator of nutrient availability in semi-arid soils, we investigated the correlation between three hydrolases-1,4-glucosidase (BG), -1,4-N-acetylglucosaminidase (NAG), and leucine-aminopeptidase (LAP)with concentrations of soluble nutrients [i.e., ammonium (NH4+), nitrate(NO3-)] in soils from six Arizona wine grape vineyards and adjacent desert ecosystems. The soils represent a variety of physicochemical properties and management histories, allowing us to identify potential relationships across a variety of conditions. Results so far indicate that, when analyzed at a regional scale (i.e., all sites fitted to a single linear regression model), no correlation is seen between soil NO3- concentration and BG, NAG, or LAP activity. A weak positive correlation was found between NH4+ concentration and BG activity at a regional scale. At the scale of individual vineyards and adjacent, unmanaged desert ecosystem plots, four of six sites showed no significant correlation between NO3- concentration and BG, NAG, or LAP activity. Two sites showed correlations between NH4+ concentration and EEA; one demonstrated a positive correlation between BG and NAG with no correlation to LAP, while the other showed a negative correlation to LAP activity with no correlation to BG or NAG. No site showed correlations between both NH4+ and NO3- concentrations simultaneously. The lack of correlation at regional scales, combined with inconsistent findings at local scales, suggest that extracellular enzyme activities alone are unlikely to be a useful indicator of nutrient availability in semi-arid soils.