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Abstract
Because potential extracellular enzyme activities (EEAs) represent microbial investment into nutrient acquisition, they may be sensitive to soil nutrient conditions and land management practices. However, inconsistent relationships between EEA and bioavailable nutrients such as inorganic N and P are generally reported. I therefore asked whether bioavailable nutrient concentrations could serve as a reliable predictor of EEA in semi-arid ecosystems. I sampled six vineyards throughout the state of Arizona, USA, collecting soil samples from within the vineyard itself and outside the area of active management. A suite of soil physicochemical characteristics including nitrate, ammonium, phosphate, and the activities of two N-acquiring [N-acetyl-β-glucosaminidase (NAG) and leucine aminopeptidase (LAP)] and one P-acquiring [phosphatase (PHOS)] EEAs were measured in each sample. I found that concentrations of inorganic N were extremely poor predictors of NAG and LAP activities. Soil phosphate concentrations were similarly poor predictors of PHOS activity. The activities of the two N-acquiring enzymes were not correlated with each other, contrary to my expectations. Additionally, the measured EEAs did not show consistent patterns based on sampling location. These findings lead me to the conclusion that EEAs are unreliable indicators of nutrient limitation or land-use in semi-arid (agro)ecosystems.