Water-based enzyme assays are more sensitive than buffer-based assays to tillage and site in the U.S. Corn Belt

Abstract

Assay methodology can influence the sensitivity of soil enzyme activities to agroecosystem management practices. Using soils from six long-term tillage experiments encompassing maize-based agroecosystems and three soil orders (e.g., Mollisols, Alfisols, Ultisols) across the U.S., we quantified the sensitivity of acid phosphomonoesterase (AC-PME), alkaline phosphomonoesterase (AK-PME), β-glucosidase (BG), N-acetyl-β-D-glucosaminidase (NAG), and arylsulfatase (SUL) activities to tillage practices (conventional vs none) and site measured by water-based assays versus buffer-based assays. To explain soil enzyme activity sensitivity differences between assay methodologies, we assessed (1) relative differences in activities measured with buffer- rather relative to water-based assays, (2) differences between soil and assay pH, and (3) activities on a soil versus soil organic carbon (SOC) basis. On a soil mass basis, enzyme activities were ≈2-fold more sensitive to tillage and to site using water- relative to buffer-based assays. Water-based NAG activity was most sensitive to tillage and site, whereas buffer-based SUL activity was least sensitive to tillage and buffer-based AK-PME activity was least sensitive to site. The difference between assay pH and soil pH were 1.5- to 6-fold lower for water-based assays, though pH divergences did not fully explain differences between water- and buffer-based activities. Buffer-based assays produced lower AC-PME (−32%) and AK-PME (−53%) activities but higher BG (+23%), NAG (+43%) and SUL (+87%) activities relative to water-based assays. Tillage effects on activities were primarily driven by SOC, except for buffer-based AK-PME activities. However, water-based activities maintained an enhanced sensitivity to site. We recommend the use of water-based assays to maximize the sensitivity of soil enzyme activities to tillage and to site.