Evaluation of β-Glucosidase Activity as a Soil Quality Indicator for the Soil Management Assessment Framework

The Soil Management Assessment Framework (SMAF) was developed to assess conservation effects on soil,and uses multiple soil quality indicator measurements to compare soil functioning. Our objective was to develop a SMAF-compatible scoring equation for soil beta-glucosidase (BG) activity using published data sets representing different: soils and management. The resulting equation was an S-shaped curve: y = a/[1 + bexp(-cx)], where x is the measured BG activity (mg p-nitrophenol [PNP] released kg(-1) soil h(-1)), a and b arc constants, and c is a factor modified by soil classification, texture, and climate. Data from a study conducted near Mandan, ND were used to test the model for sensitivity to crop management systems. Soil organic C (SOC) content at the site measured 247 to 687 g kg(-1), while BG activity ranged from 33 to 675 mg kg(-1) h(-1). Using SMAF, SOC indicator scores ranged from 0.25 to 0,73, while BG activity scores varied from 0.17 to 0.93. As the work progressed, it became apparent that when BG activity values were normalized to the SOC content, the resulting ratio could indicate C sequestration trends, with ratios of 10 to 17 g PNP kg(-1) SOC h(-1) reflective of systems in equilibrium. Ratios >17 were mostly from recently altered management systems with SOC contents trending upward, while ratios <10 were generally from soils that were expected to continue to lose soil C. The application of a sensitive C cycling enzyme activity such as BG should improve the SMAF soil quality assessments for soil functions where soil metabolic activity or C-cycle enzyme activity play a role.