Use of β-Glucosidase Activity As An Indicator in Watershed Soil Quality Assessments.

Soil quality assessment is a process for understanding long-term effects of soil and crop management. Such assessments at the field and watershed scales use a number of key indicators that are measured at points across the area to be assessed.  For the USDA Conservation Effects Assessment Project (CEAP), the Soil Management Assessment Framework (SMAF) has been the tool of choice to complete the soil quality assessment. Amongst the key soil quality indicators of microbial activity β-glucosidase (BG; EC 3.2.1.21; systematic name: β-D-glucoside glucohydrolase), which is involved in cellulose decomposition and one of the few indicators with SMAF scoring curves. In this paper, we will review the development of SMAF-BG scoring curves which are S-shaped: 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 are constants, and c is a factor modified by soil classification (ability to sequester carbon), texture, and climate. Discussion will include the results from near-surface soil samples across 17 cropland watersheds in GA, IA, ID, IN, KS, MD, MO, MS, NH, OH, OK, PA, PR, and TX (>3500 samples).  Preliminary results show that BG activity ranged from 3.9-704.9 mg PNP kg^-1 h^-1 in the 0-5 cm depth and 3.2-303.0 at 5-15 cm, with means of 114.5 and 63.4, respectively. Scoring BG activity with the SMAF equations allows for a comparison to like soils and the levels of BG activities that can be expected. The mean BG score was 0.31, with results from each watershed varying widely depending on tillage, manure amendments, cover crops, crop at time of sampling and cropping systems.