Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2009
Publication Date: 12/1/2009
Citation: Stott, D.E., Andrews, S.S., Liebig, M.A., Wienhold, B.J., Karlen, D.L. 2009. Evaluation of ß-Glucosidase Activity as a Soil Quality Indicator for the Soil Management Assessment Framework (SMAF). Soil Science Society of America Journal. 74(1):107-119. Interpretive Summary: Five years ago, the Conservation Effects Assessment Program (CEAP) was initiated to determine the effectiveness of conservation management practices in maintaining or improving soil, air and water quality. While the initial emphasis has been on water quality, a project was recently initiated to assess the soil quality on the USDA-ARS CEAP Experimental watersheds. Soil quality cannot be measured directly because it is a broad, integrative concept. Instead, a variety of proxy measurements are analyzed, which together provide clues about how the soil is functioning. These measurements are called soil quality indicators. Although the quantity and quality of data may differ, the process of soil quality evaluation follows the same basic steps regardless of the method used: indicator selection and interpretation. Once selected, indicators must be measured and results analyzed. To be useful, this must be followed by appropriate changes in management practices, when needed. Indicators should be reevaluated periodically to monitor trends. Assessment tools are needed to evaluate management affects on critical soil functions. The Soil Management Assessment Framework (SMAF) model is being developed to meet this need (available at http://soilquality.org/). SMAF uses measured soil indicator data to assess management effects on soil function using the three step process of indicator selection, indicator interpretation, and integration into an index. Currently SMAF has 11 indicators representing mostly soil physical and chemical characteristics. Soil enzyme activities have been suggested as indicators as they reflect critical processes in several nutrient cycles in the soil. The objective of this work paper was to develop an indicator scoring curve for SMAF for a soil enzyme activity, which may give early indications of trends in soil organic carbon (SOC) accumulation. We wished to develop an indicator curve for an enzyme that was involved in the C cycle. We chose ß-glucosidase (BG) for several reasons: 1) It mediates the last rate limiting step in cellulose degradation. Thus, is important in soil organic matter decomposition and nutrient cycling; 2) Responds to changes in tillage and residue management; 3) Is among the most prevalent soil enzyme activities reported in the scientific literature; and 4) Would be considered along with other organic matter indicators such SOC, total nitrogen, total Kjeldahl nitrogen, particulate organic matter, and microbial biomass. ß-glucosidase indicator curve would contribute to the following SMAF soil functions: nutrient cycling, biodiversity and habitat, filtering and buffering, and physical stability and support. A more-is-better indicator scoring function was used. A scoring curve relates the measured activity to the highest potential activity for soils of similar soil taxonomic classification, soil texture and climate. Data sets published in the scientific literature were used for curve development and initial validation. A field study in Mandan, ND, was used as additional validation. As the work on the soil quality assessment of the USDA-ARS watersheds progresses, these curves will continue to be validated or altered as necessary. As a by-product of this work, it became apparent that when BG activity values were normalized to the SOC content, the resulting ratio could indicate trends in carbon sequestration, with ratios of 10 to 16 mg p-nitrophenol (PNP) released g-1 SOC h-1 incubation reflective of a system in equilibrium (native vegetation, long-term pasture, and some long-term no-till). Ratios greater than 16 were from recently altered management systems with SOC contents trending upward, while ratios below 10 were soils that were continuing to lose soil carbon. The impact of this work is that the usefulness of the SMAF has been broadened to include a soil biochemical parameter. The BG activity indic
Technical Abstract: Soil quality cannot be measured directly because it is a broad, integrative concept. Instead, a variety of proxy measurements are analyzed, which together provide clues about how the soil is functioning. These measurements are called soil quality indicators. Currently, as part of the Conservation Effects Assessment Program (CEAP), soil quality within the USDA-ARS CEAP experimental watersheds is being assessed using the Soil Management Assessment Framework (SMAF). To date, SMAF includes 11 indicators primarily representing soil chemical and physical characteristics. Our objective was to develop a SMAF compatible scoring equation for soil ß-glucosidase (BG) activity using published data sets. The resulting equation was a sigmoidal s-shaped growth curve: y = a/(1 + be(-cx)), where x was the measured BG activity in mg p-nitrophenol (PNP) released kg-1 soil h-1 incubation, a and b were constants, and c was a factor modified by soil classification, soil texture, and climate. The equation was initially developed and validated by using independent published data sets. Additional data from a crop sequencing study conducted near Mandan, ND, was used to test the model for sensitivity to soil and crop management systems. Soil at the study site was a Temvik-Wilton silt loam association (fine-silty, mixed, superactive, frigid Typic and Pachic Haplustolls). Soil organic carbon (SOC) content at the site ranged from 247 to 687 g kg-1, while BG activity exhibited greater variation, ranging from 33 to 675 mg PNP released kg-1 h-1 (r = 0.81). Using SMAF, SOC indicator scores ranged from 0.25 to 0.73, while BG activity scores ranged from 0.17 to 0.93, thereby reflecting the wider variation observed in measured values (r=0.83). As a by-product of this work, it became apparent that when BG activity values were normalized to the SOC content, the resulting ratio could indicate trends in carbon sequestration, with ratios of 10 to 16 mg PNP released g-1 SOC h-1 incubation reflective of a system in equilibrium (native vegetation, long-term pasture, and some long-term no-till). Ratios greater than 16 mg PNP released g-1 SOC h-1 were from recently altered management systems with SOC contents trending upward, while ratios below 10 were soils that were continuing to lose soil carbon. The impact of this work is that the usefulness of SMAF has been broadened to include a soil enzyme activity parameter. Additionally, BG activity, when normalized to the SOC content, provides an early indicator of carbon sequestration trends as management systems are changed. This will impact researchers and land managers.