Soil Microbial Exoenzyme Activity Changes in Response to Amending Agricultural Soil with Corn Stover Biochar.

The increasing use of biochar as an agricultural soil amendment has raised concerns about its possible effects on soil microbial populations. Hydrolytic exoenzyme activity, as an indicator of metabolically active soil microorganisms, was investigated by sampling soils from a corn field amended with varying rates of corn stover biochar or left unamended in Aurora, NY. Activity of fluorescently labeled substrate analogues for β-D-glucosidase, β-D-cellobiase, aminopeptidase and phosphatase were monitored. All standard enzyme extraction methods proved inadequate for use in the biochar-amended soils, due to their strong adsorptive properties. Recovery of the enzymatically liberated fluorophores, 4-methylumbelliferone (MUF) and 7-amino-4-methylcoumarin (MCA), was reduced by 54% and 63% respectively, in biochar-amended soils. We overcame this limitation by measuring the equilibrium adsorption isotherms for both fluorophores and using a correction factor. The biochar-amended soils had higher activities of aminopeptidase and phosphatase, but lower activities of β-D-glucosidase, β-D-cellobiase. These changes in enzyme activities suggest that P and N use is increased relative to C mineralized and indicate a functional shift in microbial activity in response to adding biochar. The high adsorption capacity of biochar for the low-molecular weight compounds, MUF and MCA as well as for genomic DNA (see Jin et al., this volume), and the decreased activity of C mineralizing enzymes likely contribute to the observed long-term preservation of labile C in other soils high in black carbon (e.g., terra preta soils of Brazil).  Soil exoenzyme activities proved to be useful indicators of changes in microbial metabolic potential in response to biochar amendment. The relevance of these changes to observed crop yield improvement, C sequestration and changes in gas emissions reported for biochar-amended soils requires further investigation.