Validation of Soil Enzyme Activity Assay for a Biogeochemical Cycling Index in Biochar Amended Soils

Biochar offers several benefits as a soil amendment, including increased soil fertility, carbon sequestration, and water-holding capacity in nutrient-poor soils. Here, we performed a series of enzyme assays on pine biochar-amended soils, comparing multiple enzyme activities (EAs) simultaneously determined in the same soil sample vs. the sum of individual EAs involved in the C, N, S, and P cycles to provide information of the impacts of biochar on biogeochemical cycling. The combination of these four EAs has been considered an indicator of soil health due to their role in the reactions that release bioavailable nutrients in the cycling of C (β-glucosidase), N and C (β-glucosaminidase), P (acid phosphomonoesterase), and S (aryl-sulfatase) in soils. Comparisons of the theoretical EAs and the CNPS activity assay approaches in the biochar-modified soil revealed similar activity trends with the different concentrations of added biochar. Two years after adding biochar, study results showed the amended soils did not retain more pNP substrate than the un-amended control soils in three different pH buffers (5.5, 5.8, and 6.5) commonly used in EA reactions. Finally, we performed a third experiment to determine if the biochar previously added to the EAs interfered with the reactions’ enzyme or substrate. The results indicated that greater activity was measured using the combined assay, which suggests the CNPS activity method was less affected by biochar than the individual EAs. Our findings indicate that the potential soil biochemical-health index, CNPS activity (combination of four enzymes) assay is more robust than the individual EAs and can be used as an alternative tool to monitor soil functioning.