Fig. 7

Soil enzyme activities—A arylsulfatase (ARS), B urease (URE), C phosphatase (PHOS), D N-acetyl-β-D-glucosaminidase (NAG), E β-glucosidase (GLU)—of substrate treatments amended with biochar made of sewage sludge, sawdust, and zeolite. Mean ± standard deviation (error bars) calculated from independent values (n = 36), different letters express the statistical differences at significance level p ≤ 0.05