Enzymatic activity as a parameter for the characterization of the composting process

Enzymatic activity (EA) was explored as a possible tool for composting characterization. Three composts (yard wastes, cotton wastes and a mixture of the two) were sampled during different phases of the process and divided in two fractions. The first was immediately analysed for microbial biomass C (B-C) and EAs (beta-glucosidase, arylsulphatase, acid and alkaline phosphatase). The second fraction was air-dried prior to analysis for the same EAs and for organic C (C-ORG), total N (N-TOT), dissolved organic C (DOC), extractable C (C-E) and humic-like C (CH). BC decreased throughout the composting period (149 days), whereas EA in moist fractions stabilized between 50 (beta-glucosidase, alkaline phosphatase) and 90 (arylsulphatase, acid phosphatase) days of composting. EA was always reduced by air-drying (beta-glucosidase: 40-80%; arylsulphatase: 10-50%; acid phosphatase: 10-70%; alkaline phosphatase: 50-90%), but this effect was less prominent as composting proceeded, especially for beta-glucosidase and alkaline phosphatase. EA in air-dried samples displayed the same trend as in moist ones, except that there was a marked difference (47-66%) between initial and final activities of all four enzymes. EAs in air-dried compost and content of humic-like substances showed a similar trend: a marked increase in the first 90 days of the process and no significant variations afterwards. This suggests that the formation of humic-enzymatic complexes has taken place and indicates that this process occurs almost totally during the first stage of composting. EA steadiness in air-dried samples occurred concurrently with the achievement of compost stability, as indicated by the conventional indexes (i.e. C-H, C-ORG/N-TOT). Therefore, the development of a stable enzyme activity in air-dried compost could represent a simple measure of compost stabilization in routine analysis of composting process. (C) 2004 Elsevier Ltd. All rights reserved.