Effects of organic amendments on soil carbon fractions, enzyme activity and humus–enzyme complexes under semi-arid conditions

The objectives of this study are to evaluate the changes in the pool of organic carbon fractions, microbial biomass, and the activity of total enzymes and humus–enzyme complexes of a semi-arid soil amended with organic wastes of distinct origin during a period 360 days. The experiment was carried out during 360 days under laboratory conditions using soil microcosms (500 g) amended with two different doses (5 and 10 g) of different materials: sewage sludge from a wastewater treatment plant (SS), compost from that sludge (CSS), the organic fraction of municipal solid wastes (MSW), and compost from MSW (CMSW). The different carbon fractions, such as the total organic carbon (TOC), water-soluble carbon (WSC), and microbial biomass carbon (MBC), increased in amended soils compared to the control soil without amendment, as well as dehydrogenase and hydrolytic enzymes (β-glucosidase and urease) activities. For instance, after 360 days the total organic carbon reached 1.41% in soil amended with the high dose of SS and water-soluble carbon content reached 56 mg kg−1 in soil amended with high dose of CMSW. The immobilized enzymes in the soil humic extracts exhibited different behaviors compared to total activity, depending on the origin of the organic material which suggests neo-formation of humic–enzyme complexes. Addition of CMSW or SS increased the activity of β-glucosidase linked to humic substances.

► A soil was incubated with sludge, organic fraction of solid wastes and their composts.
► Humic–enzyme complexes versus total enzyme activity were compared.
► Long-term monitoring during 360 days was carried out.
► Humic–immobilized enzymes exhibited different behaviors compared to total activity.