Original articleInfluence of pruning waste compost maturity and biochar on carbon dynamics in acid soil: Incubation study

•Biochar did not affect soil respiration and did not cause priming of organic matter.•Priming was observed in case of immature compost and its mixture with biochar.•Synergism in biological activity was found between immature compost and biochar.•No synergies were observed between biochar and mature compost.•Immature compost and biochar had negative synergistic effect on ryegrass growth.

Compost is the most common organic fertilizer supplying nutrients and organic carbon to soil as well as improving soil physical, chemical and biochemical properties. On the contrary, biochar application to soil usually does not add nutrients, but can have effect on nutrient transformations and microbial community and alleviates soil acidity. Although these two products of organic residues recycling have different function in soil, their co-application could result in synergistic effect on soil biochemical properties. Therefore, the aim of present study was to determine how the application of biochar and compost in two stages of maturity (one month old after bio-oxidative phase; and final mature compost), applied alone or together, affects soil pH, water soluble carbon and nitrogen contents, carbon and nitrogen mineralization, microbial biomass and enzymes activities in acid soil in a short-term (60 days) incubation study. Additionally, same treatments were tested in a ryegrass growth assay. Application of all organic materials increased soil pH, which probably resulted in microbial community changes and overall decrease of microbial biomass carbon. Soil respiration was increased after application of immature compost (903 μg CO2-C g−1) or its mixture with biochar (823 μg CO2-C g−1), but we did not observe significant increase in respiration after biochar application respect to control (402 μg CO2-C g−1). Biochar decreased β-glucosaminidase activity and increased the activity of dehydrogenase. The higher values in β-glucosidase and dehydrogenase activities, as well as soil respiration, when immature compost and biochar were applied together, showed the synergism between these materials. Ryegrass growth was stimulated by all organic amendments, but combined application of immature compost and biochar resulted in growth increment lower than only biochar or only compost application. Adequate stabilization of pruning waste compost avoided priming of SOM induced by biochar co-application.