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The use of the residual material from waste aerobic digestion and biochar as amendments is currently discussed in the literature concerning the positive and negative effects on soil quality. We assessed the suitability of digestate (D) from biogas production and green biochar (B) to improve soil biological activity and antioxidant capacity and investigated whether there is an interaction between digestate and biochar applied to soil in combination. In a short-term (100-days) laboratory incubation, we monitored soil chemical and biological parameters. We compared soil amendments with 1% D (D1), 5% D (D5), 1% B (B), digestate–biochar combinations (D1+B and D5+B), and soil with no amendment. In D5, CO2 production, antioxidant capacity (TEAC), and dehydrogenase activity (DH-ase) and the contents of microbial biomass C, DOC and alkali-soluble phenols increased to the highest level. The biochar increased the total organic C (TOC) and TEAC of soil but decreased DOC, CO2 production, microbial biomass C, and DH-ase. The addition of biochar to digestate reduced soluble compounds (DOC and phenols), thus limiting the amount and activity of the soil microbial biomass (CO2 production and DH-ase). After 100 days of incubation D5+B showed the highest TOC content (82.8% of the initial amount). Both applied alone and in combination with digestate, the biochar appears to enrich the soil C sink by reducing CO2 emissions into the atmosphere.

Pyroligneous acid (PA) is produced during the combustion of woody biomass and is a complex aqueous fraction resulting from the thermochemical rupture of the components of vegetable biomass. We evaluated the effect of PA on the soil microbial community and activity in order to assess the applicability of this acid in soil and to gather further information on the mechanisms of its toxicity or stimulation. Five concentrations of PA solution (0, 0.5, 1, 2 and 5%) were selected to monitor the biochemical parameters of the soil. The respirometric test showed that the increase in the evolved carbon dioxide-carbon (C) was not due to a release of the native organic C from the soil, but only from the organic compounds of PA. The highest values of microbial biomass content were found in the soil treated with the lowest PA doses, but decreased with increased doses. At higher application doses (2 and 5%), there was a decrease in most enzymatic activities and a loss of soil quality. When PA was applied in doses of up to 1%, our results indicated no negative effects on soil biology and that there was even an improvement.

Purpose: Biochar addition has been recognized as a potential way to improve soil quality. However, questions remain regarding the influence of biochar on soil biological activity. In order to mitigate the possible negative effects of biochar on soil biological activities, it can be enriched by amendments such as compost. Since there is no unanimity on the advantages of biochar when mixed with amendments, it is important to ascertain how the impacts of biochar on soil biological activity could be changed by the addition of compost.Materials and methods: A 360-d aerobic incubation was carried out of a soil treated with biochar, green compost, vermicompost, biochar+green compost and biochar+vermicompost. The biochar was produced from pruning residues of fruit trees by slow pyrolysis at 550 °C. The green compost was taken to the CERMEC facility (Massa Carrara, Italy) and the vermicompost was produced mainly from farmyard manure and green waste by the Centro di Lombricoltura Toscano (Pisa, Italy). The pH, total and dissolved organic C, microbial biomass, dehydrogenase and alkaline phosphatase were monitored. The metabolic quotient, specific enzyme activities and the metabolic potential were calculated.Results and discussion: After 360-d incubation the green compost and vermicompost significantly lowered the alkaline soil pH by about one unit, increased total and dissolved organic C, microbial biomass, microbial quotient, alkaline phosphatase and specific alkaline phosphatase, dehydrogenase and specific dehydrogenase, and metabolic potential. The improvement in the biological activity was more notable and permanent with vermicompost than green compost. The biochar lowered soil pH by about one unit, showed the lowest loss of the total organic C (3.9%), did not change the amounts of dissolved organic C and microbial biomass, induced scarce effects on biological activities. When mixed with biochar, composts significantly induced higher C mineralization, dissolved organic C, microbial biomass, dehydrogenase, and did not change the metabolic quotient, specific alkaline phosphatase and specific dehydrogenase activities. The metabolic potential of control was more than halved by the green compost (2.89) and was not changed by the vermicompost.Conclusions: The mixing of green compost, and especially vermicompost with biochar increased some biological parameters in the used calcareous soil compared with the biochar-only treatment.Biochar could have benefits for carbon sequestration. The specific enzyme activities (alkaline phosphatase and dehydrogenase) were more suitable indicators than the respective absolute activities and metabolic potential for detecting the effects of amendments on soil microbial activity.

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