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Biochar is an environmental soil application tool used for climate change mitigation and carbon (C) sequestration that can be used in forest soils. Soil physical and chemical properties can be altered by biochar amendment, which can affect soil microbial communities. A limited number of forest soil studies have shown biochar can affect soil microbial communities, but few studies exist in temperate, northwestern USA forests. We investigated the effects of biochar amendment to soil in three western USA managed forest sites. Sites were amended with 0, 2.5, or 25 Mg ha−1 biochar, applied to the soil surface. DNA was extracted from soil samples collected from two depths, three to five years following amendment (dependent on site). Double-barcoded 16S rRNA (bacteria) and LSU rRNA (fungi) amplicons were sequenced on an Illumina MiSeq platform. Resulting sequencing data was analyzed for community richness, diversity, phyla relative abundance, and composition. For fungi, sequencing data represented six phyla. Biochar amendment did not affect fungal community measures, but site and sampling depth or their interaction did. Bacterial sequencing data represented 33 phyla. Biochar did not affect richness or diversity measures. However biochar did affect Bacteroidetes and Acidobacteria phyla relative abundance either alone, or when dependent on site or sampling depth. As with fungi, site and sampling depth or their interaction influenced richness, diversity, phyla relative abundance, and community composition. Results indicate that biochar amendment to northwestern USA forests is not detrimental to soil microbial community composition. [Display omitted] • Biochar amendment is not detrimental to soil microbial community composition. • Biochar amendment does not affect fungal community measures. • Biochar affects Bacteroidetes and Acidobacteria phyla relative abundance. • Site and sampling depth affect fungal and bacterial community measures. [ABSTRACT FROM AUTHOR]

Organic soil amendments benefit agricultural soils depleted in soil organic matter because they improve soil chemical and biological properties. Biochar and compost, used as organic amendments, differ in their contents of total vs. available nutrients and may therefore differ in their effects on soil properties. The effects of these amendments have seldom been assessed in coarse-textured temperate soils and in no-tillage agriculture. In this study, we conducted a 6-month laboratory experiment with a coarse-textured temperate soil with a history of conventional farming to determine the effects of biochar, compost, and their combination, which were spread evenly on the soil surface, on microbial activity and biomass, and nutrient release and leaching. Both biochar and compost increased microbial activity and nutrient release compared to the no-addition treatment, but compost effects were relatively short term (six months). Biochar and compost had additive effects on all properties when added in combination. Biochar addition to soil increased soil pH, microbial biomass, and the abundance of fungi, G+ bacteria, and actinobacteria after 6 months of incubation compared to the compost treatment and the no-addition treatment. Although biochar was expected to reduce loss of nutrients through leaching, the short exposure time and disturbance of the soil probably hindered its capacity to adsorb nutrients and to thereby limit leaching; as a consequence, the biochar acted only as a slow-release nutrient fertilizer during the 6-month incubation. [Display omitted] • Compost increased microbial activity and nutrient release during the short term. • Biochar increased microbial activity and nutrient release during the long term. • Biochar increased microbial biomass, abundance of fungi and bacteria, and soil pH. • Biochar did not reduce loss of nutrients through leaching. [ABSTRACT FROM AUTHOR]

Co-applying biochar and paper mill biosolids (PB) improves the potential of these by-products as soil amendments. Assessing the response of soil microbial communities to the co-application of biochar and PB is of great importance to further understanding their effectiveness. This hypothesis was tested in a 112-day incubation study performed separately on two acidic soils (Kamouraska clay and St-Antoine sandy loam) using two PB types (PB1 with C/ N = 24 and PB2 with C/ N = 13), co-applied with one of three rates (0%, 2.5%, and 5% w/w) of pine (Pinus strobus L.) biochar. An unamended control and a mineral-fertilized treatment were used as a reference. In the Kamouraska clay soil, co-applying 5% biochar and any PB increased microbial activity compared to PB-only application, as indicated by the fluorescein diacetate (FDA) hydrolysis assay. In both soils, co-applying biochar and PB did not affect microbial diversity and community structure compared with PB only. The application of any PB, however, increased microbial activity in both soils, while only PB2 shifted microbial community structure. Within each amendment and in both soils, microbial activity was lower at day 112 than at days 28 and 56, while major temporal shifts were observed only for bacterial community structure. Indeed, the difference in bacterial community structure between amendments was more pronounced at day 112. This study suggests that co-applying biochar and PB can improve soil properties than application of PB alone by increasing soil microbial activity without side effects on microbial diversity and community structure, especially in acidic clay soils. • Co-applying biochar and paper mill biosolids (PB) increased soil microbial activity. • Co-applying biochar and PB did not affect soil microbial community structure. • The incubation time affected soil microbial activity and community structure. [ABSTRACT FROM AUTHOR]