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Despite the use of biochar as a soil amendment has been widely studied as a promising strategy for climate change mitigation, to date, due to the extremely heterogeneous nature of this material, there is not a general agreement of the scientific community about its soil amendment potential. In the present paper, a comprehensive approach has been adopted to test the short-term effects of biochar alone or in combination with compost on soil biochemical properties related to C and N cycling functions, microbial community, and lettuce (Lactuca sativa L.) yield in a Mediterranean environment. With this purpose, a randomized block field trial was set up within an experimental area of about 500 m2. The four treatments tested were: 1) Control (C), no treatment; 2) Compost (CMP); 3) Biochar (BIO); 4) Compost-Biochar mixture (CMP + BIO in the ratio 9:1 w /w). After 45 days of cultivation, the biomass of well-developed lettuce plants was increased from 2- to 2.5-fold in all amended soils with respect to the control one. The CMP amended soils showed a higher effect with respect to the BIO amended soils on enzymatic activities (increase in β-glucosidase, SEI, synthetic enzymatic index, and butyrate esterase) and microbial community structure evaluated by the Ester-linked Fatty Acid Methyl Ester (El-FAME) profiling (increase in G- bacteria, total fungi, and AMF). The increase in the activity and abundance of the microbial community in CMP amended soils could indicate that the organic matter provided by the compost is more readily mineralized than that added with the biochar. Moreover, the BIO amended soils showed the lowest qCO 2 and ecoenzymatic C:N ratio and the highest q MIC (1.4 %), indicating a C immobilization process into the microbial biomass and a greater limitation for N with respect to C, offset by an increase in N-cycling enzyme activity. In the short term, even if the number of well-developed plants increased in all treated soils, a boost of soil C and N cycling was evident with CMP and BIO amendments respectively. Conversely, the combination of CMP with BIO (CMP + BIO) did not provide any synergic effect, probably because requiring longer periods. For these reasons repeated combined treatments can be suggested to reach a cumulative effect on soil quality over time using a balanced C:N ratio in the mixture amendment. • The number of well-developed plants (Lettuce sativa L.) increased in all treated soils. • Microbial viable biomass and its enzymatic activity were increased by compost treatment. • Biochar treatment induced C immobilization into microbial biomass and promoted N cycling. • The combining of compost and biochar seems to have had no synergistic effect. [ABSTRACT FROM AUTHOR]

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]

Biochar is increasingly considered an efficient amendment to improve soil fertility and store carbon in soils for climate change mitigation. However, few field studies have evaluated biochar impacts on soil fauna and especially earthworms in temperate areas despite their importance for soil structure and laboratory-based evidence of potential adverse effects. We investigated the effects of three biochars (from rapeseed, miscanthus and compost reject) in mixture with a green waste compost on the abundance and composition of earthworm communities during a three-year field experiment in northern France. Compost alone or biochar-compost mixtures were applied at 11.7 t dry matter ha−1 and compared to equivalent mineral fertilization. Biochar-compost mixtures did not affect total earthworm density and biomass, while contrasting effects concerning specific ecological groups were observed. Compost addition increased juvenile endogeic earthworm density roughly one month after amendment while no adverse short-term effects of biochar-compost mixtures were observed. Two years after amendment application, anecic earthworm density was higher for the mixture with compost and biochar from compost rejects, which also resulted in the highest soil organic carbon concentrations. We conclude that biochar-compost mixtures at low but more economically viable biochar application rates could be used as an increasingly understood beneficial soil amendment under temperate climate conditions without adverse effects on earthworm communities. [ABSTRACT FROM AUTHOR]