Role of biochar as an additive in organic waste composting

The use of biochar in organic waste composting has attracted interest in the last decade due to the environmental and agronomical benefits obtained during the process. Biochar presents favourable physicochemical properties, such as large porosity, surface area and high cation exchange capacity, enabling interaction with major nutrient cycles and favouring microbial growth in the composting pile. The enhanced environmental conditions can promote a change in the microbial communities that can affect important microbially mediated biogeochemical cycles: organic matter degradation and humification, nitrification, denitrification and methanogenesis. The main benefits of the use of biochar in composting are reviewed in this article, with special attention to those related to the process performance, compost microbiology, organic matter degradation and humification, reduction of N losses and greenhouse gas emissions and fate of heavy metals

Biochar's role in mitigating soil nitrous oxide emissions: a review and meta-analysis

More than two thirds of global nitrous oxide (N2O) emissions originate from soil, mainly associated with the extensive use of nitrogen (N) fertilizers in agriculture. Although the interaction of black carbon with the N cycle has been long recognized, the impact of biochar on N2O emissions has only recently been studied. Herein we reflect on proposed hypotheses to explain N2O decrease with biochar, linking them to specific mechanisms for N2O formation and consumption in soil. Moreover, to assist in elucidating key mechanisms in which biochar may act in mitigating emissions of N2O, we undertook a meta-analysis using published literature from 2007 to 2013. This quantitative analysis used 30 studies with 261 experimental treatments. Overall, we found that biochar reduced soil N2O emissions by 54% in laboratory and field studies. The biochar feedstock, pyrolysis conditions and C/N ratio were shown to be key factors influencing emissions of N2O while a direct correlation was found between the biochar application rate and N2O emission reductions. Interactions between soil texture and biochar and the chemical form of N fertilizer applied with biochar were also found to have a major influence on soil N2O emissions. While there is clear evidence that, in many cases, emissions of N2O are reduced, there is still a significant lack in understanding of the key mechanisms which result in these changed emissions. As such, we have guided readers with suggestions to address specific research gaps, which we anticipate will enhance our knowledge and understanding of biochar's N2O emission mitigation potential

Biochar as an additive in composting: impact on process performance and on the agronomical quality of the end product

[EN] There is growing interest on the use of biochar in composting and also on its effect as growing media constituent. This study addresses the main effects of the use of biochar as an additive in organic waste composting in terms of the enhancement of the process and the quality and agronomical value of the end product. Biochar presents favorable physicochemical properties (such as large porosity, surface area and cation exchange capacity) that can enhance the environmental conditions for microbial growth in the composting piles and, consequently, affect key microbially-mediated processes such as organic matter degradation and nutrient cycling. Key impacts of biochar on composting performance, compost maturity and stability are reviewed, with special attention to those agronomical aspects related to organic matter humification, reduction of N losses and nutrient bioavailability. The interaction of biochar and compost as growing media constituents is also evaluated.Sánchez-Monedero, M.; Sánchez-García, M.; Castejón-Del Pino, R.; Fornes Sebastiá, F.; Belda Navarro, RM.; Lidón, A.; Cayuela, M. (2021). Biochar as an additive in composting: impact on process performance and on the agronomical quality of the end product. Acta Horticulturae (Online). 1317:175-187. https://doi.org/10.17660/ActaHortic.2021.1317.21175187131

Changes in soil humic pools after soil application of two-phase olive mill waste compost

The use of appropriate amendments derived from two phase olive mill wastes (TPOMW) can represent a suitable option to maintain and restore C levels in agricultural soils under Mediterranean climates. We evaluated soil organic matter stabilisation pathways among different humic pools in a Calcaric Cambisol amended with 2% (40 Mg ha− 1) of TPOMW composting mixtures of different composition and at different degrees of stabilisation: starting mixture, after 14 weeks of composting (thermophilic stage) and after 30 weeks (mature compost). Non-humified soil organic C and two different fractions of humic acids (HA), namely free HA (biochemically stabilised) and bound HA (biochemically and chemically stabilised) were obtained by sequential extraction with NaOH and alkaline Na4P2O7 after 90 and 150-days incubation. HA were characterised by thermal analysis, size exclusion chromatography (HPLC-SEC), FTIR and 13C CPMAS-TOSS NMR. Amendments promoted incorporation of altered lignin structures, carbohydrate moieties and N-containing compounds into free HA and to a lesser extent into the bound HA, and increased the proportion of high MW fractions. There was an average increase of 40% for non-humic C in the free C fraction even after 90 days of incubation under optimum conditions for mineralisation. Augmentation of bound C resulted in an average increase of about 0.7 Mg ha− 1 of humic C in amended soils. This increase is important as it contributes to one of the more inert soil C pools and could represent a useful indicator for soil C stabilisation.This work was supported by the Spanish Ministry of Science and Innovation, research projects Ref. CTM2005-05324 and CTM2009-14073-C02-02.Peer reviewe