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In Europe, the agricultural biogas sector is currently undergoing fast developments, and cattle manure constitutes an important feedstock. Batch dry digester processes with leachate recirculation prove to be particularly interesting for small-scale plants. However, their startup being relatively slow, the process could be facilitated by co-digestion with energy crops. In this study, Miscanthus x giganteus was chosen for its high biomass yields and low input requirements. The carbohydrate accessibility of this lignocellulosic biomass is limited but may be improved with alkali pretreatment. The efficiency of lime (CaO) pretreatment with low water addition on the biochemical methane potential (BMP) of miscanthus was investigated through two experimental designs (CaO concentrations ranged between 2.5 and 17.5% and pretreatment lasted 1, 3, or 5 days). The pretreated miscanthus was then co-digested with cattle manure in dry leach bed reactors. CaO pretreatments led to a 14–37% improvement of miscanthus BMP, and a 67–227% increase in the first-order kinetics constant; a high contact time was shown to favor methane production. According to these results and to industrial requirements, miscanthus was pretreated with 5 and 10% CaO for 5 days, then co-digested with manure in dry leach bed reactors. Nevertheless, the promising results of the BMP tests were not validated. This could be related to the high water absorption capacity of miscanthus.

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Lignocellulosic biomass is hardly degraded during anaerobic digestion. Amongst a large panel of pretreatments used to improve the biodegradability of this substrate, alkaline pretreatments are recognized as the most efficient to remove lignin and therefore improve the methane production of these substrates. This article uses different histological stains (FASGA, phloroglucinol, Mäule reagent, Congo red), immunolocalization and histological quantification on pretreated internode stem tissues section in order to decipher the mechanism of alkaline pretreatment action (CaO and NaOH) in the anatomical and lignocellulosic matrix scale of Sorghum Biomass 140 hybrid and of Miscanthus x giganteus Floridulus. A significant delignification of all tissues was observed (sclerenchyma, parenchyma and xylem) except in the epidermis and in the internal part of the perivascular sclerenchyma. The degradation of lignin under the effect of alkaline pretreatment is accompanied by a massive unmasking of cellulose and a reduction of crystalline cellulose. This induced an increase of anaerobic digestion kinetics for both biomass and of methane yield for miscanthus. Miscanthus is rich in G-type lignins located mainly at the level of the perivascular sclerenchyma of the external internode zone which was more degraded by alkalies than the S-type lignin, this may explain the improvement of miscanthus methane potential.

Sorghum currently contributes to the species portfolio that is supporting bioenergy production including anaerobic digestion. Although agro‐morphological ideotypes maximizing biogas production have been recently proposed, there is a crucial need to refine our understanding of the impacts of the stem composition and structure on this processing trait in order to ensure genetic gains in the mid to long terms. This study aims to assess the potential of Sorghum bicolor ssp bicolor stem genetic diversity to maximize genetic gains for biogas production and define a biogas stem ideotype. In this context, a panel of 57 genotypes, encompassing most of the stem composition variability available in cultivated sorghum, was characterized over five sites. Simultaneous histological and biochemical characterizations were performed. A high broad sense heritability associated with a moderate genetic variability was detected for stem biogas potential ensuring significant genetic gains in the future. In addition, the development of a stem histological phenotyping pipeline made it possible to describe the genetic diversity available for the internode anatomy and the repartition of key cell wall components. Consistently with previous studies, moderate to high heritability was observed for stem biochemical components. Genetic correlation, hierarchical clustering, and multiple stepwise regression analyses identified soluble sugar content as the first main driver of biogas potential genetic variability. Nevertheless, breeding programs should anticipate that biogas yield improvement will also rely on the monitoring of the cell wall components and their distribution in the stem jointly with the soluble sugar content. According to the assets of sorghum in terms of adaptation to environmental stresses and the present results regarding the identification of stem ideotypes suitable for different value chains, this species will surely play a key role to optimize the economic and environmental sustainability of the agrosystems that are currently facing the effects of climate change.