The rapidly developing agro-industry generates huge amounts of lignocellulosic crop residues and animal manure worldwide. Although co-composting represents a promising and cost-effective method to treat various agricultural wastes simultaneously, poor composting efficiency prolongs total completion time and deteriorates the quality of the final product. However, supplementation of the feedstock with beneficial microorganisms can mitigate these negative effects by facilitating the decomposition of recalcitrant materials, enhancing microbial enzyme activity, and promoting maturation and humus formation during the composting process. Nevertheless, the influence of microbial inoculation may vary greatly depending on certain factors, such as start-up parameters, structure of the feedstock, time of inoculation, and composition of the microbial cultures used. The purpose of this contribution is to review recent developments in co-composting procedures involving different lignocellulosic crop residues and farm animal manure combined with microbial inoculation strategies. To evaluate the effectiveness of microbial additives, the results reported in a large number of peer-reviewed articles were compared in terms of composting process parameters (i.e., temperature, microbial activity, total organic carbon and nitrogen contents, decomposition rate of lignocellulose fractions, etc.) and compost characteristics (humification, C/N ratio, macronutrient content, and germination index). Most studies confirmed that the use of microbial amendments in the co-composting process is an efficient way to facilitate biodegradation and improve the sustainable management of agricultural wastes. Overall, this review paper provides insights into various inoculation techniques, identifies the limitations and current challenges of co-composting, especially with microbial inoculation, and recommends areas for further research in this field.

The primary purpose of this study was to investigate the influence of Cellulomonas flavigena and Streptomyces viridosporus, as a bacterial inoculant, on the compostability of post-extraction lavender waste. The major physicochemical, microbiological, and biological properties of the composting materials were monitored for 161 days. The technology developed was shown to improve the compostability of recalcitrant herbal residues. The use of lavender waste beneficially affected the composting process by extending the thermophilic phase, accelerating the degradation of organic matter, and elevating the viable counts of useful microorganisms; however, adverse effects were also observed, including an increased carbon-to-nitrogen ratio (19.05) and a decreased germination index (93.4%). Bacterial inoculation was found to preserve the nitrogen content (2.50%) and improve the efficiency of biodegradation. The Salmonella- and Escherichia coli-free final composting products were mature, stable, and ready for soil application. To the authors’ knowledge, no previous research has investigated the compostability of lavender waste. Likewise, this is the first study that has used strains of C. flavigena and S. viridosporus in combination to facilitate a composting process.

Because of the increasing use of herbal essential oils (EO) in pharmacology, medicine, and food processing, global EO production grows at a rapid rate, generating large amounts of plant residues worldwide. The use of herbal residues for co-composting with various wastes offers a novel option for eco-friendly and cost effective waste management. However, the biologically active compounds remaining in the herb residues may affect microbial activity in the composting biomass, thereby influencing final compost quality. In this review, we summarize and evaluate current literature on the occurrence and survival of pathogens in composts, the beneficial effects of using herbal residues as bulking agents, and the potential antipathogenic effects of six herb species belonging to the Lamiaceae family as follows: lemon balm (Melissa officinalis L.), oregano (Origanum vulgare L.), lavender (Lavandula angustifolia Mill.), rosemary (Rosmarinus officinalis L.), thyme (Thymus vulgaris L.), and sage (Salvia officinalis L.).