The problems arising from the limited availability of natural resources and the impact of certain anthropogenic activities on the environment must be addressed as soon as possible. To meet this challenge, it is necessary, among other things, to reconsider and redesign agricultural systems to find more sustainable and environmentally friendly solutions, paying specific attention to waste from agriculture. Indeed, the transition to a more sustainable and circular economy should also involve the effective valorization of agricultural waste, which should be seen as an excellent opportunity to obtain valuable materials. For the reasons mentioned above, this review reports and discusses updated studies dealing with the valorization of agricultural waste, through its conversion into materials to be applied to crops and soil. In particular, this review highlights the opportunity to obtain plant biostimulants, biofertilizers, and biopolymers from agricultural waste. This approach can decrease the impact of waste on the environment, allow the replacement and reduction in the use of synthetic compounds in agriculture, and facilitate the transition to a sustainable circular economy.

The correct development of the composting process is essential to obtain a product of high value from organic wastes. Nowadays, some composting mixture parameters (i.e., air-filled porosity, moisture and the C/N ratio) are used to optimize the composting process, but their suitability is still debated because the literature reports contrasting results. This paper aimed to find other parameters that control the correct development of composting. The relationship between these and the compost quality was then verified. Twelve different composting mixtures were prepared using different organic wastes and bulking agents and were aerobically treated in a 300 L composter. The physico-chemical and chemical parameters of initial mixtures were analyzed, with particular regard to the total and water-extractable forms of organic C and N and their ratios and correlated with the temperature measured during composting. A positive correlation between temperature parameters during the active phase and soluble forms of N in the initial mixtures was found. A high total organic C to soluble N ratio in the composting mixtures was correlated with the low quality of the compost produced. Based on the results, a minimum content of WEN (water-extractable N) (0.4% w/w) or a TOC/WEN (total organic C/WEN) ratio in the range of 40–80 was recommended to ensure the correct development of the process and to produce compost of high quality.

Although the application of organic amendments to arable soils is considered to be a suitable tool for improving soil fertility and enhancing carbon (C) stocks, more research is required on the influence of input of organic matter on the activity and structure of the soil’s microbial community. The aim of this work was therefore to make a comparative study of the effects of organic materials with different degrees of stabilization and source (an untreated pig slurry, the solid fraction of the digestate from the anaerobic fermentation of pig wastes, a livestock-derived organic matter compost, and an urban waste compost) on the size, activity, and structure of the microbial community in two arable soils. These effects, studied through a laboratory incubation experiment, were related to the quantity and quality of organic matter added, as well as to the rapid changes in the more labile water-soluble organic matter fraction. Particular attention was devoted to the short-term variations after organic amendment, during which changes in CO₂ emissions, microbial biomass C, and water-extractable organic C pools were most pronounced. Phospholipid fatty acid profiles and 16S rDNA sequence analyses evidenced changes in the microbial community structure of amended soils. Modifications of the structure of bacterial communities after amendment, generally involving declining proportions of Gram-positive bacteria (Actinobacteria and Firmicutes) and an increase in abundance of Gram-negative bacteria (Acidobacteria, Bacteriodetes, and Proteobacteria), were both quality and quantity dependent, with effects being proportional to the mineralizable organic C content of the added materials.