Innovative Solutions for Sustainable Agriculture: From Waste to Biostimulants, Biofertilisers and Bioenergy

Due to its intensive nature, modern agriculture has a significant environmental impact; therefore, in the coming years, the main challenge will be to address numerous problems related to its lack of environmental, social, and economic sustainability. For instance, about one-third of anthropogenic greenhouse gas (GHG) emissions derive from agri-food systems [1]. This has been estimated by considering the production and subsequent release of GHG from farms and livestock-related activities, the pre- and post-production phases of food, and the associated land-use changes due to agricultural expansion [1]. In addition, it has been documented that agricultural activities are increasingly energy-intensive, with consumption accounting for almost 30% of global energy demand [2]. A significant portion of this energy demand is met by non-renewable sources, such as fossil fuels [2]. Cultivation systems also place significant pressure on natural resources; approximately 92% of humanity’s water footprint is linked to agriculture [3]. The biodiversity of agricultural environments is also affected by the use of land and ecosystems for intensive agriculture, with significant declines in species richness [4]. Furthermore, the ongoing climate change is characterized by sudden and extreme events and is also contributing to rapid soil degradation and progressive salinization [5]. These phenomena are significantly hampering the productivity of cropping systems by compromising the health of agricultural land [6,7]. It has been documented that since 1980, the climate crisis has progressively reduced yields of major crops; corn and wheat have shown reductions in production ranging from 4 to over 5.5% [8]. Without adequate countermeasures, climate change is projected to reduce the yields of some crops worldwide by 3–12% by mid-century and by 11–25% by the end of the century [9]. Another critical factor is the use of synthetic fertilisers. In fact, in addition to being a source of GHG, they can have a substantial environmental impact and cause many problems for natural ecosystems due to their mobility. Consumption of these products has increased significantly over the last fifty years, enabling significant growth in agricultural production, but at the same time causing significant environmental pollution [10]. All the situations depicted above should be framed in terms of a compelling demand for a sustainable increase in primary production to match the growth of the world population [11].

Among the various approaches to reduce the environmental impact of agriculture, the valorisation of agro-industrial waste through a circular model offers innovative and effective solutions. Many wastes and by-products have a chemical composition that indicates a wealth of substances that, once recovered, can be used in various applications and also provide valuable materials for agriculture itself [12]. Among the substances of particular interest in waste are biostimulants [13]. These substances, when applied to plants, soil, or the rhizosphere, improve crop performance by stimulating biochemical, physiological, and adaptive processes that enhance nutrient use efficiency and increase tolerance to environmental stresses [6,13,14,15]. In addition, agro-industrial residues from various sources can be used to produce compost, which can serve as valuable organic fertilisers, helping improve soil organic matter, optimise plant nutrition, and enhance the nutrient cycle [16]. To complement the above approach, the anaerobic digestion (AD) of agro-industrial residues or residual biomass represents another important aspect of circularity [17]. By means of AD, it is possible to further valorise residues by converting them into clean bioenergy, enabling the further exploitation of the bio-derived resource and facilitating a significant transition from fossil fuels to alternative and renewable forms of energy [18]. Moreover, once AD is completed, recent studies have shown that digestate remains a source of added value, as it can be further used as a biofertiliser for plant nutrition [17].

Considering the aspects listed above and the critical problems presented, this Special Issue aimed to gather scientific contributions that explore multiple avenues for applying and even transforming waste from various sources into valuable bio-inputs to make agriculture more sustainable, productive, and resilient to environmental pressures. On the whole, some main lines have been identified that highlight how the circular approach, based on the valorisation of waste biomass, aims to mitigate agricultural impact without losing sight of the quality of primary production. This circular-based approach represents an alternative to the linear model of resource utilisation and, as shown in this Special Issue by the contributions, some of the most suitable and promising solutions to improve the environmental sustainability of agriculture are the valorisation of residues to obtain biostimulants (microbial and non-microbial), compost, fertilizers and the conversion of waste biomass into bioenergy.

A series of scientific contributions have generally focused on the use of biostimulants and biofertilisers of non-microbial and microbial origin. These studies aimed to improve various aspects of cropping systems, including yield, crop quality, and tolerance to stressors. At the same time, these contributions emphasised the need to find solutions that reduce the use of synthetic fertilisers and plant protection products. Afonso et al. [19] studied the use of biostimulants on sweet cherries before harvesting as a strategy to improve fruit yield and quality and reduce the use of conventional agrochemicals. To this end, the authors studied the effect of biostimulants based on glycine betaine and Ecklonia maxima extract on two different sweet cherry cultivars. In general, biostimulant treatments improved the parameters studied, although responses varied by cultivar. Therefore, the authors propose applying the substances mentioned above to achieve significant improvements in the key quality characteristics of sweet cherries, offering clear advantages in terms of economic and environmental sustainability. Valente et al. [20] investigated the effect of inoculating common wheat with Methylobacterium symbioticum, aiming to reduce nitrogen input to the crop. The study showed that, in some cases, the bacterium promoted root length density, delayed leaf senescence, and improved photosynthetic activity, stomatal conductance, and photosystem II efficiency, even at reduced nitrogen levels. Therefore, the results indicate that Methylobacterium symbioticum improved nitrogen metabolism, offering a viable approach to reducing chemical fertilisation. A further contribution investigated the effectiveness of the nitrogen-fixing bacterium Gluconacetobacter diazotrophicus [21]. The trials evaluated the application of GIBI029 with or without fertilisation, and the results were compared with control plants unfertilised or fertilised with nitrogen and phosphorus. Yields and fruit numbers were higher with GIBI029 alone, with results similar to those of plants subjected to complete nitrogen and phosphorus fertilisation. This study highlights how this strategy can reduce or replace synthetic N and P use, offering a green alternative with no environmental impact. Nowak et al. [22] addressed the problems caused by the pathogenic fungus Rhizoctonia solani, which is typically controlled with pesticides. The authors proposed the bacterium Priestia megaterium (KW16), obtained from Poa pratensis L., for oilseed rape. Preliminary in vitro tests showed that KW16 inhibited the growth of R. solani. In healthy plants, KW16 improved plant growth, while in infected plants, it contrasted the pathogen effects. These results position KW16 as a powerful biological alternative to synthetic fungicides. This first group of works can be associated with a review that detailed the problems associated with the prolonged and intensive use of various chemical inputs in agriculture, as well as the environmental and agronomic challenges they have generated [23]. This contribution, therefore, highlighted the need for viable, sustainable alternatives. Among the proposed solutions, particular emphasis was placed on the potential of microalgae and cyanobacteria, which offer several specific advantages. These include the ability to capture CO₂, assimilate essential micro- and macroelements, and, thanks to their metabolic versatility, produce bioactive substances with biostimulant and biocontrol properties. Finally, this review also highlighted gaps in the use of these microorganisms and offers perspectives for the development of sustainable agriculture.

Another group of scientific articles in this Special Issue focused on the use of organic residues and mineral-organic matrices to improve fertilisation and soil organic matter, to reduce the use of synthetic fertilisers and offer solutions to mitigate the environmental impact of contaminants. Ramos-Romero et al. [24] addressed the problem of valorising liquid waste from whey (CW) derived from cheese production by co-composting it with solid organic waste, such as crop residues and cow manure, using a turned-pile composting system. The results showed that CW had a high organic load and macronutrient concentrations, and low concentrations of heavy metals. It was observed that co-composting CW with agro-zootechnical waste was a sustainable strategy for producing compost with stabilised and humified organic matter and considerable agricultural value. Li et al. [25] tested Chinese astragalus and buried rice straw, using three nitrogen levels (0%, 60%, and 100% of the usual dose) to improve rice cultivation. The results indicated that the best combination for rice was obtained when astragalus was grown with straw and 60% nitrogen, yielding levels equal to those of the crop grown with 100% conventional nitrogen. In addition, the authors found that this combination stimulated microbial biomass and increased soil ammonium content. This strategy is therefore viable for maintaining crop yield and reducing the risk of nitrogen loss to the environment. Another study published in this Special Issue aimed at valorising Cupuaçu, a fruit native to the Amazon, widely used in the food industry, and which generates large quantities of shell and seed residues [26]. After characterising these residues, the authors tested their potential as substrates for seedling production. Chemical analyses showed that these residues contained significant amounts of macro- and micronutrients, exceeding those in the control soil. In addition, the substrates produced from the residues had lower densities and higher porosity than the soil. Jarosz et al. [27] studied the influence of mineral-organic mixtures containing zeolite compounds obtained from fly ash and lignite or leonardite on the organic matter of a sandy loam soil in pot experiments on maize plants. SOM and the evolution of compounds were analysed, as well as the effect of these mixtures on soil carbon reserves. The addition of these materials improved SOM stability and significantly increased total organic carbon and nitrogen content, indicating greater soil fertility. Aguilar et al. [28] investigated the effect of Ni on foliar fertilisation with urea, as this element is a cofactor of the urease enzyme. The results showed that the addition of Ni increased urease activity, improving urea assimilation, photosynthetic activity, and pigment content, and stimulating the biosynthesis of nitrogen-containing compounds. Loffredo et al. [29] studied the use of compost derived from digestate in two agricultural soils in southern Italy. In particular, the authors of this study found that compost increased the adsorption of the fungicide penconazole, the herbicide S-metolachlor and BPA, with very limited desorption. The results confirm the key role of organic matter (native and anthropogenic) in retaining xenobiotics and limiting their transfer to water and crops.

The last two contributions of this Special Issue paid attention to waste valorisation through its conversion into biochar and biogas. Chun et al. [30] obtained biochar from poultry manure and then tested it on Salicornia herbacea. The biochar was produced by pyrolysis at different temperatures. The biochar obtained at 500 °C was then applied to the soil in different doses. At the end of the trial, the authors found that biochar derived from poultry manure improved soil health and Salicornia performance. The last contribution by Di Mario et al. [31] focused on the valorisation of various biomasses to produce biogas. The authors evaluated biogas production from olive pomace, the pulp obtained from it via an ionic liquid-based procedure, and olive mill wastewater processed via freeze-drying (FDOW). The FDOW showed the highest biogas production, while the pulp produced the lowest. To address this issue, the author co-digested the pulp with the brewery’s spent grain (BSG), thereby enhancing biogas production. The same happened in the case of FDOW and BSG co-digestion.