Over the last several years, the models for organic waste management have changed to implement circular economy in the productive cycle. In this context, new scenarios have emerged, where the management of different organic waste streams by composting is conducted with decentralized models that manage organic wastes in a more local way. However, in these new models, the standardization of the process control and of the end-product characteristics is necessary to guarantee the quality and agronomic value of the compost obtained, avoiding potential risks for human health and the environment. Thus, the aim of this work was to study two different scenarios of community composting of the organic fraction of municipal solid waste separately collected in order to guarantee the effectiveness and reproducibility of the composting processes and the quality of the composts obtained. For this, the development of the process and the characteristics of the composts at agronomic, hygienic–sanitary and environmental levels were assessed in real conditions and during three cycles of the process. The results obtained show high similarity among the different composting cycles, indicating an important degree of reproducibility among the processes. In addition, the composts obtained showed a good sanitary quality, absence of phytotoxicity and low contents of potentially toxic elements, which guarantee their use in agriculture without posing any risk to human health and to the environment.

Circular economy in the context of municipal organic waste management has boosted the emergence of novel composting scenarios, such as community composting and decentralized urban composting in small installations, which favors localized management and valorization of organic waste streams. However, there is little information about the agronomic use of the composts obtained from these new organic waste management systems as an alternative for inorganic fertilization in crop production. In this work, municipal solid waste-derived composts from two decentralized composting scenarios (CM1 and CM2 from community composting, and CM3 and CM4 from decentralized urban small-scale composting plants) were applied and mixed in the top layer of a calcareous clayey-loam soil to assess their effects as alternative substitutes for conventional soil inorganic fertilization (IN) during two successive cultivation cycles of lettuce (Lactuca sativa L.) grown in pots with the amended soils. These treatments were also compared with an organic waste (goat–rabbit manure, E) and a control treatment without fertilization (B). The effects of the fertilizing treatments on the crop yield and quality, as well as on the properties of the soil considered were studied. In general, the application of the different composts did not produce negative effects on lettuce yield and quality. The compost-derived fertilization showed similar lettuce yields compared to the inorganic and manure-derived fertilizations (IN and E, respectively), and higher yields than the soil without amendment (B), with increases in the initial yield values of B, for the first cycle from 34.2% for CM1 to 53.8% for CM3, and from 20.3% for CM3 to 92.4% for CM1 in the second cycle. Furthermore, the organically amended soils showed a better crop development, obtaining higher values than the control treatment in the parameters studied. In addition, the incorporation of the organic treatments improved the soil characteristics, leading to 1.3 and 1.2 times higher organic matter contents in the soils with CM2 and in the soils with CM1, CM3, and E, respectively, compared to the control soil without fertilizing treatment (B), and 2.0 and 1.8 times greater organic matter contents, respectively, compared to soil with inorganic fertilization (IN). Therefore, the use of municipal solid waste-derived composts from these new organic waste management systems, such as the decentralized composting scenarios studied (community composting and urban decentralized small-scale composting plants), is presented, not only as a sustainable valorization method, but also as an alternative for the use of inorganic fertilizers in lettuce cultivation, while enhancing soil properties, contributing to increasing the circularity of agriculture.

The sustainable management of biowaste, mainly food and pruning waste, is currently a challenge due to the increase in its production. The CaMPuSTAJE program, which has been implemented on the campus of the Public University of Navarre (UPNA) since 2019, is an excellent example of how the institution is addressing its strategic interests in sustainable waste management. The principal aim of this program is to manage the biowastes generated by the campus canteens through a simple community composting facility, involving UPNA students and graduates. This program aims to promote experiential learning and applied research in sustainability and circular economy, managing their own waste in a circular and local way. Thus, four composting sets of the CaMPuSTAJE program were evaluated by monitoring the process and the main chemical properties of the composting samples. Also, final composts were fully characterized to ensure the process reproducibility and efficiency and the absence of any hazard in the end-products. The final composts showed a significant agronomic quality, had low content of potentially toxic elements, and were free from phytotoxicity, thus being able to be reintroduced as an organic amendment at the university campus itself.

The loss of organic matter in soils, in conjunction with water scarcity and salinization constitutes a grave problem in the Mediterranean region. The utilization of composts derived from novel decentralized models, such as agrocomposting, community composting and decentralized urban composting, is hypothesized as an effective strategy with the potential to enhance the physical, chemical and biological characteristics of the soil. The application of compost has been demonstrated to enhance the resilience of plants to abiotic stresses, including drought and salinity. In this context, eight decentralized model composts were employed as organic amendments in a degraded soil to assess whether they could mitigate the stress experienced by the Mediterranean shrub R. officinalis under conditions of drought or salinity for one month. Therefore, measurements of gas exchange, emission of volatile organic compounds and reflectance during stress were performed in order to determine the degree of adaptation of the plants. Composts from community composting were found to be more effective in alleviating the effects of abiotic stress, while the ineffectiveness of composts from decentralized urban composting depended on their characteristics (e.g. humic substances and electrical conductivity). An initial increase in monoterpene emission rates was observed, which decreased at the end of the experimental time in a positive correlation with leaf RWC. Stress also led to a reduction in photosynthesis (A) and stomatal conductance (gs). Reflectance was more affected under drought conditions and by the application of compost with a high electrical conductivity, which likely hindered chlorophyll synthesis. Thus, the present results have enabled the selection of appropriate composts for utilization in agriculture under stress conditions considering plant biomass values and compost characteristics (EC, humic substances content, and the K/Na ratio).

The large-scale use of compost in arable cropping systems is often limited by the large quantities required to meet the crop’s nutritional needs. Palletization can increase the nutrient density of organic fertilizers and improve their logistical feasibility by reducing storage, transport and application costs. This study evaluated the agronomic and environmental performance of compost pellets derived from pig slurry solids and olive pomace, using them as an alternative nitrogen source for wheat (Triticum aestivum L.) cultivated under Mediterranean conditions. A field experiment was conducted during the 2022–2023 growing season, with four treatments arranged in 24 m² replicated plots: an unfertilized control (C); pelletized compost (PSCOP); fresh pig slurry (PS); and mineral fertilization based on monoammonium phosphate and urea (IN). Excluding the control treatment, all fertilized plots received a uniform nitrogen rate of 150 kg N ha⁻¹. Soil chemical properties and nutrient availability (Pext, NH₄⁺-N and NO₃⁻-N) were evaluated at the beginning and end of the experiment, while wheat yield and grain quality were assessed at harvest. Greenhouse gas (GHG) emissions were monitored throughout the cropping season to evaluate environmental impacts. The results showed that the wheat yields achieved with PSCOP were comparable to those obtained with PS, although they remained lower than those achieved with mineral fertilization. Grain quality was not adversely affected by the application of PSCOP. Furthermore, PSCOP resulted in lower GHG emissions than mineral fertilization, with values closer to those observed in the unfertilized control. These findings suggest that pelletized organic fertilizers such as PSCOP may be a promising way to enhance nutrient circularity and reduce reliance on synthetic fertilizers and maintain crop productivity and limit environmental impact in Mediterranean agricultural systems.

The rapid growth of insect farming generates significant volumes of insect frass, a novel organic by-product. Utilizing frass-based organo-mineral fertilizers represents a sustainable alternative to synthetic fertilizers, yet long-term agronomic efficacy and residual effects require further field validation. This study evaluated pelletized organo-mineral formulations for conventional and organic agriculture, derived from Hermetia illucens and Tenebrio molitor frass. Eight field treatments were evaluated: an unfertilized control; an inorganic NPK fertilizer (150 kg N ha⁻¹); a biostabilized municipal solid waste compost at two nitrogen rates (150 and 300 kg N ha⁻¹); and two novel insect-frass-derived organo-mineral fertilizers (VHTI and VHTO) applied at a standard product dose (1000 kg ha⁻¹) and an N-equivalent dose (150 kg N ha⁻¹). Application of organo-mineral and organic treatments at the same nitrogen dose (150 kg N ha⁻¹) doubled cabbage yield compared to the unfertilized control and significantly outperformed the conventional inorganic treatment. These formulations demonstrated enhanced nitrogen and phosphorus use efficiencies, reflecting improved synchronization between nutrient release and crop demand. Furthermore, organo-mineral formulations significantly increased soil available phosphorus without adversely affecting key soil quality parameters. The Integrated Soil–Crop Response Index (ISCRI) confirmed a superior overall agroecosystem response for these treatments. These findings provide important theoretical insights into nutrient synchronization mechanisms in organo-mineral fertilizers, alongside practical applications for sustainable cabbage production. Consequently, frass-based pelletized fertilizers represent a highly sustainable, circular, and efficient alternative to optimize horticultural yield and nutrient use efficiency while enhancing soil health, ultimately supporting the valorization of insect farming by-products as viable substitutes for synthetic fertilizers.