Recycling organic wastes into farmland faces a double challenge: increasing the carbon storage of soil while mitigating CO₂ emission from soil. Predicting the stability of organic matter (OM) in wastes and treatment products can be helpful in dealing with this contradiction. This work proposed a modeling approach integrating an OM characterization protocol into partial least squares (PLS) regression. A total of 31 organic wastes, and their products issued from anaerobic digestion, composting, and digestion-composting treatment were characterized using sequential extraction and three-dimension (3D) fluorescence spectroscopy. The apportionment of carbon in different fractions and fluorescence spectra revealed that the OM became less accessible and biodegradable after treatments, especially the composting. This was proven by the decrease in CO₂ emission from soil incubation. The PLS model successfully predicted the stability of solid digestate, compost, and compost of solid digestate in the soil by using only the characterized variables of non-treated wastes. The results suggested that it would be possible to predict the stability of OM from organic wastes after different treatment procedures. It is helpful to choose the most suitable and economic treatment procedure to stabilize labile organic carbon in wastes and hence minimize CO₂ emission after the application of treatment products to the soil.

The established database represents one of the largest and most diverse collections of digestates compositions, with a total of 806 digestates derived from two main collections: (i) 608 digestates from French agricultural anaerobic digestion (AD) plants, which include detailed physicochemical characterization, trace metal elements and organic contaminants concentrations, and potential C and N mineralization during soil incubation in controlled conditions as agronomic indicators, and (ii) 198 digestates collected from an international literature review, with physicochemical characterization and some trace metal elements concentrations. An additional dataset of process metadata was also set-up, including feedstock compositions, operational conditions and post-treatments descriptions. A preliminary description of both collections is provided using descriptive statistics. This dataset can help to better understand the variability in digestates quality based on feedstock compositions, operational conditions and post-treatments, thereby improving their agronomic benefits.

The quality of microalgae-bacteria biomass as an organic soil fertilizer may depend on the biomass’s microbial composition, morphology, and growth history. This study aims to characterize the molecular complexity and bioaccessibility of organic matter, nitrogen, and phosphorus from microalgae-bacteria aggregates with different morphologies (flocs and granules) grown under nutrient-abundant and starvation conditions. A biochemical fractionation method was used based on sequential chemical extraction and fluorescence spectroscopy. Microalgae-bacteria aggregates were cultured and collected from photobioreactors using contrasting growth conditions to generate (i) loose flocs, (ii) consolidated flocs, (iii) smooth granules, and (iv) filamentous granules. The organic matter, nitrogen, and phosphorus from consolidated flocs were mostly extractable, accounting for up to 94% of their total content. In contrast, the organic matter from loose flocs was up to 50% non-extractable. The extractability of loose flocs was improved under starvation conditions. All microalgae-bacteria aggregates showed a low structural complexity, corresponding to an abundance of simple microbial-related constituents like tyrosine and tryptophane. Differences between the gradients of bioaccessibility for each microalgae-bacteria structure were related to the abundance of microorganisms and their metabolic products. The findings of this study have implications for the development of sustainable and environment-friendly organic fertilizers. Graphical Abstract

Exogenous organic materials (EOMs) are increasingly used as substitutes for mineral fertilizers and as tools to restore soil health within a circular bioeconomy context. However, the great diversity of EOMs in terms of origin, composition, chemical properties, and contaminant concentrations challenges their safe and efficient use in agriculture. The aim of the study was to establish a framework for the clustering of EOMs properties from several EU countries, enabling their categorisation according to their origin, characteristics, and chemical properties and trace elements (TE) profile. For that purpose, a dataset with chemical characteristics and TE concentrations from 118 EOMs was constructed from a database previously published. The EOMs included a wide range of organic residues and waste streams from agricultural, industrial, and urban origins representative of the diversity of European EOMs. Clustering analyses were carried out to distribute EOMs among clusters (i) of chemical properties based on their characteristics (dry mass, C‐to‐N ratio, pH, and concentrations of organic C, N, NNH ₄ , P, K, Ca, and Mg), and (ii) of TE profile based on their concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn. Five practical scenarios of EOMs applications mixing amending and fertilizing EOMs were considered to estimate input fluxes of C, N, P, K, and TEs over a period of 10 years in contrasted areas (agricultural settings and peri‐urban areas). The present study clustered the EOMs according to their soil improver characteristics that is, from NK fertilizing to liming and organic amending properties. The clustering analysis on TEs classified EOMs according to their TE profiles, with (i) smaller concentration of TEs (i.e., three quarters of EOMs), and (ii) larger concentration of all TEs and especially for Cu and Zn. The various practical scenarios simulating the repeated applications of local EOMs from contrasted areas showed that input fluxes were in line with commercial organic fertilizer inputs and below the goal of 170 kg N per hectare per year, while TE input fluxes respected the French regulation thresholds, even in the scenarios including EOMs containing also greater TE levels.