The use of deinking paper sludge (DPS) as a fertilizer instead of sending it to landfill could play a role in reducing greenhouse gases and improving soil properties. The objectives of this study were (1) to evaluate the changes in the physical (permeability and structural stability), chemical (particularly soil pH), and biological (microbial metabolic quotient (qCO2), microbial biomass soil CO2 emissions) of a calcareous agricultural soil following two successive annual amendments with three treatments (0, 30, and 60 Mg DPS ha−1—control, DPS30, and DPS60, respectively); and (2) to determine whether the addition of N-fertilizer to these treatments (controlF, DPS30F, and DPS60F, respectively) causes changes to soil fertility. The DPS application increased soil organic matter (+0.80%: DPS60 vs. control; and +0.35%: controlF vs. DPS60F), available phosphorus (+23.14 mg kg−1: DPS60 vs. control; and +14.34 mg kg−1: DPS60F vs. controlF), potassium (+0.6 g kg−1: controlF vs. DPS30F), and calcium (+0.28 g kg−1: DPS60 vs. control). The 60 Mg DPS ha−1 rate improved permeability and structural stability, regardless of the presence or absence of N-fertilizer. On the other hand, the 60 Mg DPS ha−1 rate without N-fertilizer lead to a decrease in total mineralization rate and qCO2, thereby indicating a reduction in CO2 emissions. The rate of 60 Mg ha−1 DPS could be effectively used to enhance the permeability and stability (soil restoration) and mitigate CO2 emissions, whereas the 30 Mg ha−1 rate could be used as fertilizer to improve the fertility of calcareous soils.

Thermal composting is an important and useful way to transfer raw organic matter into value-added product rich in humic substances. Furthermore, thermal composting is a very promising way to reduce deinking paper sludge pollutions, which are difficult to remove. The objective of this study was to investigate the behaviour of the composting process of deinking paper sludge with poultry manure over 14 months. Two composts were used: C1 (70/30: deinking paper sludge/poultry manure) and C2 (50/50: deinking paper sludge/poultry manure). The compost stability and maturity were assessed via physico–chemical and thermal analyses: thermogravimetric analysis, differential thermal analysis and diffuse reflectance infrared Fourier transform spectroscopy. Diffuse reflectance infrared Fourier transform spectroscopy analysis provided chemical information for the presence of aromatic, phenolic, aliphatic and polysaccharidic structures. Thermogravimetry analyses revealed that both deinking paper sludge composts were rich in humic acids. The ratio E4/E6 (fulvic acids/humic acids) was also carried out to characterise the maturity index of composts. After 14 months, the compost C2 is more stable, mature and rich in humic acids than compost C1. Furthermore, the addition of poultry manure to the deinking paper sludge-based composts enhanced the formation of humic substances. From 0 to 14 months of composting, the concentration of polychlorinated biphenyls decreased from 0.2 to 0.1 mg kg−1 and from 0.6 to 0.2 mg kg−1 for C1 and C2, respectively.

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