Nanominerals and biochar as additives in the composting of agricultural waste: effects on GHG emissions, composition and biodegradability of end-products on grassland soils.

The utilization of additives is a strategy commonly used in composting operations to enhance the physicochemical properties and optimize the process. However, little is known about the impact of nanominerals, biochar and their combination during composting. The objective of this research was to evaluate the effects of iron oxide/halloysite nanominerals and oat hull-biochar as additives in the physicochemical properties of an aerobic composting process, the emission of greenhouse gases (GHG) and the composition of end-products. In order to analyze the biodegradability of composting end-products in grassland soils, an incubation experiment was also carried out. Wheat straw, lupine and beef manure were mixed (C/N: ~25) with iron oxide (Fe) or halloysite (Ha) nanoparticles (2% w/w), oat hull-biochar (B) (7% w/w) and their combination (BFe, BHa). pH, EC, C/N ratio, NH₄-NO₃ contents and the emission of CO₂ and CH₄ were analyzed. After 128 days of aerobic composting process, the end-products and their NaOH soluble fraction were characterized by using spectroscopic analysis that included E₄/E₆ ratio and solid state nuclear magnetic resonance (¹³CNMR). To analyze the biodegradability of produced compost in grassland soils, a respiration experiment (60 days) using Respicond Apparatus IV, combined with δ13C isotopic analysis was conducted. A decrease of final C/N ratio was observed in all treatments that was lower in B treated compost. Nitrate concentration increased as composting progressed, and compost supplied with Ha showed the higher final content of NO₃ (5800 mg kg^-1) and NH₄ (220 mg kg^-1). The addition of B significantly decreased the mean emission of both CO₂ (~400 g CO₂ m² d^-1) and CH₄ (~4.5 g CH₄ m² d^-1). Nanominerals significantly decreased the final E₄/E₆ ratio (<6) and the addition of B increased the aromaticity (twice), the alkyl-C/O alkyl-C ratio and the hydrophobicity which are parameters associated to stabilized end-products. In soil, the incorporation of additives reduced the loss of C (<5% after 60 days of incubation). Treatments supplied with B and Ha showed a higher mean residence time (8 and 5 years respectively) than compost without additives. These results suggest that the addition of halloysite and biochar to composting operations have significant effects on C stabilization and biodegradability of compost in grassland soils, that is relevant in the production of C sequestrant amendments. Acknowledgments to FONDECYT N° 3170677.