Biodegradation of sunflower hulls with different nitrogen sources under mesophilic and thermophilic incubations

The decomposition of sunflower hulls and mixtures of sunflower hulls and other crop residues was examined under mesophilic (25°C) and thermophilic (50°C) temperatures during a 45-day incubation. Four treatments were set up: sunflower hull, sunflower hull+5% alfalfa, sunflower hull+5% vetch, sunflower hull+0.1% urea, to asses the efficiency of composting. Changes in total organic C (TOC), oxidizable C, N, pH, electrical conductivity (EC), and different fibre fractions—cellulose, hemicellulose and lignin—were determined. TOC decreased by 13–21% at 25°C and 25–41% at 50°C and the largest loss of C was for hulls amended with vetch residues and urea. Loss of oxidizable C was not affected by either the temperature or the treatments. The largest loss of cellulose occurred at 50°C in all the treatments. The hemicellulose content of the sunflower hulls alone and in the treatment with urea was significantly different with respect to the other treatments, whereas lignin content showed either a small increase (1.6% in sunflower hulls amended with alfalfa at 50°C and 1.8% in sunflower hulls with urea at 25°C) or a decrease of between 2.5% and 19% in the other treatments. The C/N ratio never fell below 50:1 and the highest decrease was for the thermophilic incubation. Increases in EC and pH values accounted for 3 and 1.5 units, respectively, and occurred after a 45-day incubation for mesophilic and thermophilic conditions. In general the incubation at 50°C facilitated the decomposition of sunflower hulls with high C/N ratios and little N addition.