Improving pig manure and rice husk compost technology and quality by wood charcoal addition

Abstract: In order to promote waste recycling and investigate the effect of hardwood-based biochar additives in the process of pig manure composting, the pig manure was mixed with rice husk and further amended with oak wood charcoal at 0%, 2.5%, 5.0%, 7.5%, and 10.0% in dry weight before being aerobically composted for 60 days. The parameters, such as temperature, water content, pH, electrical conductivity (EC), organic carbon, C/N ratio, total nitrogen, NH4+-N, NO3--N, NH3, etc., were detected through chemical analysis. The organic matter degradation process was evaluated by the Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electronic Microscopy (SEM) methods. The FTIR analysis showed that the contents of the compounds with -OH, -CH3, and -CH2 groups decreased with time, while the contents of the compounds with -C=O, C-O-C, -COO, and aromatic groups increased with time during the 60 days of composting. The compost sample SEM analysis proved that the mixture particle size decreased with the progress of time and the amount of wood charcoal. During the 60 days composting, as the wood charcoal addition amount increased, the organic carbon contents in all the treatments decreased 12.23%, 13.77%, 14.88%, 15.36%, and 15.86%, respectively; the C/N ratio decreased 47.80%, 54.98%, 56.97%, 60.03%, and 65.73%, respectively. Compared to the control, the addition of wood charcoal had no significant effect on pH variation, and the final compost pH kept the balance at 8.0. Wood charcoal addition extended high-temperature processing times by 2-5 days, increased the water contents, and decreased the EC value during composting. During the 60 days composting, as the wood charcoal addition amount increased, the moisture content decreased 15.9%, 13.0%, 11.1%, 10.1%, and 7.8%, respectively; while the EC value in the wood charcoal added treatment were decreased 0.07, 0.15, 0.23, and 0.28 mS/cm than that of the control, respectively. Addition of oak wood charcoal additives could enhance the organic matter degradation during the composting. Wood charcoal addition increased the NH4+-N concentration at the beginning composting stage and the NO3--N concentration at the later composting stage. Compared to the control, after 60 days of composting the final NO3--N concentrations in charcoal adding treatments increased 55.86%-89.32%, the final total nitrogen concentrations in charcoal adding treatments increased 20.55%-53.07%, and the total nitrogen contents in the final compost were nearly 14.92, 16.69, 17.32, 18.05, and 18.53 g/kg, respectively. The addition of wood charcoal had no significant effect on potherb mustard (Ardisiasquamulosapresl) germination but had a positive effect on the germination index evolution. The potherb mustard germination index values increased with composting time and were higher in the wood charcoal adding treatments than in the control. After 60 days of composting, the potherb mustard germination rate increased 17.6%-41.2% and germination index ranged from 1.02 to 1.44 in the charcoal adding treatments. The potherb mustard germination index value variation proved that the addition of wood charcoal in pig manure composting favors the detoxication of the compost. The study implicated that adding wood charcoal in pig manure compost could accelerate the composting maturity, reduce nitrogen loss, and improve the quality of compost. Wood charcoal was a potential additive in pig manure composting.