Effect of Biochar as an Additive in Co-Composting: Impacts on Physicochemical Properties, Enzyme Activity, and Substrate Quality

The growing demand for sustainable alternatives to non-renewable growing media components, such as sphagnum peat, has driven interest in compost–biochar combinations as high-quality horticultural substrates. Nevertheless, the effects of incorporating biochar at the onset of co-composting on process dynamics and the quality of the resulting end-products remain poorly understood and insufficiently characterized. This study evaluated the influence of hardwood residue-derived biochar as an additive on the co-composting of corn stover and swine manure under greenhouse conditions over 94 days. Three treatments were established: a Control (C; without biochar), CB1 (10% v/v biochar), and CB2 (20% v/v biochar), each with three replicates. Physicochemical properties (pH, electrical conductivity, water holding capacity, bulk density, particle size distribution, total organic carbon), enzyme activities (FDA hydrolysis, dehydrogenase, urease, arginine ammonification, acid phosphatase, arylsulfatase), compost stability (Solvita® test), and phytotoxicity (germination index) were monitored throughout the process and at process completion. Results showed that biochar addition significantly influenced physicochemical and biological parameters in a dose-dependent manner. The CB2 treatment reduced electrical conductivity by 46% relative to the Control (1.87 vs. 3.45 mS/cm) and increased water-holding capacity by 32% (479 vs. 364%), while all treatments met the Chilean composting standard NCh 2880. Enzyme activities were generally higher in the Control and CB1 treatments, indicating that high biochar rates may limit microbial activity due to recalcitrant carbon inputs, yet a 20% biochar addition improved compost maturity and reduced phytotoxicity. These findings indicate that, within the tested range (10–20% v/v), biochar incorporation appears to enhance the quality, supporting its potential as a sustainable component in growing media formulations as an alternative to peat.