Biochar derived from wheat straw was added to pig manure in amounts equivalent to 5%, 10%, or 15% (w/w, wet weight). The ratios of NH4+/NO3 and of UV light absorption at a wavelength of 254 nm (SUV254) and dissolved organic carbon (DOC) indicated that compost with 10-15% biochar became more mature and more humified within 42 days of composting, and the content of DOC and the concentration of NH4+ in such compost decreased by 37.5-62.0% and 4.0-20.9%, respectively, compared to the corresponding levels in the control. Addition of biochar lowered the pH and increased electrical conductivity by 7.0-37.5% compared to the control and also increased the concentrations of water-soluble nutrients including PO43 (5.6-7.4%), K+ (14.2-58.6%), and Ca2+ (0-12.5%). It is therefore recommended that straw biochar be added to pig manure at 10-15% by weight. (C) 2015 Elsevier Ltd. All rights reserved.