Effect of Biochar on Urea Hydrolysis Rate and Soil ureC Gene Copy Numbers

The influence of biochar on soil urea hydrolysis process is associated with its potential ability on soil nitrogen amelioration. The objective of this study was to investigate the effect of biochar on urea hydrolysis and identify the main factors that drive their consequent behavior. The short-term incubation experiment was conducted to quantify the soil ureC gene copies and urea hydrolysis rate (UHR) as well as the gross N mineralization rate in response to biochar or its extracts pyrolyzed at 300 °C (BC300) and 600 °C (BC600). Biochar soil incorporation enhanced the ureC gene copies, and consequently increased the UHR by 39.49% (BC300) and 39.86% (BC600). Biochar addition also accelerated the gross organic N mineralization and NH4+-N immobilization rates, which proved that biochar stimulated ureolytic microorganism abundance in response to urea hydrolysis. In comparison to control, BC300 extracts increased soil ureC gene copy number by 23.18%, while BC600 extracts showed an inhibition effect of 29.65%. Property analysis revealed that BC300 extracts contained lower concentrations of water-soluble phenols but higher concentrations of water-soluble carbon than BC600, which indicated that the stimulated soil ureolytic microorganism abundance was primarily attributed to an increase in biochar labile carbon. Meanwhile, the inhibition effect may be offset by biochar high pH thereby promoting the urea hydrolysis in BC600-amended soil. This study indicates that the considerable labile carbon in biochar stimulated the thriving of ureolytic microorganisms and, combined with liming effect, buffered the negative effect from phenols or other toxicities, and eventually increased the soil UHR.