Chemically and biologically activated biochars slow down urea hydrolysis and improve nitrogen use efficiency

Biochar is considered a potential technology to enhance chemical fertilizer use efficiency through intensification of the interactions between nutrients and the functional groups on biochar surfaces. However, little is known about how the application of activated biochars mixed with urea influences nitrogen (N) mineralization and crop performance in paddy fields. Here, a sawdust-derived fresh biochar (FBC) (ca. 400 °C) was activated chemically with 15% hydrogen peroxide and biologically with a nutrient solution mixed with a soil inoculum to obtain a chemically activated biochar (CBC) and a biologically activated biochar (BBC), respectively. The chemical and surface properties of FBC, CBC, and BBC were evaluated using spectroscopic methods, i.e., Fourier transform infrared spectroscopy and 13C nuclear magnetic resonance, and potentiometric charge determination. The N retention capacity of biochars and their interaction with urea hydrolysis were examined in a laboratory incubation experiment. Additionally, a field experiment was carried out in a paddy field with the biochars unmixed or mixed with urea at a 1:1 ratio. Our results showed that negative surface functional groups and negative charges were increased on both activated biochars, especially CBC. Both activated biochars contributed to a significant reduction in urea-biochar suspension pH and increased N retention in the incubation experiment. Despite the enhanced surface properties of the activated biochars, no similar increases in rice biomass and grain yield were observed for these biochars in the field experiment. However, rice biomass, grain yield, apparent N use efficiency, and agronomic N use efficiency were significantly higher with the application of the three biochars compared to no-biochar application. Altogether, the results indicate that the application of urea mixed with biochar could enhance crop performance, especially in the case of activated biochar, which would enhance N retention in the soil, reducing N loss.