Abstract
Biochar is considered as 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 on how application of activated biochars mixed with urea would influence nitrogen mineralization and crop performance in rice paddy fields. Here, a fresh sawdust biochar (FBC) (~400°C) was activated chemically with 15% H 2 O 2 and biologically with a nutrient solution mixed with a soil inoculum to obtain a chemically (CBC) and a biologically (BBC) activated biochar, respectively. The chemical and surface properties of these biochars were evaluated by spectroscopic methods (FTIR and 13 C-NMR) and by potentiometric charge determination. The N retention capacity of biochars and their interaction with the hydrolysis of urea were examined in a laboratory-based incubation. Additionally, a field trial was carried out in a paddy rice plot with these biochars after mixing at a ratio of 1:1 with or without urea. Our results showed that both activated biochars increased negative surface functional groups and negative charges after the activation process, especially in the case of CBC which was more effective than BBC. Both activated biochars contributed to a significant reduction in urea-biochar suspension pH while increasing N retention in a batch incubation. Despite the enhanced surface properties of the activated biochars, a similar increase in biomass and grain yield was observed for the three biochars in the field experiment. Similarly, apparent nitrogen recovery efficiency and agronomic efficiency were significantly higher in the three biochars than in control. Altogether, the results indicate that the application of urea mixed with biochar could enhance crop performance, especially in the case of activated biochars that would also enhance N retention in the soil preventing eventual N losses.
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