Over use of N fertilizers, most commonly as urea, had been seriously concerned as
a major source of radiative N (Nr) for severe environment impacts through leaching,
volatilization, and N2O emission from fertilized croplands. It had been well known
that biochar could enhance N retention and use efficiency by crops in amended croplands.
In this study, a granular biochar-mineral urea composite (Bio-MUC) was obtained by
blending urea with green waste biochar supplemented with clay minerals of bentonite
and sepiolite. This Bio-MUC material was firstly characterized by microscopic analyses
with FTIR, SEM-EDS and STEM, subsequently tested for N leaching in water in column
experiment and for N supply for maize in pot culture, compared to conventional urea
fertilizer (UF). Microscopic analyses indicated binding of urea N to particle surfaces
of biochar and clay minerals in the Bio-MUC composite. In the leaching experiment
over 30 days, cumulative N release as NH4+-N and of dissolved organic carbon (DOC)
was significantly smaller by >70% and by 8% from the Bio-MUC than from UF. In pot
culture with maize growing for 50 days, total fresh shoot was enhanced by 14% but
fresh root by 25% under Bio-MUC compared to UF. This study suggested that N in the
Bio-MUC was shown slow releasing in water but maize growth promoting in soil, relative
to conventional urea. Such effect could be related mainly to N retention by binding
to biochar/mineral surfaces and partly by carbon bonds of urea to biochar in the Bio-MUC.
Therefore, biochar from agro-wastes could be used for blending urea as combined organo/mineral
urea to replace mineral urea so as to reduce N use and impacts on global Nr. Of course,
how such biochar combined urea would impact N process in soil-plant systems deserve
further field studies.