Abstract:
This study mechanistically addressed for the first time, the contradiction between the application of many
biochars to paddy soil and increased arsenic (As) release as employed by most of previous studies. Three types of
biochar containing natural and chemical forms of Si: (i) unmodified rice husk biochar (RHBC), (ii) RHBC
modified with Si fertilizer (Si-RHBC), and (iii) RHBC modified with nanoparticles of montmorillonite clay (NMRHBC)
were applied in As-contaminated paddy soil to examine their potential to control the mobility of As in the
soil-microbe-rice system. Both Si-RHBC and NM-RHBC decreased As concentration in porewater by 40–65 %,
while RHBC decreased by 30–44 % compared to biochar unamended soil from tillering to maturing stage. At
tillering stage, RHBC, Si-RHBC and NM-RHBC amendments significantly decreased As(III) concentration in the
rice rhizosphere by 57, 76 and 73 %, respectively compared to the control soil. The immobilization of As is due
to: (i) lowering of microbe mediated As release from iron minerals, (ii) oxidation of As(III) to As(V) by aioA gene, and (iii) adsorption on a Si-ferrihydrite complex. The decrease of more toxic As(III) and its oxidation to less
mobile As(V) by Si-rich biochar amendments is a promising As detoxification phenomenon in the rice rhizosphere.
