Reduced colloidal phosphorus release from paddy soils: A synergistic effect of micro-/nano-sized biochars and...

Colloidal phosphorus (CP) has high mobility and great loss risk; their biogeochemical processes are influenced by agricultural management such as redox oscillation and biochar-amendment application. This study monitored CP concentration in pore-water, soil P species and P adsorption capacity, to investigate CP release from paddy soils as affected by the interactive effects of oxygen status (continuous anoxic/oxic for 12 days, CA/CO; intermittent anoxic for 2, 4, 6, 8, 10 days during the 12-day cycle, IA_2-10) and management (soil only, CK; bulk/micro/nano-sized biochar with various properties: SB_Bulk, SB_Micro, and SB_Nano). Compared to the control (0.25-0.84 mg L^-1, CK-CA), the single intermittent anoxic treatment (CK-IA) reduced CP concentrations by 45 %, due to the rise of Eh and pH and the decline of the degree of P saturation along with the increased soil Fe/Al-P and organic-P. Longer anoxic duration under the CK-IA reduced CP release, probably donated from massive production of redox-stable amorphous Fe/Al-bound P. The single biochar treatment (SB-CA: SB_Bulk-CA > SB_Micro-CA > SB_Nano-CA) decreased CP release by 37 % as compared to the CK-CA, ascribed to the increased soil pH, Eh, and P adsorption capacity. The combined treatment (SB-IA: SB_Bulk-IA₂ > SB_Nano-IA₁₀) synergistically reduced CP release by 68 % in comparison with the CK-CA, due to the increase of adsorption through interactions of soil Fe/Al/Ca- and organic-P. Therefore, nano-sized biochar and long intermittent anoxic duration are recommended for reducing CP release from paddy soils.