Impacts of soil incorporation of pre-incubated silica-rich rice residue on soil biogeochemistry and greenhouse gas fluxes under flooding and drying.

Incorporation of silica-rich rice husk residue into flooded paddy soil decreases arsenic uptake by rice. However, the impact of this practice on soil greenhouse gas (GHG) emissions and elemental cycling is unresolved particularly as amended soils experience recurrent flooding and drying cycles. We evaluated the impact of pre-incubated silica-rich rice residue incorporation to soils on pore water chemistry and soil GHG fluxes (i.e., CO2 , CH4 , N2 O) over a flooding and drying cycle typical of flooded rice cultivation. Soils pre-incubated with rice husk had 4-fold higher pore water Si than control and 2-fold higher than soils pre-incubated with rice straw, whereas the pore water As and Fe concentrations in soils amended with pre-incubated straw and husk were unexpectedly similar (maximum ~0.85μM and ~450μM levels, respectively). Pre-incubation of residues did not affect Si but did affect the pore water levels of As and Fe compared to previous studies using fresh residues where straw amended soils had higher As and Fe in pore water. The global warming potential (GWP) of soil GHG emissions decreased in the order straw (612±76g CO2 -eqm-2 )>husk (367±42gCO2 -eqm-2 )>ashed husk=ashed straw (251±26 and 278±28gCO2 -eqm-2 )>control (186±23gCO2 -eqm-2 ). The GWP increase due to pre-incubated straw amendment was due to: a) larger N2 O fluxes during re-flooding; b) smaller contributions from larger CH4 fluxes during flooded periods; and c) higher CH4 and CO2 fluxes at the onset of drainage. In contrast, the GWP of the husk amendment was dominated by CO2 and CH4 emissions during flooded and drainage periods, while ashed amendments increased CO2 emissions particularly during drainage. This experiment shows that ashed residues and husk addition minimizes GWP of flooded soils and enhances pore water Si compared to straw addition even after pre-incubation.