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

Abstract

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., CO, CH, NO) 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 CO-eqm)>husk (367±42gCO-eqm)>ashed husk=ashed straw (251±26 and 278±28gCO-eqm)>control (186±23gCO-eqm). The GWP increase due to pre-incubated straw amendment was due to: a) larger NO fluxes during re-flooding; b) smaller contributions from larger CH fluxes during flooded periods; and c) higher CH and CO fluxes at the onset of drainage. In contrast, the GWP of the husk amendment was dominated by CO and CH emissions during flooded and drainage periods, while ashed amendments increased CO 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.

Black carbon yields highest nutrient and lowest arsenic release when using rice residuals in paddy soils

“…In case of P, this higher immobilization in the black carbon and ash treatment may be attributed to the higher redox potential during incubation experiments probably due to changes in microbial community or lower microbial activity as a result of lower availability of labile carbon after burning and the potential higher amount of iron minerals formed during burning (see above). Compared to previous studies 9 , 10 , showing a negative effect of rice straw burning (leading to a mixture of ash and charred material) on P mobilization (after application in paddy soils), the data of the present study clearly shows that burning to black carbon leads to a much higher P mobilization compared to ash, which our data suggest for the first time when using rice straw as source material. Hence, burning conditions have a very strong influence on later nutrient availability, when using straw-based amendments, which our data suggest for the first time when using rice straw as source material.…”

“…Depending on burning temperature and completeness of mineralization, black carbon (~350 °C burning temperature), biochar (400 to 700 °C fixed-bed slow pyrolysis) or ash (full oxidation) forms. Numerous studies have shown the release of individual nutrients from either fresh straw (P, Si, sulfide 3 – 5 ), black carbon (Si, P 6 , 7 ), biochar (Si 8 ), ash (Si, P), or a mixture of ash and charred material (Si, P, Fe, and As 9 , 10 ). Rice straw is more prone to microbial decomposition compared to black carbon or ash, and hence induces a lower redox potential in the paddy soil pore water which promotes mobilization of Fe and P 11 .…”

“…However, typically, biochar is not produced by open field burnings practiced by farmers (see above), instead black carbon and ash are produced in different amounts depending on burning conditions 13 , 14 . However, despite knowledge about the effect of straw application and application of straw burned to a mixture of ash and charred material on element availabilities in paddy soils 9 , 10 , little is known about the different effects of black carbon and ash.…”

Alleviation of arsenic accumulation in rice by applying silicon-rich rice husk residues in Bangladesh soil

“…Rice husk contains less arsenic and labile carbon and provides more silicon to soil porewater compared to rice straw which releases more arsenic and less silicon in soil porewater than fresh husk or huskash (Penido et al, 2016). In addition, incorporation of rice husk generates less methane emissions, which is a potential greenhouse gas, from rice paddies than incorporation of straw (Penido et al, 2016;Gutekunst et al, 2017). Therefore, rice husk is advantageous over rice straw as a sustainable solution to mitigate arsenic contamination in rice worldwide.…”

Networking our science to characterize the state, vulnerabilities, and management opportunities of soil organic matter

“…There are many practices influencing SOC storage in croplands. These include tillage management (in some cases, Powlson et al, 2014); crop rotations and cover crops (Poeplau & Don, 2015); improving crop production through fertilization and irrigation management; selection of high residue yielding crops; crop intensification by removing bare fallow management in a cropping system; application of silica residues to reduce greenhouse gas emissions (Gutekunst, Vargas, & Seyfferth, 2017); and application of organic amendments with manure or biochar.…”