Biogeochemical impacts of silicon-rich rice residue incorporation into flooded soils: Implications for rice nutrition and cycling of arsenic

Abstract

Aim Soil incorporation of Si-rich rice residues may aid smallholder rice farmers in improving crop yields and may affect As uptake. Here, the biogeochemical impacts of rice residue incorporation into flooded soil without plants were evaluated. Methods Various particle sizes of fresh rice straw (FS), fresh rice husk (FH), rice straw ash (RSA) and rice husk ash (RHA) residues were incorporated into soil (1 % w/w) in a flooded pot experiment. Pore-water chemistry was monitored weekly and dissolved CH 4 concentrations and genes specific for methanotrophy and methanogenesis in DNA extracts of soil were evaluated. Results FH-amended soil had the highest level of dissolved Si, followed by FS and then ash (RHA or RSA). Particle size had little effect on the dissolved Si concentration for any residue tested. No amendments had any substantial effect on pore-water pH. FS-amended soil had much higher As, Fe, and CH 4 concentrations in pore-water compared to ash and FH, and its extracted DNA also had higher amplifications of genes indicative of methanogenesis. Conclusion FH, RHA, or RSA are attractive amendments for smallholder rice farmers to increase plantavailable Si without exacerbating CH 4 emissions, which may improve rice nutrition through the beneficial impacts of Si on combating biotic and abiotic stress including decreased arsenic uptake.

Understanding arsenic dynamics in agronomic systems to predict and prevent uptake by crop plants

“…Rice plants take up high concentrations of silica, constituting up to 10% of dry matter in the straw and husk of the plant 198 . As mentioned earlier (Section 6) the silicon membrane transporter (Lsi1) is the main route of arsenite entry in to rice root cells, and provision of silicon causes competitive inhibition of arsenite uptake.…”

“…Fertilization of rice paddy soils with silicon is a potential mitigation strategy for preventing or reducing arsenic uptake by rice through competitive inhibition of arsenite uptake 199 . The use of synthetic silicon fertilizers, such as calcium silicate or silica gel is prohibitively expensive for smallholder farmers in developing countries, however reusing the silicon-rich parts of the rice plant that remain after harvesting and grain processing may provide a sustainable solution that also addresses the ongoing issue of silicon depletion of the soil 198 . Soil incorporation of fresh rice husks, or the ash that remains after burning the husk and straw for energy (which is a common practice for smallholder farmers), can provide silicon without increasing methane production and decreases either total or inorganic arsenic in rice grain 200 .…”

Managing Water and Soils to Achieve Adaptation and Reduce Methane Emissions and Arsenic Contamination in Asian Rice Production

“…Penido et al [204] examined the incorporation of fresh rice straw (FS), fresh rice husk (FH), rice straw ash (RSA), and rice husk ash (RHA), at the rate of 1% of the weight of soil, in a flooded pot experiment. Soils amended with the fresh materials (FS and FH) had higher levels of dissolved silicon than those amended with the ash materials (RSA and RHA), while the FS-amended soil had the highest concentrations of arsenic, iron, and methane.…”

“…Soils amended with the fresh materials (FS and FH) had higher levels of dissolved silicon than those amended with the ash materials (RSA and RHA), while the FS-amended soil had the highest concentrations of arsenic, iron, and methane. The authors conclude that amending soils with FH, RSA, or RHA might enable smallholder rice farmers to increase plant-available silicon, while not causing higher levels of methane emissions [204]. Although silicon is not considered an essential element, it has been shown to reduce biotic and abiotic stresses, including arsenic uptake and toxicity [205][206][207][208][209][210][211][212][213][214].…”

“…Research is needed to determine methods for accelerating the composting of rice straw, and for utilizing biochar derived from rice straw as soil amendments [214,313]. Incorporating the ash of rice straw and husks into paddy soils also can enhance silicon availability, thus improving rice plant performance, while not increasing methane emissions [204,314,315].…”