Influence of Conventional and Non-Conventional Si Amendments/Fertilizers on the Release of As from Paddy Soils and the Uptake of As and Si By the Rice Plant.

Influence of conventional and non-conventional Si amendments/fertilizers on the release of As from paddy soils and the uptake of As and Si by the rice plant

¹Balint R., ¹Celi L., ¹Zanzo E., ¹Passarella I., ²Abi-Ghanem R., ¹Barberis E., ³Romani M., ¹Martin M.

¹Soil Biogeochemistry, Dept. of Agricultural, Forest and Food Sciences, University of Torino, Largo Braccini 2, 10095 Grugliasco (TO), Italy.

2 Bio Huma Netics, Dept. of Research and Development, Gilbert 85233, AR U.S.A

³Rice Research Centre, Ente Nazionale Risi, Strada per Ceretto 4, 27030 Castello d’Agogna, (PV), Italy.

Arsenic (As), an ubiquitous and toxic metalloid, accumulates in rice to a higher extent with respect to other crops due to cultivation in flooded fields under anaerobic conditions.  This results in the mobilization of As at the solid/liquid interface. Inorganic As is more hazardous and toxic than its organic forms.  The uptake of As by the rice plant and translocation into the grain are performed by the same transporters as for silicon (Si), which has been identified as an essential plant nutrient. This test was done to see whether high concentrations of available Si in solution may compete with As uptake at the root level by rice.  However, higher levels of Si have been believed to contribute to the release of some As bound to soil components, such as Fe (hydr)oxides, by competing for the binding sites. In this work, several conventional and non-conventional Si amendments/fertilizers were tested by mesocosm and macrocosm experiments for their ability to decrease inorganic As (III and V) release, uptake, and accumulation in the rice grain, while supplying crops with an available Si source. The specific products tested were:  a liquid fertilizer named Sili-Max®, a granular fertilizer known as Agrosil®, some Si beads, a rice husk ash, and a biochar derived from rice straw.  After submersion,the control soil and the soil mixed with the various amendments all showed an increase in the amounts of soluble As, over time, with no significant differences among treatments.  This indicates that the main mechanism controlling the As release into solution was the progressive reductive dissolution of Fe (hydr)oxides and As(V) reduction to As(III), rather than Si competition with As. These tests also proved that higher concentrations of Si in solution and in plant tissue during the separate growth stages were effective in preventing inorganic As uptake and accumulation in the rice grains.  The best results in the prevention of inorganic As accumulation were found using the Sili-Max®, followed by the Si gel beads, while the other amendments/fertilizers showed poorer performances.  Interestingly, the addition of Sili-Max® resulted in an increase of the organic As fraction in the rice grain, which requires further investigations. This work provides new insights into the effectiveness of relatively low cost materials potentially useful for their dual assignment as Si fertilizers and as inorganic As uptake inhibitors for rice crops.