From Soil to Rice: Understanding the Dynamics of Metal(loid) and Nutrient Retention and Release under Different Management and across Soils through Advanced Analytical Techniques

Abstract

Rice is a staple crop for billions of people around the world but is subject to unique stresses from the flooded soils it is traditionally grown in which can threaten the production of safe and sustainable rice. Soil microbes use alternative terminal electron acceptors under suboxic/anoxic conditions in flooded soils. Some of these chemically reduced species are phytotoxic such as Mn(II), Fe(II), and S2-, or have increased mobility and toxicity such as As(III), and some are potent greenhouse gases such as CH4. One way to limit soil reduction is occasionally allowing the soils to drain in alternate wetting and drying (AWD), though this practice can increase the mobility of the toxic metal Cd which can accumulate in rice. Rice plants incorporate large amounts (up to 10%) of Si in their tissues which help with insect and disease resistance. Arsenic (As(III) and dimethyl arsenic acid (DMA)) is chemically analogous to silicic acid and shares the efficient Si transport system. Maintaining high soil Si levels can decrease transporter activity and As uptake. Rice plants have also adapted to survive in flooded soils through aerenchyma tissues which allow O2 to diffuse from the atmosphere, into their roots, and out into the soil. This oxygenates the rhizosphere, where microbes oxidize Fe(II) to Fe(III) oxides which form a reddish “Fe plaque” on the root. Iron plaque is thought to be a barrier to As uptake due to its high anion adsorption strength. It is unknown whether Fe plaque interferes with uptake of nutrient anions such as Si, P, and Se. In this project, we examined how different management strategies for Si-rich rice husk (a milling byproduct) impacts As and Cd uptake and greenhouse gas emissions alongside different water management techniques. We also studied Fe plaque formation on rice roots over time and in diverse paddy soils to test whether nutrient oxyanions (Si, P, Se) are retained on plaque similarly to As. We used a combination of lab, greenhouse, field, and synchrotron techniques. Our results suggest that the effectiveness of rice husk to provide Si to plants decreases with higher burning/pyrolyzing temperatures, and husk and husk biochar are not effective at decreasing Cd uptake. Pyrolyzing husk at low temperatures did decrease CH4 emissions while still building soil carbon and may be optimal from a climate perspective. Our Fe plaque experiments suggest that As is retained in plaque to a far greater extent than other anions, likely because it can adsorb as As(III) or re-oxidize to As(V). Phosphorus limitation was seen to drive greater root exploration to account for some adsorption to plaque. Finally, we found that the rhizosphere/plaque was enriched in (mostly inorganic) As compared to the bulk soil, and that As is not retained in the rhizosphere region in silty, high As soils.