Simultaneous Minimization of Arsenic Mobilization and Nitrous Oxide Emission in Rice Paddy Soils

Abstract

Rice represents a major food source for billions of people worldwide. Traditional water-logged rice cultivation induces reducing conditions under which natural, toxic arsenic can accumulate in rice grains causing health issues for humankind. Under nitrogen (N) fertilization, microbial denitrification causes the formation of nitrite as intermediate and dinitrogen gas or nitrous oxide (N2O), a potent greenhouse gas, as final reaction products. Iron(II) can react abiotically with nitrite during the process of chemodenitrification, leading to N2O production. During both biotic iron(II) oxidation and chemodenitrification, iron(III) minerals form, which serve as highly reactive sorption templates for nutrients or contaminants, such as arsenic. The application of less or no N-fertilizer in waterlogged rice cultivation will shift redox conditions towards iron(III) reduction ultimately remobilizing arsenic. In the framework of this dissertation, we identified conditions under which we have lowest arsenic mobilization and greenhouse gas emissions (i.e., N2O and methane) to minimize health- and climate-related risks. In a microcosm study with paddy soil from Vercelli, Italy, that was carried out over 129 days, we applied nitrate fertilizer at different concentrations and timepoints. We found that arsenic was rapidly scavenged by iron(III) minerals after fertilizer application, yet, also rapidly mobilized after nitrate depletion. Only the highest rate of nitrogen application resulted in sustained, long-term retention of arsenic on iron minerals. At the same time, N2O emissions were similarly high irrespective of fertilizer concentrations added. This illustrates that timing and frequency of fertilizer application is crucial for controlling arsenic mobility and N2O emissions not only under lab settings, but also likely under more natural conditions. Under N fertilization, iron(III) mineral formation was caused by iron(II)-oxidizing microorganisms, such as Gallionellaceae that were more abundant under nitrate fertilization compared to non-fertilization. By cultivation techniques, we were able to enrich a first lithoautotrophic nitrate-reducing, iron(II)-oxidizing culture from a paddy soil – called “culture HP” – which is dominated by Gallionellaceae. We quantified the extent of nitrate reduction, iron(II) oxidation and identified N2O as the main product during denitrification. By combining experimental data with environmental systems analysis, we were able to quantify that 99.5% of the produced N2O was biologically derived and that enzymatic iron(II) oxidation accounted for 99.8% of the total iron(II) oxidation. Further, we were able to show that labile, bioavailable organic carbon sources (i.e., acetate) tremendously impacted the microbial community composition shifting the enrichment culture towards more mixotrophic or heterotrophic denitrifiers (Dechloromonas sp., Acidovorax sp., Zoogloea sp., and Parvibaculum sp.). This study systematically investigated the dynamics of arsenic mobility and N2O emissions under diverse nitrate fertilization regimes in rice paddy soils. By enriching microbial key players responsible for nitrate-dependent iron(II) oxidation in paddy soils, we provided insights into important microbial processes. Our findings underscore the significance of biotically derived N2O emissions, emphasizing that these emissions cannot be overlooked in lab microcosm experiments, in microbial cultures and likely also in rice paddy soils. Through an interdisciplinary framework that integrated field sampling, laboratory experiments, molecular biology techniques, and numerical reaction modeling, we elucidated the intricate interplay of biotic and abiotic reactions, microbiome composition, and their collective impact on arsenic mobility and greenhouse gas dynamics in nitrate-fertilized paddy soils.