Arsenic mobilization and nitrous oxide emission modulation by different nitrogen management strategies in a flooded ammonia-enriched paddy soil

Abstract

Elevated arsenic (As) mobilization and increased nitrous oxide (N₂O) emission are two primary environmental concerns existing in flooded paddy soils. In this study, dissolved As(III), N isotope-labeled Na¹⁵NO₃, and/or ¹⁴NH₄Cl were incorporated into a microcosm incubation to determine the effects of N fertilization regimes on As mobilization and N₂O emission in a flooded paddy soil. Because nitrate had a higher redox potential and comprised a higher proportion of N substrate, As(V) was preferentially formed due to enhanced nitrate-dependent microbial As(III) oxidation. Thus, As availability was correspondingly attenuated due to the improved production of less mobile and toxic As(V). After 2-d incubation, more than 90% and 98% of soluble As(III) were immobilized in the As + NH₄⁺-N + NO₃^--N and As + NO₃^--N treatments, respectively. Following nitrate depletion (after 2 d), microbial As(V) and Fe(III) reductions were gradually enhanced, which was attributed to stimulation of anaerobic ammonium oxidation (anammox) coupled to Fe(III) reduction, known as Feammox, by the abundance of NH₄⁺. By the end of the incubation period (10 d), the As + NO₃^--N treatment led to higher As immobilization of originally added As(III) (ca. 61%) than the As + NH₄⁺-N + NO₃^--N treatment (42%). The As + NH₄⁺-N + NO₃^--N treatment prominently mitigated N₂O emission compared to the As + NO₃^--N treatment, which was ascribed to anammox, inducing accumulation of byproducts from incomplete denitrification. High-throughput sequencing indicated that the relative abundances of denitrifiers (e.g., Azoarcus, Ochrobactrum, and Thiobacillus denitrificans) increased in the As + NO₃^- treatment, whereas quantitative polymerase chain reaction results indicated higher 16S rRNA gene copy numbers for anammox and Feammox (Acidimicrobiaceae bacterium A6) bacteria in the As + NH₄⁺ + NO₃^- treatment. Collectively, the experimental results demonstrated that N fertilization can be a feasible As remediation strategy while providing an effective strategy for mitigating N₂O emission from paddy soils at the same time.