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

Microcosm incubations were conducted to determine the effects of two different N fertilization regimes on arsenic (As) mobilization and nitrous oxide (N2O) emission in flooded paddy soils using 15N-isotope tracing techniques. Treatments consisted of an ammonia-nitrate combined amendment (4 mM 14NH4+ + 6 mM 15NO3−) versus a nitrate (10 mM 15NO3−) amendment. Because nitrate has a higher redox potential and comprises a higher proportion of N substrate, As(V) is preferentially formed due to enhanced nitrate-dependent microbial As(III)-oxidation. Thus, arsenic availability in flooded soils is correspondingly attenuated due to the enhanced production of less mobile and toxic As(V). After a 2 d-incubation, more than 90% and 98% of soluble As(III) was immobilized in ammonia-nitrate and nitrate treatments, respectively. Following nitrate depletion (after 2 days), microbial As(V)/Fe(III) reduction was gradually enhanced, which was attributed to feammox reactions that were stimulated by an abundance of NH4+. By the end of the incubation period (10 days), the nitrate treatment led to higher As immobilization (~61% of originally added 0.1 mM As(III) was immobilized) versus the ammonia-nitrate treatment (42%). N2O emission was prominently mitigated by the ammonia-nitrate treatment compared to the nitrate treatment, which was ascribed to anammox inducing accumulation of byproducts from incomplete denitrification. High-throughput sequencing indicated that the abundances of denitrifiers (e.g., Azoarcus, Ochrobactrum and Thiobacillus denitrificans) were enriched in the nitrate treatment, whereas qPCR results indicated higher 16S rRNA gene copy numbers for anammox and feammox bacteria (Acidimicrobiaceae bacterium A6) in the ammonia-nitrate treatment. Collectively, the experimental results demonstrated that N fertilization can be a feasible As-remediation strategy while at the same time providing an effective strategy for mitigating N2O emission from paddy soils.