Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments

Abstract

Estuarine and coastal environments are assumed to contribute to nitrous oxide (N 2 O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N 2 O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N 2 O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N 2 O molecule increased due to increasing nitrate concentration, suggesting the changes in N 2 O production pathways. Enhanced N 2 O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N 2 O production by 11−25%, whereas it decreased bacterial N 2 O production by 16−33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13−28% of N 2 O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N 2 O production. Collectively, these results suggest that increasing nitrate concentration can shift N 2 O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N 2 O production and have widespread implications for N 2 O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.

Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments

“…In this study, sediments were anaerobically incubated, N 2 O was only produced from denitrification, including bacterial, fungal, and chemodenitrification. 14,25 Relative contributions of the three processes to N 2 O production in the ambient group (without microbial inhibitor) were estimated using three endmembers of N 2 O isotope mass balance. 13,14,25 Relative contribution, SP and δ 18 O were estimated according to following equations:…”

Fe(II) Oxidation Shaped Functional Genes and Bacteria Involved in Denitrification and Dissimilatory Nitrate Reduction to Ammonium from Different Paddy Soils

“…Ferrous iron has been reported to strongly affect the performance of denitrification and DNRA. A paddy soil with ∼40 mmol/kg Fe­(II) showed faster nitrate reduction and emitted less N 2 O than did the other paddy soil with one-half of the Fe­(II) concentration, and chemodenitrification, the abiotic reaction between Fe­(II) and NO 2 – , accounted for 6.8% ∼ 67.6% of the total N 2 O emissions in paddy soils and estuarine/coastal sediments. , The addition of 5 mM Fe 2+ to 15 N-NO 3 – experiments with estuarine sediments results in a reduction in denitrification, accounting for only 6% of nitrate reduction, and increases in DNRA and Fe 2+ removal rates. , On the other hand, both denitrification and DNRA rates in lake sediment microcosms are found to be stimulated by Fe­(II) addition. , However, the Fe­(II) addition did not enhance NH 4 + production in our previous paddy soil microcosms under nitrate-reducing conditions, but changed the dominant genera from Acidaminobacter, Proteiniclasticum, and Alkaliphilus to Azospira, Zoogloea, and Dechloromonas . Metagenomic analysis of the NO 2 – -to-NO genes, nirK and nirS , and NO 2 – -to-NH 4 + genes, nrfA and nirB , indicates that most of the bacteria associated with nirS and nirB were affiliated with β-proteobacteria in the lake sediment microcosms after long-term Fe­(II) inputs, which are similar to the reported nitrate-reducing Fe­(II)-oxidizing bacteria. , …”

“…A paddy soil with ∼40 mmol/kg Fe(II) showed faster nitrate reduction and emitted less N 2 O than did the other paddy soil with one-half of the Fe(II) concentration, 19 and chemodenitrification, the abiotic reaction between Fe(II) and NO 2 − , accounted for 6.8% ∼ 67.6% of the total N 2 O emissions in paddy soils and estuarine/coastal sediments. 20,21 The addition of 5 mM Fe 2+ to 15 N-NO 3 − experiments with estuarine sediments results in a reduction in denitrification, accounting for only 6% of nitrate reduction, and increases in DNRA and Fe 2+ removal rates. 22,23 On the other hand, both denitrification and DNRA rates in lake sediment microcosms are found to be stimulated by Fe(II) addition.…”

Elucidation of the Mechanism of Greenhouse Gas Generation by Abiotic Transformation of Nutrient Ions Flowing into Closed Water Areas with Little Phytoplankton

“…Among the greenhouse gases, the formation mechanism of N 2 O is not well understood. − A clear point is that nitrogen fertilizers consisting of ammonia produced by the Haber–Bosch process are one of the main sources of N 2 O . However, as the population is expected to grow in the future, it will be difficult to reduce the amount of nitrogen fertilizer used. − Processing of nitrogen by microorganisms in rivers requires substrates, reactant inputs, suitable redox conditions, and intermediate residence times and is therefore thought to occur primarily in hyporheic zones rather than in surface waters. , Therefore, in order to establish a way to reduce N 2 O, we need to know how N 2 O is produced from nitrogen fertilizer. − For example, the dissimilatory reduction of nitrate to ammonium (DNRA) in soil does not produce N 2 O. , Additionally, denitrification and nitrification, which is used as a purification method for septic tanks, is thought to be less likely to generate N 2 O . However, much of the nitrogen fertilizer that remains in the soil without being absorbed by plants is broken down by nitrifying and denitrifying bacteria.…”