Goethite Contributes to Increased N2O Emissions From Autotrophic Nitrification in Paddy Soils

Abstract

Background

Iron (Fe) oxides play a crucial role in biogeochemical processes governing nitrous oxide (N₂O) emissions. However, the precise impacts of Fe oxides on various N₂O production pathways remain inadequately understood.

Aim

To elucidate how goethite influences the relative contributions of both principal biotic pathways, specifically autotrophic nitrification, heterotrophic nitrification, and denitrification, as well as abiotic processes, to N₂O production.

Methods

Sterilization and inhibition experiments were carried out in acidic (pH 5.5) and alkaline paddy (pH 7.9) soils with/without goethite addition under N-fertilizer and non-N-fertilizer conditions. Low-concentration acetylene and high-pressure O₂ were used to distinguish biotic nitrification and denitrification pathways, whereas high-pressure steam sterilization differentiated biotic and abiotic N₂O production.

Results

On the basis of the comparison of sterilized and unsterilized setups, we found that under the condition of 60% water-holding capacity (WHC), N₂O emissions from the acidic and alkaline paddy soils were mainly from biological processes (>82%), whereas abiotic reactions made a minimal contribution. Nitrification (including autotrophic nitrification and heterotrophic nitrification) produced 14.2–17.3 µg N kg⁻¹ in the acidic soils and 71.4–79.7 µg N kg⁻¹ in the alkaline soils, accounting for 48%–55% and 79%–84% of total emissions, respectively. Moreover, autotrophic nitrification contributed 78%–83% to N₂O production in the alkaline soil, which was significantly higher than the 38%–47% in the acidic paddy soil. Denitrification also played a substantial role in the acidic paddy soil, contributing 29%–49% to N₂O production. Goethite addition altered the contributions of these pathways. In the untreated soils, the contribution of non-denitrification pathways (such as autotrophic and heterotrophic nitrification) to N₂O production was 38% and 78% in the acidic and alkaline paddy soils, respectively. After supplementing with goethite, these contributions increased to 47% and 83%. Conversely, the contributions of denitrification to N₂O production decreased from 49% to 20% in the untreated the acidic and alkaline paddy soils to 42% and 15% in the goethite-supplemented soils.

Conclusions

Goethite addition under 60% WHC primarily enhances N₂O production by promoting autotrophic nitrification, particularly in alkaline paddy soils, while reducing denitrification's contribution in acidic soils. These findings advance our understanding of the role of Fe oxides in soil nitrogen cycling and N₂O release, providing insights for mitigating greenhouse gas emissions in agricultural ecosystems.