Nitrogen Application Stimulates Methane Emissions

Abstract

Methane (CH₄), a potent greenhouse gas, has a global warming potential 27 times higher than carbon dioxide on a 100-year scale and contributes 30% of human-induced global warming (IPCC 2021; WMO 2022). Rice paddies emit 24–31 Tg CH₄ year⁻¹, accounting for ~9% of anthropogenic CH₄ emissions (IPCC 2021). Meanwhile, rice is the most important stable food globally, emphasizing the urgency to reduce CH₄ emissions (Bao et al. 2024). Nitrogen (N) fertilizers are crucial to global rice production (van Grinsven et al. 2022), and elucidating the intricate impacts of N application on CH₄ emissions from rice paddies through various pathways remains a significant challenge (Tang et al. 2024). First, N fertilization typically promotes plant growth (Cai et al. 2023; Feng et al. 2023), thereby increasing the availability of substrates conducive to CH₄ production (Qian et al. 2023), ultimately leading to more CH₄ emissions. Second, ammonium (NH₄⁺) suppress CH₄ oxidation by competing for the binding sites on CH₄ monooxygenase (Qian et al. 2023), thereby further enhancing CH₄ emissions. Conversely, N application can enhance the growth of rice roots, subsequently facilitating the transport of oxygen into the rhizosphere (Jiang et al. 2017), which may increase the growth and metabolic activity of soil methanotrophs, potentially promoting CH₄ oxidation and thus decreasing CH₄ emissions. Therefore, the intricate interplay between N fertilization and the CH₄ cycle introduces complexity, making it challenging to anticipate the net effects of N fertilization on CH₄ emissions from rice paddies.

Numerous studies have endeavored to quantify the impacts of N fertilization on CH₄ emissions from rice paddies. However, the findings have been inconsistent and are profoundly influenced by a range of local factors, including soil characteristics, plant species, agricultural management strategies and climatic conditions (Liao et al. 2021). Despite these investigative endeavors, the precise magnitude of N fertilization's contribution to CH₄ emissions remains uncertain, presenting significant challenges in predicting global CH₄ emissions and developing targeted mitigation strategies.

Recently, Tang et al. (2024) conducted the first global-scale study to quantify the effects of N fertilizers on CH₄ emissions from rice paddies, a topic that is both intriguing and highly topical. The study employed a multifaceted approach that integrates a comprehensive meta-analysis with serial verification methodologies, including field, pot and incubation experiments, to provide novel insights into the role of soil pH in modulating the effects of N fertilization on CH₄ emissions from rice paddies worldwide. The meta-analysis synthesized 153 most relevant observations from 41 studies to identify the key driver of N effects on CH₄ emissions. A crucial aspect of this analysis was its focus on continuous, multi-season fertilization experiments conducted under field conditions, reflecting the long-term nature of N fertilization practices. The finding revealed soil pH as the most important predictor of N fertilization effects on CH₄ emissions from rice paddies, accounting for a greater proportion of variation than various environmental, plant and soil factors. As soil pH increased, the impacts of N fertilization on CH₄ emissions consistently decreased. Linear model selection analysis further confirmed soil pH as a key moderating factor, with the most parsimonious model exclusively incorporating soil pH. In addition, field experiments investigated the impact of soil pH and N fertilization on CH₄ emissions. The results supported the meta-analysis findings, demonstrating that increasing soil pH through liming reduced CH₄ emissions. Pot experiments provided insights into the microbial processes underlying N effects on CH₄ emission at different soil pH levels.

Furthermore, the incubation experiments, conducted with soil samples across China's rice paddies, explored whether the impact of N fertilization on CH₄ production varies with soil pH. The experiments consistently showed that N fertilization had stronger effects on CH₄ emissions in acidic soils compared with alkaline soils. In acidic soils, N fertilization increased the availability of NH₄⁺, a key substrate for methanogens, thereby promoting CH₄ production. In alkaline soils, ammonia (NH₃) predominates over NH₄⁺, and ammonia volatilization occurs at a higher rate, reducing the availability of NH₄⁺ and subsequently CH₄ production. Additionally, alkaline soils often correlate with a more oxidized environment, suppressing the activity of methanogenic archaea and further reducing CH₄ production.

In their paper, Tang et al. (2024) also explored the impact of N fertilization on yield-scaled CH₄ emissions, which is essential for balancing food security and climate change mitigation. The results indicated that N fertilization slightly increased yield-scaled CH₄ emissions but that this effect decreased with increasing soil pH. Notably, soil pH remained the most important predictor of N effects on yield-scaled CH₄ emissions among various factors. The researchers acknowledged several potential mechanisms explaining the role of soil pH in modulating the impact of N fertilization on CH₄ production. These include changes in substrate availability for methanogens, microbial community composition and soil redox conditions. The study emphasized the importance of considering soil pH when developing mitigation strategies to reduce CH₄ emissions from rice agriculture. Despite its comprehensive nature, the study has some limitations. Firstly, it may have a geographical bias due to a scarcity of data from Southeast Asia and Brazil, where acidic soils predominate. However, the researchers noted that this bias is unlikely to affect the accuracy of their global extrapolation, as their meta-analysis was based on climate factors, soil factors and agricultural practices rather than specific geographical locations. Additionally, the study focused on the effects of N fertilization on CH₄ emissions, but rice cultivation also requires a range of nutrient supplements, such as phosphorus and potassium, which also influence CH₄ emissions. In their closure, Tang et al. (2024) also highlighted that N fertilization impacts the environmental footprint of rice in multiple ways. Specifically, the application of N fertilizer to rice paddies enhances the volatilization of NH₃ and the emission of nitrous oxide. These processes not only contribute to aerosol formation but also substantially decrease nitrogen use efficiency, resulting in a perpetuating cycle that necessitates increased N fertilizer application.

This study furnishes mechanistic elucidation through rigorous data synthesis, presenting novel and profound insights into the pivotal role of soil pH in modulating the influence of N fertilizer on CH₄ emissions from rice paddies worldwide. We expect that future research endeavors will successfully accomplish the multiple goals of sustaining rice yields while concurrently minimizing greenhouse gas emissions and N losses, through the adoption and implementation of advanced and intelligent rice field management strategies.

Kaikai Fang: conceptualization, writing – original draft, writing – review and editing.

The author declares no conflicts of interest.