The significance of nitrogen to global rice production, especially in China where fertilizer application rates frequently reach two to three times the global average, has long placed the spotlight on agricultural management practices aimed at curbing nitrogen loss. Until now, efforts to mitigate gaseous nitrogen emissions have centered on optimizing fertilizer use. However, one fundamental challenge has been accurately distinguishing soil-emitted N₂ from the atmospheric background, which has obscured precise source attribution. By addressing this challenge, the research team has uncovered that a substantial fraction—approximately 72% to 75%—of N₂ emissions stems from the mineralization of soil organic nitrogen rather than the applied synthetic fertilizers themselves.

To unlock these revelations, the team deployed an innovative in situ methodology that combined ^15N isotope tracing with membrane inlet mass spectrometry (MIMS). This sophisticated approach allowed for continuous, simultaneous monitoring of multiple nitrogen gases—dinitrogen (N₂), ammonia (NH₃), and nitrous oxide (N₂O)—across the entire rice growing season. More importantly, it enabled researchers to partition the gaseous emissions according to their nitrogen sources precisely. This methodological breakthrough overcame long-standing analytical limitations and provided unprecedented resolution in field-scale nitrogen transformation measurements.

These detailed observations revealed a complex temporal pattern in nitrogen losses associated with rice cultivation. Early in the growing season, volatile ammonia emissions were closely tied to the application of synthetic fertilizers, mainly urea, which rapidly hydrolyzes to produce ammonium (NH₄⁺). However, as the season progressed, the dominant pathway shifted toward dinitrogen emissions primarily originating from nitrogen released through the microbial breakdown of soil organic matter. Nitrous oxide emissions, another potent greenhouse gas, were found to be produced by both fertilizer-derived and soil-derived nitrogen, indicating a more complicated interplay between soil microbiota and fertilizer inputs.

The researchers propose a novel conceptual framework termed the “microbial nitrogen pump” to explain these dynamics. This mechanism suggests that, following fertilizer application, soil microbes swiftly assimilate ammonium to fulfill their growth demands, inducing a stoichiometric imbalance between carbon and nitrogen within the microbial biomass. To restore this equilibrium, the microbes accelerate the decomposition of native soil organic matter, leading to the mobilization and mineralization of soil organic nitrogen. The mineralized ammonium, derived from this “old nitrogen,” subsequently undergoes nitrification and denitrification processes, culminating in the release of dinitrogen gas into the atmosphere. This challenges the conventional notion that fertilizer nitrogen directly transforms into gaseous losses.

Intrinsically, the microbial nitrogen pump thus acts as an indirect driver of substantial nitrogen losses, with fertilizer serving as an activator of soil nitrogen pools rather than the primary nitrogen source lost as gas. This insight has profound implications for nitrogen management in flooded rice systems because it shifts the focus from fertilizer input alone to the soil microbial and organic matter interactions that govern nitrogen cycling at a more fundamental level.

Beyond merely advancing scientific understanding, the study also identifies promising practical applications. Notably, hybrid rice cultivars demonstrated enhanced efficiency in nitrogen uptake and microbial nitrogen utilization. These varieties reduced yield-scaled nitrogen gas emissions by approximately 43% without sacrificing grain productivity, highlighting a viable pathway to achieve both environmental sustainability and food security. This synergy underscores the importance of integrating crop breeding strategies with soil microbiome management to optimize nitrogen use efficiency.

The implications of this research extend far beyond rice paddies in China. By establishing soil organic nitrogen as the dominant source of dinitrogen emissions, the study calls for a reassessment of global nitrogen budgets and the models that predict greenhouse gas emissions from agricultural systems. Current models may underestimate the contribution of soil organic nitrogen mineralization to nitrogen gas fluxes, which could lead to errors in shaping climate policies and agricultural guidelines worldwide.

Furthermore, this work offers a new theoretical and methodological foundation for future research focused on sustainable agricultural practices. The combination of isotope tracing and real-time gas measurement tools presents an opportunity to explore nitrogen cycling intricacies in various agroecosystems, potentially leading to innovations that curb nitrogen losses while maintaining or even enhancing crop yields.

Importantly, understanding the microbial nitrogen pump also opens avenues to manipulate soil microbial communities and organic matter dynamics to mitigate nitrogen loss. For example, adjusted fertilization regimes that minimize microbial disruption or practices that maintain the soil organic nitrogen reservoir could reduce N₂ emissions. Additionally, breeding or engineering crop varieties capable of more efficient nitrogen uptake could further diminish the indirect losses triggered by microbial mineralization.

In conclusion, this pioneering study by Prof. YAN Xiaoyuan and colleagues heralds a new era in agricultural nitrogen research. By demonstrating that soil organic nitrogen, rather than fertilizer nitrogen, predominantly drives dinitrogen emissions in flooded rice systems, they challenge longstanding assumptions and highlight the central role of microbial processes in agricultural nitrogen cycling. Their findings not only provide theoretical insights but also practical strategies to enhance nitrogen use efficiency and reduce environmental impacts, offering hope for more sustainable food production systems globally.

[Subject of Research]: Not applicable
[Article Title]: Soil organic nitrogen rather than fertilizer drives dinitrogen losses in flooded rice systems
[References]: YAN Xiaoyuan et al., Proceedings of the National Academy of Sciences, 2026.
[Image Credits]: YAN Xiaoyuan
[Keywords]: Soil science, Environmental sciences, Soils, Pedology, Soil chemistry, Fertilizers, Nitrogen cycle

Recent groundbreaking research published in National Science Review offers compelling evidence identifying periphyton as a previously overlooked yet pivotal microbial nitrogen sink in flooded rice paddies. Led by Dr. Yonghong Wu of the Institute of Soil Science, Chinese Academy of Sciences, the study unveils the crucial role of this thin biofilm – a composite of algae, bacteria, and extracellular polymeric substances – that thrives at the soil-water interface. This microbial consortium creates a densely packed microhabitat endowed with robust capabilities for nutrient uptake, biogeochemical transformation, and temporary nitrogen storage, thereby bridging the enigmatic nitrogen budget gap.

The research team employed a multifaceted approach combining an extensive nationwide field survey and innovative ^15N isotope tracer experiments to quantify the extent of fertilizer nitrogen interception by periphyton and elucidate its subsequent fate. Over four years, from 2016 to 2019, they meticulously sampled periphyton from 840 rice paddies spanning more than 93% of China’s vast rice-growing regions. Measurements of periphyton biomass and nitrogen content allowed them to extrapolate findings with spatial precision at provincial and national scales. Parallel to this, ^15N-labeled urea tracing experiments were conducted across three climatically distinct regions – temperate, subtropical, and tropical zones – to directly monitor fertilizer nitrogen incorporation into periphyton throughout rice’s cultivation season.

Findings from both nationwide sampling and isotope labeling converged on a striking conclusion: periphyton sequesters a substantial fraction of applied fertilizer nitrogen. Data reveal that periphyton accounts for between 6 to 24% of fertilizer inputs across different provinces, with a national average capture of approximately 12%. This translates into an astonishing 0.8 teragrams of nitrogen annually retained in China’s paddy periphyton biomass alone, a magnitude strikingly equivalent to the previously unaccounted nitrogen pool identified in long-term nitrogen budget analyses. This quantification closes a critical knowledge gap that has challenged agronomic nitrogen cycling models for decades.

The mechanistic insights from ^15N tracer assays unambiguously support this interpretation. The proportion of fertilizer nitrogen assimilated by periphyton was quantified at 9.3 ± 1.6% in tropical rice fields, 11.4 ± 1.6% in subtropical zones, and peaked at 21.3 ± 3.8% in temperate regions. The mean across all sites reached around 14%, underscoring periphyton’s widespread influence on nitrogen partitioning within paddy ecosystems. This robust consistency between empirical data sets across varied environmental contexts strongly corroborates the role of periphyton as a ubiquitous and quantifiable component in paddy nitrogen cycling processes.

Beyond sequestration capacity, the chemical nature of nitrogen stored within periphyton reveals it functions primarily as a transient and potentially recyclable nitrogen reservoir. Ammonium (NH_4^+) emerged as the dominant inorganic nitrogen species within the periphyton matrix, surpassing nitrate (NO_3^−) levels by at least an order of magnitude. Intriguingly, the concentration gradients of ammonium mirrored climatic zones, decreasing progressively from temperate to subtropical to tropical fields. This pattern suggests that microbial metabolic processes within periphyton are highly temperature-sensitive, influencing nitrogen dynamics and turnover rates in regionally distinct ways.

Further isotopic partitioning revealed multiple downstream fates of nitrogen once incorporated into periphyton. Approximately 19 to 24% of this nitrogen returns to the soil residual nitrogen pool, reinforcing soil fertility and potential crop availability. A notable fraction, ranging from 8 to 29%, escapes via ammonia volatilization, representing an atmospheric loss pathway. Additionally, denitrification-related gaseous emissions accounted for 7 to 16% of nitrogen loss. The residual nitrogen remains sequestered temporarily within periphyton biomass, with eventual release possible through periphyton decomposition. These redistributive pathways highlight the complex nitrogen cycling role periphyton plays, acting both as a sink and a source within paddy ecosystems.

In synthesizing these findings, the study profoundly revises conceptual frameworks traditionally employed to evaluate nitrogen fate in flooded rice systems. Incorporating periphyton dynamics as a short-lived nitrogen reservoir and redistribution hub closes the longstanding nitrogen budget gap. This enhanced understanding paves the way for improved nitrogen management strategies that harness periphyton’s nutrient cycling functions. Targeted water management practices and optimized fertilization timing geared towards synchronizing periphyton nitrogen release with peak crop demand could bolster internal nitrogen recycling efficiency. Such refinements hold promise to reduce unnecessary nitrogen fertilizer applications and mitigate accompanying environmental trade-offs, including greenhouse gas emissions and nutrient pollution.

This research underscores the necessity of integrating microbial biofilms like periphyton into holistic nutrient management paradigms. Previously marginalized, these complex microbial consortia exert outsized influence on agroecosystem nutrient fluxes and sustainability. As global agriculture grapples with the imperative to increase production while minimizing environmental harm, elucidating the multifaceted roles of periphyton offers a novel lever for enhancing nitrogen use efficiency. Advancing this line of research could inform scalable, climate-resilient interventions to sustainably intensify rice production systems worldwide.

Moreover, the interdisciplinary methodology blending landscape-scale field surveys with high-resolution isotope biogeochemistry provides a powerful template for future studies scrutinizing hidden nutrient pools. By unmasking cryptic nitrogen sinks, such investigations can sharpen nitrogen cycle models and contribute to precision agriculture. The evidence presented invites reconsideration of nitrogen budgets in other flooded agroecosystems and aquatic environments where periphyton biofilms are prevalent but underappreciated in nutrient accounting.

In conclusion, the discovery of periphyton’s role in closing nitrogen budget deficits represents a paradigm shift in understanding nitrogen cycling in rice paddies. It not only resolves a long-standing scientific puzzle but also highlights practical opportunities to refine fertilizer management and mitigate environmental impacts. As researchers and practitioners embrace this expanded nutrient cycling perspective, periphyton may emerge as a vital ally in achieving sustainable intensification of rice production, safeguarding food security while protecting ecological integrity.

Subject of Research: Nitrogen cycling and fertilizer use efficiency in rice paddy agroecosystems

Article Title: Periphyton closes the nitrogen budget gap in rice paddies

Web References:
DOI: 10.1093/nsr/nwag016

References:
Wu, Y., et al. (2023). Periphyton closes the nitrogen budget gap in rice paddies. National Science Review. DOI: 10.1093/nsr/nwag016

Image Credits: ©Science China Press

Keywords: nitrogen cycle, periphyton, rice paddies, fertilizer nitrogen, nitrogen budget, isotope tracer, nitrogen use efficiency, ammonia volatilization, denitrification, agroecosystems, microbial nitrogen sink, nutrient cycling