Abstract
Paddy fields are major contributors to agricultural greenhouse gas emissions. Applying ~1% biochar by topsoil weight (high single, HS) effectively reduces greenhouse gas emissions from paddy fields, but long-term impacts are unclear. Here we present 8-year field experiments showing HS reduces CO2 equivalent per hectare by 59% and yields a net benefit of US$1,810 per hectare. However, its effectiveness declines over time due to the decreased soil carbon content and methanotrophic activity triggered by higher soil ammonium concentrations. To counteract this, the annual-low method, involving yearly biochar recycling, surpasses the HS approach with a 52% CO2 reduction and yields a net benefit of US$2,801 (35%) per hectare—highlighting the economic and environmental viability of annual-low biochar use in sustainable paddy field management practices.
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Data availability
The data supporting the findings of this study are available within the article and the Supplementary Information. Source data are provided with this paper.
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Acknowledgements
This research was supported by the National Natural Science Foundation of China (42407006, 42077032, 42325707 and 41571241), Postdoctoral Talent Funding Program (2023C03G2072956) and the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAC02B01). We gratefully acknowledge the financial support from the China Scholarship Council (202106320251) and the Doctoral Rising Star Program of Zhejiang University. We thank X. Fan for the constructive suggestions.
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W.W. designed the study. Q.N. and B.G. prepared and analysed the data. Q.N. and B.G. wrote the first draft of the paper. Q.N. and W.C. conducted the long-term field experiment. Y.Q. reviewed this article. Q.N., D.R.S. and J.M. conducted metagenomics analysis. Q.N. provided visualization support. All authors contributed to writing the paper.
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Extended data
Extended Data Fig. 1 Cumulative CH4 emission over 8 years with different amendment strategies.
Different letters above the bar present the significant differences among treatment for each year. There is no interaction between the year and the treatment. Data are presented as mean values ± SEM to account for variability within the replicates (n = 3). The letters above bars denote significant differences (p < 0.05) between treatments for each year in one-way ANOVA. Post-hoc comparisons were conducted using the LSD test to determine specific group differences.
Extended Data Fig. 2 Urea application on CH4 emissions.
The effect of urea application on CH4 emissions in paddy field (a) and its decisive factors (b).The data collected from the long-term field experiment and Banger et al. The data collection method is in Supplementary Method 5. The reference was listed in the supplementary information.
Extended Data Fig. 3 Rice yield with rice straw and biochar amendments over 8 years.
CK, control treatment, no rice straw, and biochar incorporation. RS, rice straw application at 8 Mg ha-1 annually; HS, rice straw biochar application at 22.5 Mg ha-1 only in the first year; AL, rice straw biochar application at 2.8 Mg ha-1 annually. The letters above bars denote significant differences (p ≤ 0.05) between different treatments for each year in a one-sided comparison. Data are presented as mean values ± SEM to account for variability within the replicates (n = 3). The bars show standard error. The letters above bars denote significant differences (p < 0.05) between treatments for each year in one-way ANOVA. Post-hoc comparisons were conducted using the LSD test to determine specific group differences. There is no interaction between the year and the treatment.
Extended Data Fig. 4 Methanogens and methanotrophs community structure.
Community structure variation of methanogens (a-c) and methanotrophs (d-f) between AL and HS, observed in 2016 (a,d), 2018 (b,e) and 2021 (c,f). Methanogen and methanotroph abundances were derived from 16S rRNA sequencing (V34 for methanogens, V45 for methanotrophs) in 2016 and 2018, and from metagenome sequencing in 2021. Sampling methods are detailed in Supplementary Method 1.
Extended Data Fig. 5 Methanotrophs structure within biochar amendments.
The relative methanotrophs abundance of between AL and HS, observed from years of 2016 (a), 2018 (b), and 2021 (c).
Extended Data Fig. 6 CO2-eq reduction and the net benefit over 8 experimental years in RS.
a, GHG emissions reduction after rice straw incorporation at 8 Mg ha-1. b, the GHG emissions reduction over 8 years in RS. c, the corresponding net benefits after rice straw incorporation at 8 Mg ha-1. d, the total benefits over 8 years in RS.
Extended Data Fig. 7 The effect of fresh and aged biochar on CH4 mitigation.
This schematic figure illustrates the adverse effects of biochar aging on CH₄ emission reduction, highlighting the degradation of its physical structure, loss of nutrient retention, decline in root oxygen exudation capacity, and the resulting increase in soil ammonium levels. As biochar ages, its pore structure breaks down, gradually diminishing its ability to supplement nutrients. Compared to fresh biochar, aged biochar in HS is less effective at supporting root growth than in AL, leading to weakened root systems with reduced oxygen exudation capacity. This decline in root activity is directly linked to decreased aerobic methanotrophic activity. In HS, repeated agricultural practices further accelerate the breakdown of biochar’s pores, exposing more surface area and speeding up its oxidation. This process increases the concentration of oxygen-containing functional groups, such as carboxyl groups, which in turn raises soil ammonium ion levels. The elevated ammonium concentration is detrimental to methanotrophs unless they can mitigate ammonium toxicity, leading to decreased methanotroph diversity. In the long term, this inhibition may not only reduce CH₄ mitigation but could potentially promote CH₄ emissions.
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Supplementary Code 1
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One Excel workbook with multiple sheets containing all the data from the manuscript.
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Nan, Q., Speth, D.R., Qin, Y. et al. Biochar application using recycled annual self straw reduces long-term greenhouse gas emissions from paddy fields with economic benefits. Nat Food 6, 456–465 (2025). https://doi.org/10.1038/s43016-025-01124-z
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DOI: https://doi.org/10.1038/s43016-025-01124-z
