Straw return locks more carbon in paddy soils

A research team has shown that returning rice straw to paddy fields increases soil organic carbon by strengthening soil aggregates and promoting stable chemical bonds between carbon and iron oxides. Drawing on an 11-year field experiment, the researchers identified two complementary carbon-storage mechanisms: physical protection inside aggregates and chemical stabilization through iron–carbon associations. The findings clarify how straw incorporation supports long-term carbon sequestration and could guide agricultural practices that improve soil fertility, recycle crop residues, and reduce the climate footprint of intensive rice production.

Soil organic carbon is essential for maintaining soil structure, nutrient availability, crop productivity, and climate regulation. Paddy soils have considerable carbon-storage potential because repeated flooding and drainage can slow organic matter decomposition and alter mineral–carbon interactions. Returning crop straw supplies fresh carbon and is widely promoted as an alternative to residue burning or removal. However, previous research has not fully explained how straw affects different forms of iron oxide within soil aggregates or how these minerals regulate the accumulation and persistence of organic carbon. Resolving these mechanisms is necessary for developing residue-management strategies that produce durable, rather than temporary, gains in soil carbon.

A study (DOI: 10.48130/aee-0026-0015) published in Agricultural Ecology and Environment on 22 June 2026 by Yinghua Duan's & Minggang Xu's team, Chinese Academy of Agricultural Sciences, reports that long-term straw incorporation increases soil carbon through iron-mediated physical protection and chemical stabilization.

The researchers established a long-term field experiment in 2012 within a double-cropping rice system and collected soil after 11 consecutive years. They compared four management treatments: nitrogen, phosphorus, and potassium fertilization with winter fallow and no straw return; the same fertilization with straw return; fertilization combined with winter green manure, specifically Chinese milk vetch, but without straw return; and fertilization combining green manure with straw return. Soil samples were separated into aggregate-size fractions, allowing the team to measure soil organic carbon, aggregate distribution and stability, different forms of iron oxide, and iron-bound organic carbon. They also used specific ultraviolet absorbance indicators to evaluate the chemical characteristics and stability of carbon associated with iron. Compared with chemical fertilizer alone, straw return increased soil organic carbon by 13.6%. Green manure alone produced a 5.3% increase, while combining green manure and straw return raised soil organic carbon by 22.7%, indicating that the combined management system delivered the largest gain. Straw incorporation also promoted the formation of macro-aggregates larger than 2 millimeters. Mean weight diameter, an indicator of aggregate stability, rose by 57.2% under straw return compared with fertilizer alone and by 73.1% when straw was added to the green-manure system. The mineral analyses revealed that straw return increased soil iron oxides, but their distribution and functions differed by form and aggregate size. Complexed iron oxides accumulated mainly in macro-aggregates, whereas amorphous iron oxides were enriched in micro-aggregates smaller than 0.25 millimeters. Both patterns contributed to greater aggregate stability. By contrast, crystalline iron oxides responded inconsistently across aggregate fractions and were not significantly associated with aggregate stability. Iron-bound organic carbon represented 21.2%–26.7% of total soil organic carbon and was dominated by carbon associated with complexed iron. Straw return increased total iron-bound carbon by 41.0% relative to fertilizer alone and by 30.9% when added to the green-manure treatment. Higher specific ultraviolet absorbance values further indicated that straw return increased the aromaticity and chemical stability of complexed and amorphous iron-bound carbon, making this carbon more resistant to decomposition.

Overall, the study demonstrates that straw return does more than simply add organic material to paddy soil. It reorganizes the soil’s physical structure and strengthens mineral–carbon interactions, helping newly added carbon remain protected over longer periods. These findings provide a mechanistic basis for incorporating straw into sustainable rice-field management. Long-term implementation, particularly alongside green manure, may simultaneously improve soil quality, support crop production, recycle agricultural residues, and increase the contribution of paddy fields to climate-change mitigation.

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References

DOI

10.48130/aee-0026-0015

Origianl Source URL

https://doi.org/10.48130/aee-0026-0015

Funding information

This study was supported by the Innovation Program of Chinese Academy of Agricultural Sciences (CAAS-CSAL-202302), the Central Public-Interest Scientific Institution Basal Research Fund (No. Y2025YC88), and the Chinese Long-Term Experimental Network for Agricultural and Rural Development (CLENARD-DAC-AR and CLENARD-HY).

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Agricultural Ecology and Environment (e-ISSN 3070-0639) is a multidisciplinary platform for communicating advances in fundamental and applied research on the agroecological environment, focusing on the interactions between agroecosystems and the environment. It is dedicated to advancing the understanding of the complex interactions between agricultural practices and ecological systems. The journal aims to provide a comprehensive and cutting-edge forum for researchers, practitioners, policymakers, and stakeholders from diverse fields such as agronomy, ecology, environmental science, soil science, and sustainable development.