Returning crop straw to farmland is widely used to recycle nutrients and improve soil quality. New research now reveals another important benefit: straw return may help paddy soils retain more organic carbon by activating two complementary forms of protection, one physical and one chemical.
Researchers from the Chinese Academy of Agricultural Sciences investigated how long-term straw incorporation affects soil organic carbon in a double-cropping rice system. Their findings show that iron oxides play a central role in connecting straw management with carbon storage.
"Our results suggest that straw return does more than simply add organic matter to soil. It also changes the way iron minerals interact with soil particles and organic carbon, creating conditions that can support longer-term carbon stabilization," said corresponding author Yinghua Duan of the Chinese Academy of Agricultural Sciences.
The study was based on a long-term field experiment established in 2012 at the Qiyang Red Soil Experimental Station in Hunan Province, China. After 11 years, the researchers compared four management treatments involving mineral fertilizer, straw return, winter green manure using Chinese milk vetch, and combinations of these practices.
Straw return produced clear gains in soil organic carbon. Compared with mineral fertilizer alone, straw return increased soil organic carbon by 13.6%. When straw return was combined with winter green manure, soil organic carbon was 22.7% higher than under mineral fertilizer alone.
The researchers then examined how this additional carbon was protected.
One pathway involved soil aggregates, clusters of mineral particles and organic matter that can physically shield carbon from decomposition. Straw return promoted the formation of large aggregates greater than 2 mm. It also increased mean weight diameter, an indicator of aggregate stability, by 57.2% and 73.1% when straw-return treatments were compared with their corresponding treatments without straw.
Iron minerals appeared to help build this protection. Straw return increased several forms of iron oxides. Complexed iron accumulated preferentially in large aggregates, while amorphous iron oxides were enriched in smaller aggregates. Both patterns were associated with improved aggregate stability.
A second pathway involved direct chemical bonding between iron and organic carbon. Iron-bound organic carbon represented 21.2% to 26.7% of total soil organic carbon, showing that mineral-associated carbon formed a substantial part of the soil carbon pool.
Straw return significantly enlarged this pool. Total iron-bound organic carbon increased by 41.0% when straw was added to mineral-fertilized soil and by 30.9% when straw was added to plots receiving winter green manure.
Spectroscopic indicators also suggested that straw return increased the aromaticity, hydrophobicity, and molecular complexity of important iron-bound carbon fractions. These properties are generally associated with greater chemical stability.
Together, the results reveal two iron-mediated mechanisms for carbon preservation: physical protection inside stable soil aggregates and chemical stabilization through associations between iron oxides and organic carbon.
The researchers caution that the effectiveness of these mechanisms may vary among soils and climates. Iron-rich, clay-rich soils may be particularly favorable, while sandy, alkaline, or iron-poor soils may respond differently.
The findings provide a clearer mechanistic basis for using crop residues in sustainable soil management and suggest that straw return could simultaneously support soil fertility and enhance carbon retention in subtropical paddy systems.
===
Journal Reference: Li B, Huang J, Liu L, Li D, Duan Y, et al. 2026. Straw return promotes soil organic carbon sequestration through aggregate protection and chemical bonding mediated by iron oxides. Agricultural Ecology and Environment 2: e018 doi: 10.48130/aee-0026-0015
https://www.maxapress.com/article/doi/10.48130/aee-0026-0015
===
About Agricultural Ecology and Environment :
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.
