Straw return combined with potassium fertilization improves potassium stocks in large-macroaggregates by increasing complex iron oxide under rice–oilseed rape rotation system

Potassium (K) supplementation strategies are required to enhance farm productivity in rice-upland rotations, where intensive cultivation practices often result in K deficiencies. Straw return improves the adsorption of K by increasing the content of soil humic acid in macroaggregates. Iron/aluminium (Fe/Al) oxides promote soil organic carbon storage and aggregate stability by acting as binding agents. However, limited information is available on the effects of Fe/Al oxides on the distribution of aggregate-associated K stocks. A field experiment was performed in the Yangtze River Basin, an area with low K stemming from intensive cultivation, with four fertilization treatments: inorganic nitrogen-phosphorus fertilizer (NP), (NPK), inorganic NP with straw return (NP+St), and inorganic NPK with straw return (NPK+St). Results showed that the straw return (NP+St), K fertilization (NPK) and the combination of both (NPK+St) increased soil exchangeable K content (EK) by 32.6 %, 23.7 % and 53.6 % in the rice season, respectively, and increased by 49.9 %, 25.5 % and 182.0 % in the oilseed rape season, respectively, compared with that of no K addition (NP) treatment. K stocks in macroaggregates accounted for more than 90 % of the total K stocks in all treatments. Straw return and K fertilization increased EK and non-exchangeable K (NEK) stocks in large-macroaggregates (>2 mm) by increasing the aggregate-associated K content and regulating the abundance of aggregate. Redundancy analysis showed that complex iron oxide (Fep) was one of the main factors influencing soil available K. The NP+St and NPK+St treatments increased the proportion of particle size and K stocks by increasing the Fep content in large-macroaggregates. Pearson’s correlation analysis and random forest model analysis indicated that EK and NEK stocks in the large-macroaggregates were positively correlated with K uptake by rice and oilseed rape, which suggested that they were key factors influencing K uptake. Therefore, straw return increased Fep in large-macroaggregates to expand the K stock in soil and K uptake by crops under this field experiment conditions. Our results provided new insights with implications for improving soil K availability by straw return combined with K fertilization.