Chemical properties drive topsoil improvement: Long-term straw return benefits in paddy fields under contrasting tillage and depths

Returning straw to the field substantially improves soil quality and is a key agronomic practice for sustainable rice production. However, the long-term effects of straw returning on soil quality in different soil layers under different tillage practices remain insufficiently understood. Here, we performed a long-term experiment in Nanning and Hezhou to examine variation in soil quality across multiple depths (0-10 cm, 10-20 cm, 20-30 cm, and 30-50 cm) under four treatments: no-tillage (NT), no-tillage with straw mulching (NT-SMR), conventional tillage (CT), and conventional tillage with straw incorporation (CT-SR). Long-term straw return significantly improved the physical, chemical, and biological properties of topsoil, regardless of the tillage method. In comparison to the straw no return treatment, the soil quality index (SQI) in the 0-10 cm soil layer at the Nanning and Hezhou experimental sites increased by 32.3% and 35.8%, respectively; for the 10-20 cm soil layer, the corresponding SQI increments were 11.6% and 47.0%, respectively. Correlation analysis and random forest modeling indicated that the soil quality index (SQI) was significantly correlated with soil chemical properties, including total nitrogen (TN), available nitrogen (AHN), total phosphorus (TP), available potassium (AK), and organic carbon (SOC). The structural equation model showed that tillage and straw returning mainly improved soil quality indirectly by influencing chemical properties. Therefore, the CT-SR treatment is an efficient soil management measure to enhance soil quality. This study provides practical guidance for sustainable soil management in paddy fields in southern China. • The effect of straw return on soil properties is predominantly observed in the 0-20 cm soil layer. • Long-term straw return significantly improved the SQI of the 0-10 cm and 10-20 cm soil layers. • SQI was primarily affected by chemical properties.