Effects of Long-Term Different Straw Returning Methods on Soil Organic Carbon, Nutrients and Aggregate Formation in Different Soil Layers of Double Cropping Rice Field

Long-term direct returning of straw to the field improves the physical and chemical properties of the soil and is conducive to soil carbon and nitrogen retention. However, there are differences among various soil layers. This study aimed to clarify the variation characteristics of soil organic carbon, nutrients and structure at different soil depths.

The study was based on a long-term straw returning experiment established in 2009, with a double rice system as the research object. Four treatments were set up: no straw returning and no fertilizer control (CK), chemical fertilizer only (F), straw burning and returning (SBR), and full returning (SR). In 2021, samples were taken to analyze the physical properties of the soil, aggregate structure, the content of carbon, nitrogen and available nutrients in each soil layer, and the formation of double-cropping rice yield.

Compared with CK, all fertilization treatments could improve soil physical and chemical properties and carbon and nitrogen contents. Compared with SBR and F treatments, SR significantly increased the soil moisture content and total porosity of double-cropping paddy fields, while reducing soil bulk density, with a decrease of 12.0%-17.3% in early rice and 10.7%-16.0% in late rice. SR treatment significantly increased the content of large aggregates (>2 mm) and the average mass diameter and geometric diameter of soil in the 0-15 cm layer of double-cropping paddy fields, which was conducive to promoting the formation and stability of large aggregates, and the effect was the most significant in the 0-5 cm layer of soil. SR treatment also significantly increased the organic carbon content in each soil layer of 0-30 cm and promoted the increase of total nitrogen content in the 10-30 cm layer of soil, while there was no significant difference in organic carbon and total nitrogen between SBR and F treatments. At the same time, SR treatment significantly increased the content of available nutrients, such as ammonium nitrogen, alkali-hydrolyzable nitrogen and available phosphorus in the 0-10 cm layer of soil, but the nitrate nitrogen content only significantly increased in the 0-10 cm layer of soil in the early rice season. In addition, compared with F treatment, SR significantly increased the content of slow-release potassium in each soil layer in the late rice season, thereby significantly increasing the yield of late rice.

Overall, direct straw returning to the field was beneficial to increase the content of large soil aggregates in the 0-15 cm soil layer of double-cropping rice fields, promote the increase of available nutrients in the 0-10 cm soil and the content of organic carbon, total nitrogen and slow-release potassium in deep soil, and achieve stable and high yields.