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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.

Method

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.

Result

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.

Conclusion

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.

Issue

Quantitative assessment of the transport and distribution of photosynthetic carbon and exogenous nitrogen in a rice–soil system under long-term straw return: An isotope trial using ¹³C and ¹⁵N labelling

Shiqi Yang, Liming Chen, Yifan Tang, Xueming Tan, Yongjun Zeng, Xiaohua Pan, Yanhua Zeng

Journal of Integrative Agriculture (JIA) 2025, 24(10): 4063-4077

Published: 04 April 2025

Abstract

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Straw return has demonstrated significant potential for enhancing carbon (C) sequestration and nitrogen (N) uptake while concurrently promoting plant productivity. However, the specific transport and distribution of C produced by photosynthesis and exogenous N within the rice plant–soil system under straw return remains unclear. A long-term straw return pot trial experiment was conducted in a double cropping rice system, incorporating treatments of inorganic fertilizer application with straw removal (F), straw burning and ash return with reducing inorganic fertilizers (SBR), and straw return with reducing inorganic fertilizers (SR) to investigate C sequestration and exogenous N uptake using ¹³C pulse and ¹⁵N isotope tracer techniques. The SR treatment had significantly higher soil ¹³C abundance, by 24.4 and 25.4%, respectively, ¹³C concentrations in aboveground plant parts, by 18.4 and 35.8% respectively, and ¹⁵N concentrations in rice panicles, by 12.8 and 34.3% than the SBR and F treatments. This enhancement contributed to a higher total organic C concentration and increased rice grain yield in the SR treatment. Furthermore, the SR treatment had significantly higher photosynthetic C, by 9.8%, which was directly transferred to soil C. The SR treatment had a higher distribution of photosynthetic C in the leaves and stems, but a lower distribution in the panicle compared to the SBR treatment. This finding is advantageous for sequestering photosynthetic C into the soil through straw return; conversely, opposite trends were observed in ¹⁵N distribution. In addition, rice plants in the SR treatment had increased N uptake from urea and soil N sources, enhancing N recovery by 9.2 and 12.5%, respectively, and reducing soil N residues. Correlation analysis showed that the SR treatment increased the concentrations of ¹³C in leaves and roots while decreasing the ¹⁵N abundance in all rice organs, thereby contributing to an increase in rice yield. The partial least square path model suggested that the increase in rice yield under the SR treatment was primarily linked to ¹³C accumulation within the rice plant–soil system. The results suggest that straw return increases the sequestration of photosynthetic C and exogenous N in the rice plant–soil system and increases N utilization efficiency, which subsequently improves both rice and soil productivity.

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