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

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.