Evaluation of long-term organic carbon dynamics and organic matter stability in a cultivated paddy soil using a carbon and nitrogen stable isotopes-based model

Understanding soil organic matter (SOM) stability and carbon storage under long-term intensive land management systems is vital for estimating the C sequestration potentials of agricultural lands. Available methods currently used for measuring the stability of SOM, such as radiocarbon dating, are expensive. However, a costeffective and reliable stable isotope-based model was developed for grasslands which utilizes the C/N ratio and delta 15N of soils to determine the stability of SOM. We applied this model to test for the first time its usefulness for evaluating SOM stability in the top and subsoils of a subtropical paddy field cultivated for 37 years. We also compared paddy soil's SOM stability and carbon storage under different fertilizer treatments. Treatments evaluated were a control (CK), inorganic fertilizer (NPK), NPK+cattle manure (NPKM), and NPK+rice straw (NPKS). Results showed that the C/N ratios and delta 15N signatures of POM and mOM proved effective in predicting SOM stability as predicted by the model. SOM stability increased with depth across the treatments, especially in NPKM and NPKS in the deeper soil layer compared to CK and NPK. Introducing delta 13 C as an indicator further strengthened the model, revealing higher delta 13 C accumulation in the subsoils, indicating higher C sequestration. Further insights showed that unlike in the CK treatment, the use of NPKM and NPKS significantly increased SOC accumulation by increasing the proportion of mineral-protected organic matter (mOM) compared to the microbial-accessible particulate organic matter (POM). The abundance of mOM associated with a lower rate of decomposition in the paddy fields and the incorporation of higher 13 C in the subsoils under NPKM and NPKS were key factors that enhanced the long-term increase in organic C and SOM stability. Thus, the stable isotopebased model proved a reliable proxy for estimating SOM stability in paddy fields.