Temporal changes in soil organic carbon contents and δ13C values under long-term maize–wheat rotation systems with various soil and climate conditions

Understanding soil organic carbon (SOC) decomposition and its replenishment by contrasting plant residues is critical to rationally manage soil carbon pools. Long-term (15 years) field experiments in maize–wheat (Zea mays L.–Triticum aestivum L.) rotation systems at diverse sites with contrasting climates and soil properties were conducted to evaluate the temporal dynamics of the C inputs, SOC concentrations and δ13C values. In the non-fertilized Control treatments mean annual C inputs (mainly roots) at the various sites ranged from 0.39 to 1.24 Mg ha− 1, and SOC contents remained largely unchanged during the 15 years study. However, results for the fertilized treatments indicated that SOC concentration increased by 1 g kg− 1 for every 24.3 (5.4–45.2) Mg C ha− 1 from roots alone in the NPK treatment and for every 29.4 (11.1–52.6) Mg C ha− 1 from crop roots plus straws in the NPKSt treatment. Furthermore, there was a positive correlation among changes in SOC, C4-derived C and C3-derived C and the δ13C values in all treatments across the four sites. Our results suggest that the δ13C value was a useful tool to quantify temporal changes of SOC from C4 and C3 plants, even when actual changes in soil C stock were small in these wheat–maize rotation cropping systems.