Response of global farmland soil organic carbon to nitrogen application over time depends on soil type

Carbon (C) and nitrogen (N) are closely coupled to regulate the cycle of soil organic carbon (SOC) and its feedback to climate change in terrestrial ecosystem. However, the response of SOC content to N input in different soil types on time scales is still less understood. Here, we did a meta-analysis by collecting global farmland data across 60 years based on soil taxonomy, as well as our long-term field agricultural experiment from 2006 to 2019 (total number of observations, n = 301). The results showed that N addition promoted the increase of SOC content on an average of 9.1% in agrosystem, but varied considerably in different soil types (5.2%–12.0%). The SOC content of Alfisols and Mollisols soils showed a significant increasing response along the time scale. Furthermore, random forest model and structural equation model analysis indicated that climate factors and soil types indirectly affect the temporal response of SOC to N application through soil properties such as TN and C:N. This study emphasizes that the temporal response of SOC content to long-term N addition depends on soil type, with an scale-up effect in soil types with relative high SOC content (like Alfisols and Mollisols), which provides a new perspective for better understanding the C sequestration as the consequence of coupling effect of C and N in farmland system under long-term human activities.