Anthropogenic activities have raised nitrogen (N) input worldwide with profound implications for soil carbon (C) cycling in ecosystems. The specific impacts of N input on soil organic matter (SOM) pools differing in microbial availability remain debatable. For the first time, we used a much‐improved approach by effectively combining the 13 C natural abundance in SOM with 21 years of C 3 –C 4 vegetation conversion and long‐term...