The undefined N source might be overestimated by N tracer trials.

Based on data from previous N-tracer trials, Yan et al. (Global Change Biology, 26, 191-199; 2020) accounted the sources of cereal crop nitrogen (N) from fertilizer inputs (from both current and previous seasons) and possible uptake of nonfertilizer N inputs, leaving a significant part of N source undefined. We argue that the undefined N source does not reflect the net change in soil N stock and might be overestimated for three reasons: (a) higher yield in N tracer trials; (b) a low residue return rate is assumed; (c) underestimated contribution by fertilizer inputs from previous seasons due to the lack of long-term trials.