Divergent long-term effect of synthetic and organic fertilizer inputs on grain nitrogen content and its impact on nitrogen use efficiency assessment

Grain nitrogen (N) content is an important indicator of grain protein content and nutritional quality, and it is also an essential parameter for calculating nitrogen use efficiency (NUE) in cropping systems. Despite the potential influence of fertilizer, grain N content is often treated as a constant for a given crop, and few studies have evaluated its variability under different fertilization practices over the long term. In this study, we analyzed historical data from 11 long-term experiments in China to assess how fertilization practices impact grain N content and to quantify potential bias in NUE assessments arising from assuming a fixed grain N content. N inputs were not strictly equalized among treatments. N application significantly increased grain N content by 19% in wheat and 20% in maize relative to the unfertilized control. However, the increase diminished for wheat and stabilized for maize over the long term in synthetic fertilizer treatments. Compared with synthetic NPK fertilizer alone, the incorporation of manure effectively enhanced grain N content over the long term, despite a short-term reduction. Ignoring changes in grain N content led to an underestimation of wheat NUE by 11% and overestimation of maize NUE by 17%, underscoring the critical need to track grain N contents over time and across different fertilizer practices.