The soil fertility and improvement team of the Institute of Agricultural Resources and Regional Planning (IARRP), CAAS, recently achieved a breakthrough in crop fertilization which was published internationally. The team elucidated the long-term effects of different fertilization practices on crop grain nitrogen content and nitrogen use efficiency — and it also revealed the limitations of assuming a fixed grain nitrogen content. The related findings were published in the international scientific journal Resources, Conservation & Recycling under the title of the "Divergent long-term effect of synthetic and organic fertilizer inputs on grain nitrogen content and its impact on nitrogen use efficiency assessment".

Broadly speaking, grain nitrogen (N) content is an important indicator of grain protein content and nutritional quality, and it is also an essential parameter for calculating N 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. The study integrated over 20 years of observational data from 11 long-term field experiments across China to systematically assess the long-term effects of fertilization practices on grain N content and yield in wheat and maize, and to quantify potential biases in NUE estimates arising from the assumption of a fixed grain N content. 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%. These results highlight the importance of continuously monitoring grain N content and its temporal changes under different fertilization practices.

The study combines the long-term dynamic changes in grain N content with the evaluation of NUE, clarifying the limitations of using fixed grain N content under different crops and fertilization conditions. This reportedly expands the perspective for evaluating the long-term effects of fertilization, providing scientific evidence for optimizing long-term fertilization strategies while balancing grain yield, protein quality, and efficient N use. Furthermore, the study recommended establishing a grain N content monitoring system that integrates long-term field experiments with on-farm monitoring across diverse agroecological regions and fertilization practices. Regular updates of the parameters used in NUE assessments and N balance calculations would support more accurate evaluations of N use at regional and national scales and inform sustainable N management.

A framework diagram of the long-term effects of different fertilization measures on grain nitrogen content and nitrogen use efficiency 

IARRP postdoctoral researcher Ren Keyu is the first author of the paper. Researcher Duan Yinghua from IARRP's soil fertility and improvement team and Professor Zhang Xin from the University of Maryland Center for Environmental Science are the corresponding authors. Academician Xu Minggang and researcher Zhang Wenju participated in and guided the work. The research was jointly supported by the State Key Laboratory of Efficient Utilization of Arid and Semi-arid Cropland in Northern China, the Science and Technology Innovation Project of the Chinese Academy of Agricultural Sciences, the Long-term Factors Network of the Ministry of Agriculture and Rural Affairs, and the US National Science Foundation.

Citation:

Ren K, Xu M, Zhang W, Duan Y, Zhang X. Divergent long-term effect of synthetic and organic fertilizer inputs on grain nitrogen content and its impact on nitrogen use efficiency assessment. Resources, Conservation & Recycling 237, 109182 (2027).

Link: https://doi.org/10.1016/j.resconrec.2026.109182