Fates and controlling factors of nitrogen fertilizers in relation to grain yield after the application season in wheat cropping systems: A meta-analysis of 15N field trials

Understanding the fate and controlling factors of nitrogen (N) fertilizer after the application season of 15N fertilizer added in a cropping system is key for improving crop yield and N use efficiency (NUE) and reducing its loss. However, it remains unclear about the fates and controlling factors of N fertilizers after the application season in wheat cropping systems in relation to grain yield. The aim of this study was to comprehensively summarize the main controlling factors influencing N fertilizer fate and grain yield in wheat cropping systems in the different regions. The meta-analysis was used to examine the effects of agricultural management practices, climates and soil properties on fates of N fertilizer after the application season of wheat and grain yield by compiling a dataset of 207 15N field tracer observations from 64 publications worldwide. The proportions of 15N uptake, 15N soil residue and 15N loss after the application season in wheat cropping system were 38% (ranging from 10% to 78%), 33% (ranging from 9% to 70%) and 29% (ranging from 1% to 62%), respectively. Wheat yield and 15N fate were strongly affected by mean annual precipitation (MAP) and mean annual precipitation /mean annual potential evaporation (P/PET). 15N remaining in soil after the application season was as high as one third of N fertilizer applied. Optimizing N application rate and soil moisture are key to guarantee wheat grain yield and reduce N loss. The fate of 15N remaining in soil after the application season needs to be studied over even longer time. The high variations in N fate and wheat grain yield in the different regions highlight that there is more space to increase NUE, and reduce N loss to the environment at the global scale. Understanding the long-term residual N in soil at both field and regional scales is important to comprehensively evaluate its agricultural and environmental effects.