Long-term impacts of mineral and organic fertilizer inputs on nitrogen use efficiency for different cropping systems and site conditions in Southern China

The application of nitrogen (N) fertilizer is key to realize high crop yields and ensure food security. Excessive N application and relatively low N use efficiency (NUE), however, have led to substantial N losses to air and water with related impacts on biodiversity and health. We used historical data from 13 long-term experiments to unravel how the NUE depends on fertilizer strategy and site conditions such as crop rotation, soil properties and climate. During nearly 40 years of fertilization, NUE decreased when crops were fertilized with N and potassium fertilizers only, while the NUE increased when multi-nutrient or organic fertilizers were used. The highest NUE was found when 25-30% of the total N input was supplied in organic form. Among the site conditions analysed, soil pH was the most important factor controlling NUE with an optimum pH around 6. In addition to soil acidity, phosphorus availability increased NUE. Crop rotation, soil properties and fertilizer management together explained 46-85% of the variation in NUE, allowing site specific fertilizer strategies to be developed boosting NUE. Current NUE equalled on average 30% in paddy soils, 39% in upland soils and 42% in paddy upland soils. Optimizing all fertilizer inputs and soil nutrient levels might increase the NUE up to 40-47% in paddy soils, up to 40-77% in upland soils and even up to 40-87% in a paddy upland soils.