Due to the difficulty in widely applying site-specific methods in the millions of small-holder farms of China with technical or financial limitations, nitrogen input has not been accurately matched with the actual requirements for crops, resulting in excessive N surplus in soils. Loss of N via nitrification-denitrification, NH3 volatilization, nitrate leaching and runoff to atmosphere and water environment has caused serious environmental issues, such as excessive nitrate content in groundwater, eutrophication, PM2.5 and greenhouse gas emissions. Based on the thresholds of N application, restriction of excessive N input is urgently needed to ensure environmental safety. Apparent N balance is a simple form of N balance, usually expressed as the difference between nutrient input via fertilization and output via crop harvest removal that is simply calculated and easily understood. Therefore, this research raises two questions: What is the upper limit of N surplus apparent in the soil in rice paddies of China? Can the apparent N balance be used as an indicator for estimating thresholds of nitrogen input in rice paddies?

Based on observations from a large number of field experiments conducted in rice cropping systems of China, the study established the relationships between N balance and ammonia (NH3) volatilization, the yield increase ratio, and the N application rate, respectively. A dramatic increase in NH3 volatilizations and a stagnant increase in the rice yields were observed when the N surplus exceeded certain levels. Using a piecewise regression method, the seasonal upper limits of N surplus were determined as 44.3 and 90.9 kg N ha−1 under straw-return and straw-removal scenarios, respectively, derived from the responses of NH3 volatilization, and were determined as 53.0–74.9 and 97.9–112.0 kg N ha−1 under straw-return and straw-removal scenarios, respectively, derived from the maximum-yield consideration. Based on the upper limits of N surplus, the thresholds of N application rate suggested to be applied in single, middle-MLYR, middle-SW, early, and late rice types ranged from 179.0 to 214.9 kg N ha−1 in order to restrict NH3 volatilization, and from 193.3 to 249.8 kg N ha−1 in order to achieve maximum yields.

Graphical abstract

The research results were published in the international academic journal Environmental Pollution (IF 8.071). Assistant research fellow Ding Wencheng from the Innovation Team of Plant Nutrition of the Institute of Agricultural Resources and Regional Planning of the Chinese Academy of Agricultural Sciences is the first author, and research fellow He Ping is the corresponding author. This research was funded by the National Key Research and Development Program of China (2016YFD0200101) and the Basic Research Fund of the Central Public-Interest Scientific Institution (1610132021003).

http://doi.org/ 10.1016/j.envpol.2021.118091